JP2001168519A - Mixed mounting structure, mixed mounting method, and electronic device - Google Patents

Abstract

(57) [Abstract] [Problem] To prevent reflow soldering from peeling off during flow soldering in mixed mounting of reflow and flow using lead-free solder, within the range of heat resistance of components. I do. SOLUTION: On the surface A of a substrate 2, electronic components 3a and 3b are surface-connected and mounted by reflow soldering,
On the surface B of the substrate 2, the leads 10 of the electronic component 6a on the surface A are connected by flow soldering to the electrodes 7 by flow soldering. Here, the solder 5 used for the reflow soldering on the A side is Sn- (1.5 to 3.5 wt%).
Ag- (0.2-0.8 wt%) Cu- (0-4 wt%
%) In- (0 to 2 wt%) Bi is a lead-free solder having a composition of Bi, and the solder 8 used for flow soldering on the B side is Sn- (0 to 3.5 wt%) Ag.
-A lead-free solder composed of a composition of (0.2 to 0.8 wt%) Cu is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hybrid mounting structure, a hybrid mounting method and an electronic apparatus assembled by reflow soldering and flow soldering by using lead-free solder for an electronic component on a wiring board. .

[0002]

2. Description of the Related Art Conventionally, Sn-Pb eutectic solder (melting point: 1)
(83 ° C.). Due to the various excellent properties of such solder, reflow soldering is provided on one surface of the wiring board (a solder layer is provided in advance on a soldered portion, the solder layers of these soldered portions are butted, and these solder layers are heated and melted. (Reflow soldering)
To connect and mount the electronic components, and then connect and mount the electronic components on the other surface by flow soldering (contacting or immersing the soldered part in molten solder and soldering. Also called flow soldering) In the so-called mixed mounting (hereinafter, such mixed mounting is referred to as reflow-flow mixed mounting), there are few problems in the process, reliability, etc., and the heat resistance of the components is also reduced. It can withstand the temperature of soldering.

[0003] Lead-free solders that do not use lead include:
Japanese Patent Application Laid-Open Nos. 8-215880 (hereinafter referred to as Conventional Technique 1) and Japanese Patent Application Laid-Open No. 10-193169 (hereinafter referred to as Conventional Technique 2) are known.

[0004] In the prior art 1, the Sn remaining, Ag (0.5
~ 3.5wt (wt)%), Cu (0.5 ~ 2.0wt)
%), Which can lower the melting temperature, and has good wettability and mechanical strength.

[0005] Further, in the prior art 2, 1 wt% or more and 5 w
Ag of not more than t%, Bi and In of not less than 0.1 wt% and not more than 14 wt% and not less than 0.1 wt% and not more than 10 wt% and the total of both being not more than 15 wt%, and not less than 0.1 wt% and not more than 2 wt%
It describes a lead-free solder alloy containing less than wt% of Cu, with the balance being Sn and unavoidable impurities. This further lowers the melting point of the Sn-Ag based solder alloy and suppresses an increase in cost, and has both good wettability and good mechanical properties not only at room temperature but also at high temperatures.

[0006]

By the way, when electronic components are mounted at a high density, they can be mounted on both sides of the wiring board. That is, one side (A side) of the wiring board is soldered by reflow soldering. It is indispensable to mount the reflow flow in which the other surface (surface B) is soldered by flow soldering or the like. In this case, the substrate is largely warped at the time of flow soldering, but it is necessary not to melt the reflow soldering joint on the A surface in order to ensure high reliability.

Further, in this mixed mounting, a multi-pin LSI (Large Scale Integration) terminal is connected at a high density (narrow pitch connection), a defective electronic component or an electronic component broken for some reason is removed, and a new one is removed. Work (repair work) for re-installing various electronic components without melting the solder joints of adjacent components, and
What can be performed without affecting the adjacent components without heat is a necessary condition for mixed mounting.

However, the above-mentioned prior arts 1 and 2 are effective for mounting on only one side of a circuit board, but do not take into account a lead-free solder material and a soldering method that can meet the above-mentioned requirements for mixed mounting. .

An object of the present invention is to solve such a problem,
It is an object of the present invention to provide a hybrid mounting structure, a hybrid mounting method, and an electronic device which can realize, with high reliability, mixed mounting of a reflow flow, which is the strictest mounting process, using lead-free solder.

Another object of the present invention is to provide a wiring board which can be solder-bonded within a range of heat resistance of components in a mixed mounting of a reflow flow, which is the strictest mounting process, using lead-free solder. An object of the present invention is to provide a hybrid mounting structure, a hybrid mounting method, and an electronic device which enable high-reliability solder bonding without causing reflow soldering joints to be peeled even when warpage occurs.

[0011] Still another object of the present invention is to provide a connection mounting part having a thermal fatigue characteristic in a mixed mounting of a reflow flow, which is the strictest mounting process using lead-free solder.
The evaluation condition of reliability is in a severe category -55-1
An object of the present invention is to provide a hybrid mounting structure, a hybrid mounting method, and an electronic device having a joint that can be guaranteed even under temperature cycle test conditions such as 25 ° C.

[0012]

In order to achieve the above object, in a hybrid mounting structure according to the present invention, an electronic component is mounted on one surface of a wiring board by using Sn-Ag-Cu or I-Ag-Cu.
Using a lead-free solder to which at least one of n and Bi is added, surface connection mounting is performed by reflow soldering, and another electronic component is mounted on the other surface of the wiring board by a ternary system of Sn-Ag-Cu. Are connected and mounted by flow soldering using lead-free solder composed of

In the hybrid mounting structure according to the present invention, the lead-free solder composed of the Sn-Ag-Cu ternary system may be made of Sn- (0-3.5 wt%) Ag- (0.2-
0.8 wt%) Cu, and the above-mentioned Sn-Ag-Cu or In, Bi
Of the lead-free solder to which at least one of Sn- (1.5 to 3.5 wt%) Ag- (0.2 to 0.
8 wt%) Cu- (0-4 wt%) In- (0-2 wt%)
%) Bi.

An electronic device according to the present invention has a configuration including the above-described hybrid mounting structure.

In the hybrid mounting method according to the present invention, an electronic component is reflowed on one surface of a wiring board by using Sn-Ag-Cu or lead-free solder to which at least one of In and Bi is added. Surface connection mounting by soldering,
Then, another electronic component is placed on the other surface of the wiring board by Sn
A lead-free solder composed of a ternary system of -Ag-Cu is used for connection mounting by flow soldering.

Further, in the hybrid mounting method according to the present invention, the electronic component may be formed by placing Sn-Ag-Cu,
Alternatively, surface connection mounting is performed by reflow soldering using a lead-free solder to which at least one of In and Bi is added, and then another electronic component is mounted on the first of the wiring board.
Using a lead-free solder composed of Sn-Ag-Cu ternary system on the second surface opposite to the surface, the temperature of the soldered joint on the first surface is fixed by the solidification of the solder. It is connected and mounted by flow soldering while keeping the temperature below the phase line temperature.

Further, the hybrid mounting method according to the present invention provides an S-mounting method for connecting and mounting by the above-mentioned flow soldering.
Sn- (0-3.5 wt%) Ag- (0.2-0.8w) lead-free solder composed of a ternary system of n-Ag-Cu
t%) The solder has a composition of Cu 2.

Further, the hybrid mounting method according to the present invention provides a method of mounting the semiconductor device by using Sn-Ag-Cu used for connection mounting by reflow soldering, or one of In and Bi.
The lead-free solder to which the seeds or more are added is Sn- (1.5 to 3.5 wt%) Ag- (0.2 to 0.
8 wt%) Cu- (0-4 wt%) In- (0-2 wt%)
%) A solder having a composition of Bi.

As described above, the electronic component is provided on one surface of the wiring board with Sn- (1.5 to 3.5 wt%) Ag- (0.2 to 0.
8 wt%) Cu- (0-4 wt%) In- (0-2 wt%)
%) Using a solder having a composition of Bi and surface mounting by reflow soldering. Thereafter, another electronic component is formed on the other surface of the wiring board by Sn- (0 to 3.5 wt%) Ag- (0 wt. 0.2-0.8w
(t%) Since connection and mounting are performed by flow soldering using solder having a composition of Cu, mixed mounting can be realized with high reliability. At this time, the temperature of the reflow soldering joint on the one surface of the wiring board is kept at or below the solidus temperature of the solder when the other surface of the wiring board is subjected to flow soldering. This is to perform mixed mounting.

[0020]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram showing an embodiment of a mixed mounting structure according to the present invention used for an electronic device or the like, wherein 1 is a mixed mounting structure, 2 is a wiring board, and 3a is a QFP.
-LSI, 3b is SOP-LSI, 4 is electrode pad, 5
Is a soldering joint by reflow soldering (reflow soldering joint), 6a is a lead component, 6b is a chip component, 7 is an electrode, 8 is a soldering joint by flow soldering (flow soldering joint), 9, 10
Is the lead.

In FIG. 1, a hybrid mounting structure 1 having a connection structure using lead-free solder shown as this embodiment has a QFP having leads 9 in four directions on one surface (hereinafter referred to as A surface) of a wiring board 2. An LSI 3a, an SOP-LSI 3b having leads 9 in two directions, and a lead component 6a are mounted.
Further, the chip component 6b is provided on the other surface (hereinafter, referred to as surface B).
Is implemented.

The wiring board 2 is made of glass epoxy or the like having a maximum heat resistance of about 260 ° C. A QFP (Quad Fl) having a maximum heat resistance of 240 ° C.
at Package)-Lead 9 or SOP (Small Ou) of LSI 3a
The lead 9 of the t-line package) -LSI 3b, the lead of the TSOP (Thin Small Out-line Package) -not shown, the electrode of the chip component not shown, and the like are mounted on the electrode pad 4 provided on the A surface on the Sn-Ag. The surface mounting connection is performed by a reflow soldering joint by reflow soldering using -Cu or a lead-free solder to which one or more of In and Bi is added, that is, a reflow soldering joint 5.

On the other hand, an electrode 7 to which the lead 10 of the lead component 6a is connected and an electrode 7 to which the chip component 6b is connected are provided on the surface B of the wiring board 2 in advance. An insertion hole penetrating through 7 is provided. Here, the lead component 6 a is arranged on the A-plane side of the wiring board 2, and its lead 10 is connected to the electrode 7.

Then, when the mounting on the A side of the wiring board 2 is completed, flow soldering is performed on the B side (by the above-mentioned flow soldering, and the soldered portion is brought into contact with or dipped in molten solder. The lead 10 of the lead component 6a to be soldered) is inserted into the insertion hole of the electrode 7 inserted into the insertion hole of the wiring board 2 penetrating from the A side, and the tip of the lead 10 is It is set in a protruding state. Next, in such a state, S
The lead 10 of the lead component 6a is mounted and connected to the electrode 7 by a soldering joint by flow soldering using ternary lead-free solder of n-Ag-Cu, that is, a flow soldering joint 8. At this time, the chip component 6
The flow soldering joint 8 of the solder layer is formed on the electrode 7 to which b is connected, and the chip component 6b is mounted and connected to the electrode 7 by flow soldering (that is, flow soldering) using the solder layer. Is done.

As described above, the reflow-flow hybrid mounting structure of this embodiment is formed by using lead-free solder, and is used for various electronic devices. Part 3 (QFP-LS
I3a or SOP-LSI3b) on a surface A of the wiring board 2 made of glass epoxy or the like, a reflow soldering joint using Sn-Ag-Cu or a lead-free solder to which at least one of In and Bi is added. Then, the lead component 6a and the chip component 6b are mounted on the B surface of the wiring board 2 made of glass epoxy or the like by Sn-Ag-Cu.
And a flow soldering joint 8 using ternary lead-free solder.

In the above description, the case where the wiring board 2 on which the electronic components are mounted is a glass epoxy board having a narrow range of conditions for obtaining an optimum connection because of its low heat resistance and easy warpage is taken as an example. However, in the case of a ceramic substrate, the heat resistance is excellent and the warpage is small. Can also be applied sufficiently.

First, the composition of the solder used in the flow soldering process on the B side of the wiring board 2 made of glass epoxy or the like is Sn- (0 to 3.5 wt%) Ag- (0.2
Sn-A which is Cu and does not contain Bi
A ternary lead-free solder of g-Cu was selected. This is because the solder containing Bi can lower the flow soldering temperature and is advantageous from the viewpoint of the heat resistance of the wiring board 2 and the electronic component 3, but the Cu pad as the electrode pad 7 during the flow soldering is used. A lift-off phenomenon in which the fillet is peeled off from the surface occurs. The reason is that the melting temperature of the solder containing Bi has a wide range, so that the segregation of Bi easily occurs and the connection strength is weak.

In order to prevent defects due to lift-off, it is conceivable to increase the cooling rate. However, in order to perform this in an actual process, a flow soldering apparatus having a special cooling device is required. In addition, it is difficult to reliably control the cooling rate of a mounting board on which electronic components having various thicknesses and various heat capacities are mounted.

For this reason, the solder for flow soldering is Sn-Ag-Cu ternary lead-free solder containing no Bi. This Sn-Ag-Cu
As for the composition range of the ternary lead-free solder, the addition amount of Ag is defined as a range in which primary crystals of Ag ₃ Sn are not generated.
Up to 3.5 wt%, the amount of Cu added is 0.8 wt%
If it exceeds, the elongation will decrease, so the upper limit is 0.8w
t%. In addition, since Cu is slightly contained, C
When a u-pad, a Cu lead, or the like is to be soldered, the lower limit of the added amount of Cu is set to 0.2 wt% because it is effective in preventing the erosion of Cu. By using such a solder composition, a lift-off defect on the flow soldering surface can be prevented.

However, in the above composition range of the solder, the melting point is about 215 ° C. to 230 ° C. In consideration of the non-uniform distribution of the temperature in the wiring board 2, the flow soldering surface has the heat resistance of the electronic component 3. It must be heated to a temperature of about 240 ° C. Further, in the flow soldering, since the heating time is short, heating up to around 250 ° C. may be required in some cases.

Therefore, the reflow soldering joint 5 on the surface A of the wiring board 2 must have a solder composition that can withstand the flow soldering process at an early stage and that can sufficiently secure the thermal fatigue resistance during use.

Next, an example of the composition of the lead-free solder used for the reflow soldering of the surface A will be described in consideration of the above restrictions.

In reflow soldering using lead-free solder, the time of exposure to high temperature is longer than that of flow soldering and the cooling rate is slower, so that the heat resistance of the substrate 11 and the electronic components 3 becomes a problem. Come. In order not to impair the heat resistance of the current substrate 11 or electronic component 3, (a) the surface temperature of the electronic component 3 needs to be set to approximately 240 ° C. or less at the maximum. In addition, as a lead-free solder, Sn-Ag 2
A ternary system or a ternary system of Sn-Ag-Cu is conceivable, but in order for these to normally wet from the liquidus temperature, (b) a binary lead-free solder of Sn-Ag must be used as the connection temperature. At least 225 ° C., Sn—Ag—Cu ternary lead-free solder requires at least 220 ° C.

Even if a forced convection type reactor having a small variation in the furnace temperature is used as the reflow furnace, the variation in the temperature of the joint portion on the large-sized wiring board 2 of about 300 mm square is about 10 to 15 ° C. Therefore, the highest temperature is 240 ℃
Then, with a certain soldering joint, the temperature may be about 225 ° C., which is the lowest level at which wetting can be ensured. Therefore, under the condition that the heat resistance of the member can be ensured or the temperature variation in the furnace is reduced, a ternary lead-free solder of Sn-Ag-Cu can be used for reflow soldering. In the case where a bad component or a large difference in heat capacity between the components and a large temperature variation in the wiring board 2, another solder that can ensure high reliability is required.

For this reason, it is necessary to lower the reflow temperature, and Sn-Ag-
A quaternary lead-free solder of Bi-Cu was studied.

The concentration of Ag and Cu in the solder was 3 wt.
% And 0.7 wt%, and the melting point when Bi is added is shown in FIG. 2. When the Bi addition amount is 3 wt%, the liquidus temperature is 216 ° C., and when the Bi addition amount is 5 wt%, the liquid The phase line temperature drops to 213 ° C., and the reflow soldering becomes easier when the heat resistance temperature of components and the variation in the furnace temperature are considered. When Bi is added in an amount of 6 wt% or more, the reflow soldering is further facilitated because the melting point is further lowered.

However, it has been found that an increase in the amount of added Bi causes the following problem.

The first problem is that the amount of Bi added is about 5 wt%.
If it increases as described above, the elongation characteristic of the material is reduced as shown in FIG. When the elongation characteristic of the solder material is reduced, the thermal stress generated in the reflow soldering joint 5 cannot be reduced at the time of connection or use, and the reliability is greatly reduced.

The second problem is that the amount of added Bi is about 5 wt%.
If the number is larger than the above, the strength (peel strength) of the bonding interface is reduced. A 42 Alloy lead frame, which is generally used as an LSI lead, has a Sn
Sn-3wt% Ag- with respect to the plated lead and Cu used as the electrode material of the wiring board 2.
FIG. 4 shows the strength of the bonding interface (vertical peel strength) when Bi was added to the solder having a composition of 0.7 wt% Cu.
5 and FIG. 5, it can be seen that the connection strength decreases as the amount of Bi increases.

Such a decrease in the strength is particularly caused by the 42% plating of Sn-10 wt% Pb which is widely used at present.
This becomes noticeable for Alloy leads. The result is shown in FIG.
It can be seen that even when Bi is added at 2 wt%, the interface strength is significantly reduced. The cause of this strength drop is B
This is probably due to the segregation of i. In particular, it is considered that the strength decrease in the case of Pb-containing metallization is caused by the formation of Sn—Pb—Bi low-temperature phase (97 ° C.) between Bi and Sn in the solder and Pb mixed from the plating film.

FIG. 7 shows a DSC (differential scanning calorimetry) of a connecting portion of the strength evaluation sample shown in FIG.
ing calorimetry) (relationship between temperature (° C.) and heat flow (mcal / sec)). However, FIG. 7A shows the case where the amount of Bi added is 0 wt%, and FIG.
FIG. 7 (c) shows that the additive amount of Bi is 7 wt%.
Each case of t% is shown, and the addition amounts of Ag and Cu are fixed as described above.

Up to about 4% by weight of Bi in the solder, as shown in FIGS. 7A and 7B, an endothermic peak due to melting is not seen up to about 180 ° C., but Bi is reduced to about 7% by weight. Then, as shown in FIG. 7C, a low-temperature phase is observed. The formation of such a low-temperature phase hastens deterioration in a practical use environment.

The third problem is that the reflow soldering joint 5 having a weak connection strength may cause the wiring board 2 to warp during the flow soldering on the B side of the wiring board 2 depending on the size of the electronic component 3. And it causes peeling failure. An example is shown below.

A large QFP-LSI (lead: Sn plating on 42 Alloy) is reflow soldered to the A side of the wiring board 2 with Sn-3Ag-0.7Cu-5Bi solder (melting point: 204 to 213 ° C.), and then , B side, Sn-3
Using Ag-0.5Cu solder (melting point: 218 ° C.)
Flow soldering was performed at 250 ° C. Sn-3Ag
After the reflow soldering process using −0.7Cu-5Bi solder, that is, the strength of the reflow soldering joint 14 of the LSI before the flow soldering is almost the same as that of the Sn—Pb eutectic according to the 45 ° peel test. And had sufficient strength. Further, even when the temperature cycle test at -55 to 125 ° C. was performed, the reflow soldering joint 5 of the QFP-LSI 3a was at a level that could sufficiently clear 1000 cycles. However, the reflow soldering joint 14 peeled off after the flow soldering. In order to investigate the cause, EDX was applied to the fractured surface of the reflow soldering joint 14 that was peeled off during the flow soldering process (hereinafter referred to as the flow process).
(Energy dispersive X-r analysis: energy dispersive X-r
As a result of the ay spectroscopy) analysis, the destruction site was the interface between the lead 9 and the solder 14, and as shown in FIG. (5 wt%), it was found to be contained in a large amount.

From this, at the bonding interface, the temperature rises to around 204 ° C. of the solidus during the flow process, and the warpage of the wiring board 2 is also added, so that Bi segregates along the interface (concentration of concentration). Increase) and the connection strength decreases,
In this case, it is considered that peeling occurred. further,
In the case of a solder containing a large amount of Bi, the reflow soldering joint 5 is remelted during the flow process, and this phenomenon becomes more remarkable.

On the other hand, even if there is no problem such as separation at the initial stage in the reflow / flow mixed mounting process, if the concentration of Bi at the connection interface increases due to heat during the flow process, the lead of the electronic component 3 will not increase. The reflow soldering joint 14 for SOP-LSI or TSOP-LSI with high rigidity and severe stress is -55 to 125 ° C.
Under the temperature cycle conditions, the target life cannot be satisfied. That is, the connection structure is degraded by a temperature cycle associated with the ON / OFF operation of the switch in the electronic component 3 and is broken even by a weak stress. Therefore, at the time of flow soldering, it is an important condition for maintaining high reliability that the temperature of the reflow soldering joint 14 on the surface A of the large-sized LSI be kept at least equal to or lower than the solidus temperature.

From the above, it is necessary to minimize the amount of Bi added. Since the solubility limit of Bi in Sn-Ag-Cu solder to Sn crystal is about 1.5 wt%,
Adding more Bi lowers the melting point,
Bi that cannot form a solid solution precipitates and becomes brittle due to a decrease in mechanical properties, particularly, elongation that greatly affects reliability, as shown in FIG. 3, and causes a decrease in the strength of the joint interface.

FIG. 9 shows the tensile strength when Bi was added to the composition of Sn-3Ag-0.7Cu.
It can be seen that as the addition amount of i increases, the tensile strength of the solder increases and the solder becomes harder. Therefore, as is clear from FIG. 3, since the elongation does not deteriorate up to the addition amount of Bi of 2 wt%, the properties of the bulk material can be said to be excellent up to the addition amount of Bi of 2 wt%. The upper limit was found to be 2 wt%.

Conversely, by adding even a small amount of Bi, the surface tension of the solder is reduced, and the flowability is increased. Therefore, a remarkable effect is achieved in reducing the bridge of narrow pitch leads to LSI and preventing chip standing. This property can be used to improve the yield.

From the above, from the viewpoint of the properties of the bulk solder material and the bridging resistance, the amount of Bi added is set to 2.
It has been found that 0 wt% is the upper limit. No more Bi
The amount of addition leads to a decrease in the strength of the interface, and is not suitable as a solder for the surface of the wiring board 2 in the hybrid mounting structure 1 on which reflow soldering is performed.

The above is the addition of Bi. Similarly, in order to lower the soldering temperature and ensure high reliability, lead-free solder used for reflow soldering on the A side of the wiring board 2 is used. In to Sn-Ag-Cu solder
The addition was also considered.

First, looking at the effect of the addition of In on the melting point, the addition of In to the Sn-3Ag-0.7Cu solder is as shown in FIG. The melting point tends to decrease as the amount of addition to the -3Ag-0.7Cu solder increases. However, as shown in FIG. 11, even when the amount of In added to the Sn—Ag—Cu solder is increased, as shown in FIG. I knew it wasn't. In addition, as shown in FIG.
FIG. 4 also shows a plated 42 Alloy lead.
As apparent from comparison with the case of Bi, it was found that the peel strength was not remarkably reduced as in the case of Bi. Furthermore, S
An experiment was performed in which the amount of Bi and the amount of In were both changed with respect to the ternary solder of n-Ag-Cu, and even in that system, the mechanical reliability of the soldering joint was determined by the amount of added Bi. It has been clarified that the amount of In added has a function of lowering the melting point without substantially impairing the mechanical properties.

Next, the result of a detailed study of the In content in consideration of the Bi addition range will be described.

FIG. 13 is a graph showing changes in the liquidus temperature and the solidus temperature with respect to the amounts of added Bi and In when the Sn-3Ag-0.7Cu solder is used as an example.
The left half of the horizontal axis indicates the range of addition of Bi of 0 to 5 wt%, and the right half indicates the range of addition of In of 0 to 5 wt%.

FIG. 14 shows the same Sn-3 as in FIG.
FIG. 14 is a diagram illustrating a change in elongation with respect to the amount of added Bi and In, using an Ag-0.7Cu solder as an example, where the vertical axis indicates elongation and the horizontal axis is the same as FIG.

13 and 14, in both cases, the amount of Bi added to the Sn-3Ag-0.7Cu solder was 2 wt%, and the amount of In added at this time was 0 to 5 wt%.
%, The changes in the liquidus / solidus temperature and elongation are shown by broken lines in the right half of each drawing.

According to FIG. 13, for example, Sn-3Ag-
0.7Cu-2Bi-3In (Sn-3Ag-0.7C
The melting point of the solder is close to that of the Sn-3Ag-0.7Cu-5Bi solder and lower than that of the Sn-3Ag-0.7Cu-2Bi solder. From this, Sn-3Ag-
The reflow temperature of the 0.7Cu-2Bi-3In solder is
This is possible at the same level as Sn-3Ag-0.7Cu-5Bi solder.

On the other hand, according to FIG. 14, even if In is added, there is almost no deterioration in elongation, and there is little deterioration in connection strength. Therefore, the flow process on the back surface (B surface) of the wiring board 2 is initially sufficient. It is durable and has no problem in long-term thermal fatigue properties.

As described above, the above-mentioned solder composition can solve the above-mentioned problem in the hybrid mounting structure 1.

Similarly, an experiment was conducted in which the amount of In was changed in a range where the amount of Bi was up to 2 wt%. It was found that a lead-free solder composition for reflow soldering on the A side was obtained. This is In
If the addition amount is 4 wt% or more, the solidus temperature becomes low, and it becomes impossible to initially endure the flow soldering process of the back surface (B surface) of the wiring board 2. Moreover,
Since In is expensive, the cost and the fact that In is easily oxidized easily affect the storage properties and printability of the paste, and the solidus temperature is lowered due to the lower solidus temperature.
Since a low-temperature phase considered to be n-In-Pb-Bi tends to appear, there is a problem that the solder structure becomes unstable.
For the above reasons, the upper limit of the amount of In added is set to 4 wt%.

The composition range of Cu is set to a range that has an effect of lowering the melting point of the Sn—Ag crystal by several degrees without impairing the mechanical properties. If the addition amount of Cu exceeds 0.8 wt%, as shown in FIG. 15, the elongation is reduced as compared with the case where no Cu is added, so the upper limit of the addition amount of Cu is 0.8 wt%. did. Moreover, the fact that a small amount of Cu is added is intended for Cu pads and Cu leads.
It is also effective in preventing erosion of Cu. Therefore, the lower limit of the added amount of Cu is set to 0.2 wt%.

When the amount of Ag added is 1.5 wt% or less, the melting point increases and the mechanical properties decrease. If the addition amount is 3.5 wt% or more, the melting point is also increased and the primary crystal of Ag3Sn grows similarly, so that the mechanical properties are lowered and the cost is increased. Therefore, the added amount of Ag is 1.
The content was 5 to 3.5 wt%.

As described above, when the surface A of the wiring board 2 is reflow-soldered using lead-free solder and the surface B is subjected to flow soldering using lead-free solder, the reflow soldering of the surface A The composition of the lead-free solder of Sn-Ag-Cu or Sn- (1.5-3.5 wt%) Ag- (0.2-0.0 wt.%) To which at least one of In and Bi is added.
8 wt%) Cu- (0-4 wt%) In- (0-2 wt%)
%) Bi could be determined to be optimal.

Therefore, in the hybrid mounting structure 1 of the reflow flow as the embodiment shown in FIG.
Reflow soldering is performed using the Sn-Ag-Cu in the above composition range or a lead-free solder to which at least one of In and Bi is added, and then the wiring board 2 Of the above composition range with respect to side B of
By performing flow soldering using ternary lead-free solder of n-Ag-Cu, initial failure does not occur within the range of heat resistance of the wiring board 2 and the electronic components 3, 6a.
In addition, an excellent effect of having excellent thermal fatigue characteristics can be obtained over a long term.

Next, an embodiment of the mixed reflow / flow process will be described in detail with reference to FIG.

Example 1 250 mm as the wiring board 2
A 300-pin glass epoxy board is used, and a 208-pin QFP-LSI 3a or SO
The P-LSI 3b, the chip component 6a, and the like were soldered by reflow soldering. In this case, as a lead-free solder used for reflow soldering, a quaternary lead-free solder such as Sn
-3Ag-0.7Cu-5Bi solder (melting point: 204-
213 ° C.) was used as a comparative example.
Sn-3Ag-0.7Cu which is a lead-free solder
Example 1 using -2Bi-3In solder (melting point: 203 to 212 ° C) was used. The solder used was a non-cleaning type solder paste (solder particle size: average 30 μm, chlorine content: 0.05 wt%, flux content: 10%), and the preheating temperature was 150 ± 1.
Reflow soldering was performed in air at 5 ° C. and the joint temperature max was 235 ° C.

The surface temperature of the wiring board 2 at this time is max.
243 ° C. (heat resistance of glass epoxy substrate: max 260
° C), and the surface of the electronic components 3 and 6a has a maximum of 238 ° C.
(Heat resistance of parts: max 240 ° C.). The reflow furnace used is a forced circulation type infrared furnace. Also, QFP
-Lead pitch of LSI3a is 0.5mm, 0.4m
m. At this time, the reflow-soldered solder joints (soldering joints) were used in Comparative Example and Example 1.
There was no problem in any of the above.

After the above reflow soldering, the Sn-3Ag-0.5Cu solder (melting point: 218), which is the above-mentioned ternary lead-free solder, is formed on the B side of the wiring board 2 of the first embodiment.
° C), and on the B side of the wiring board 2 of the comparative example, 2
Using Sn-0.7Cu solder (melting point: 227 ° C.), which is the original lead-free solder, each has a maximum of 235 ° C. to m.
The flow soldering was performed in the range of ax260 ° C.

By this flow soldering, the wiring board 2
In the comparative example in which the A side of the above was reflow soldered with Sn-3Ag-0.7Cu-5Bi solder, the QFP-LSI3a
The lead 9 peeled off on one side was observed. It was also found that the lead joint at the same position was weak for the lead 9 that was not peeled off. Since the lead joint in the vicinity of the joint is strong, a load is applied to a certain position due to the warpage of the wiring board 2, and it is determined that the mode is such that the peeling is intensively performed there. The case where the flow temperature was lowered to lower the temperature of the surface A of the wiring board 2 was also examined. However, some places where the flow soldering surface was insufficiently melted were found, but no break was initially observed. There was no problem on the reflow soldering side of side A. However, short leads and severe T
The SOP-LSI (not shown in FIG. 1) is
The life in the temperature cycle test of １２５125 ° C. could not meet the target 1000 cycles,
Disconnected in the cycle. Therefore, it cannot withstand the reflow / flow mixed loading process, and there is a problem with the service life.
Before performing flow soldering with Sn-Ag-Cu ternary lead-free solder on the B side of the wiring board 2, reflow soldering with Sn-3Ag-0.7Cu-5Bi solder on the A side opposite to this. It was determined that it was difficult to obtain good performance as a hybrid mounting structure.

On the other hand, in the first embodiment, Sn-Ag
-Cu-Bi-In quinary lead-free solder
When the A side of the wiring board 2 was reflow-soldered with n-3Ag-0.7Cu-2Bi-3In solder, the lead was not peeled even in the subsequent flow soldering regardless of the flow soldering conditions. . Furthermore, 1000 cycles or more could be cleared in the subsequent temperature cycle test at -55 to 125 ° C. Thus, the melting point and the soldering temperature were comparable to those of the Sn-3Ag-0.7Cu-5Bi solder, and a highly reliable solder composition could be obtained.

The reflow soldering of the A side of the wiring board 2 is performed by Sn-3Ag-0.7Cu-1Bi-2In solder, and the subsequent flow soldering of the B side of the wiring board 2 is performed by Sn-3Ag-0. The same experiment as described above was performed using 7Cu solder, but in this case, there was no problem in terms of initial and long-term reliability. That is, Sn-3A
After reflow soldering with g-0.7Cu-1Bi-2In solder, flow soldering was performed at a solder bath temperature of 250 ° C. and a conveyor speed of 1 m / min using Sn-3Ag-0.7Cu solder. The reflow soldering joint 5 rose to a maximum of 195 ° C. This is equal to or lower than the solidus temperature of the solder for reflow soldering, does not re-melt the reflow soldering joint 5, and secures a sufficient bonding interface strength against the warpage of the wiring board 2. Therefore, the combination can endure mixed mounting. The surface treatment of the lead of the electronic component is performed by Sn-
Because of the 10 wt% Pb plating, the melting point was slightly lowered, but the re-melting phenomenon did not occur.

Further, even in the case of the Sn-3Ag-0.7Cu-2Bi-4In solder in which the melting point was lowered by increasing the amount of In added, the process and reliability could be similarly cleared.

Next, Sn-3Ag-0.7Cu-3In
Solder (melting point: 207-216 ° C) and Sn-3Ag-
The result of performing reflow soldering using 0.7Cu solder is shown.

The joint temperature of the Sn-3Ag-0.7Cu-3In solder is max 236 ° C., and the heat resistance can be cleared. Also, the temperature of the joint at the time of reflow soldering with Sn-3Ag-0.7Cu solder is set to a maximum of 24 when soldering is performed in a conventional infrared reflow furnace.
5 ° C., and the surface temperature of the wiring board 2 at this time is ma
x265 ° C., which exceeded the heat resistant temperature of the substrate. In addition, the surface temperature of the electronic components 3 and 6 was also max 245 ° C., and the components swelled and had an odor. However, by using a forced convection type reflow furnace with reduced temperature distribution in the furnace, it was confirmed that reflow was possible within the range of heat resistance. Therefore, for these, the flow soldering of the B surface of the wiring board 2 was performed using Sn-3Ag-0.7Cu solder, and the reliability of the soldering joint was evaluated. There was no peeling of the solder joints in these reflow-flow mixed processes. It is considered that the reason for this is that the strength of the bonding interface is high because Bi is not included. Both of the results of the temperature cycle test show that the temperature cycle test at -55 to 125 ° C.
It clears over 00 cycles or more and has excellent reliability.

From these results, the Sn-A according to the present invention was obtained.
g-Cu or a lead-free solder to which at least one of In and Bi is added, that is, Sn- (1.5 to 3.5 wt%) Ag- (0.2 to 0.
8 wt%) Cu- (0 to 2 wt%) Bi- (0 to 4 wt%)
%) In was confirmed to be a well-balanced and excellent composition in the reflow / flow mixed mounting. If reflow soldering is performed first and then flow soldering is performed, as in reflow / flow mixed mounting,
Conventionally, the material for reflow soldering performed at the beginning has the same or higher melting point than the material for subsequent flow soldering. However, in the present invention, the material for reflow soldering is used. Is low when Bi and In are added, which is a method different from the conventional method.

The solder composition for the reflow soldering on the surface A of the wiring board 2 is not limited to the case of the mixed mounting, but may be used only on the reflow surface.

Further, even if there is a mixed mounting of reflow, reflow and flow, and other separate parts, a highly reliable mounting board can be obtained by the above combination.

Further, when hierarchical soldering is necessary, hierarchical connection using Sn-58Bi solder or solder in which a small amount of Ag, Cu, or the like is added to the surrounding composition is also possible.

[0079]

As described above, according to the present invention,
The effect of achieving high reliability is the realization of mixed mounting of reflow and flow soldering, which is the strictest mounting process using lead-free solder.

Further, according to the present invention, in the mixed mounting of reflow / flow soldering, which is the strictest mounting process using lead-free solder, solder joining can be performed within the range of heat resistance of the components. Even with respect to the warpage of the substrate, the solder joint of reflow soldering performed before flow soldering does not cause peeling, and solder joining can be performed with high reliability.

Further, according to the present invention, in the mixed mounting of reflow / flow soldering, which is the strictest mounting process using lead-free solder, a temperature cycle test of -55 to 125 ° C. It is possible to realize a hybrid mounting structure having a soldering joint that can be guaranteed even under conditions.

Further, according to the present invention, the most severe mounting process using lead-free solder is used for reflow soldering.
In mixed mounting of flow soldering, heat fatigue resistance,
It is excellent in creep resistance and high temperature resistance, and can be mounted with high yield for high-density mounting.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of a mixed mounting structure according to the present invention.

FIG. 2 is a diagram showing changes in liquidus temperature and solidus temperature when Bi is added to Sn-3Ag-0.7Cu solder.

FIG. 3 is a diagram showing a change in elongation at 20 ° C. when Bi is added to Sn-3Ag-0.7Cu solder.

FIG. 4 is a view showing a peel strength between Sn-plated 42 alloy lead when Bi is added to Sn-3Ag-0.7Cu solder.

FIG. 5 is a view showing a peel strength with respect to Cu when Bi is added to Sn-3Ag-0.7Cu solder.

FIG. 6 is a diagram showing the peel strength of Sn-10Pb-plated 42 alloy lead when Bi is added to Sn-3Ag-0.7Cu solder.

FIG. 7: Sn-3Ag-0.7C with different amounts of Bi added
It is a figure which shows the DSC measurement result of u solder.

FIG. 8 is a diagram showing an EDX analysis result of a peeled portion.

FIG. 9 is a diagram showing tensile strength at 20 ° C. when Bi is added to Sn-3Ag-0.7Cu solder.

FIG. 10 is a diagram showing a solidus temperature and a liquidus temperature when In is added to Sn-3Ag-0.7Cu solder.

FIG. 11 is a diagram showing elongation at 20 ° C. when In is added to Sn-3Ag-0.7Cu solder.

FIG. 12 is a diagram showing a peel strength between Sn-3Ag-0.7Cu solder and Sn-plated 42 alloy lead when In is added to the solder.

FIG. 13 shows Bi and I on Sn-3Ag-0.7Cu solder.
FIG. 4 is a diagram showing a solidus temperature, a liquidus temperature, and a reflow temperature when n is added.

FIG. 14 shows Bi and I on Sn-3Ag-0.7Cu solder.
It is a figure which shows elongation at 20 degreeC at the time of adding n.

FIG. 15 is a diagram showing elongation at 20 ° C. when Cu is added to Sn-3Ag solder.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Mixed mounting structure 2 Wiring board 3a QFP-LSI 3b SOP-LSI 4 electrode 5 Sn-Ag-Cu-In-Bi five-element lead-free solder 6a Lead component 6b Chip component 7 Electrode 8 Sn-Ag-Cu Ternary lead-free solder 9,10 leads

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) H01L 25/10 B23K 101: 42 25/11 H01L 25/14 Z 25/18 // B23K 101: 42 (72) Inventor Shoji Ishida 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside the Hitachi, Ltd.Production Technology Laboratory (72) Inventor Koji Serizawa 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside the Hitachi, Ltd.Production Technology Laboratory (72) Inventor Tetsuya Nakatsuka 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture F-term in Hitachi, Ltd. Production Technology Research Laboratory F-term (reference) 5E319 AA03 AB01 AB05 BB01 CC22 CC33 GG20

Claims (7)

[Claims] An Sn-Ag-C is provided on one surface of a wiring board.
u or a lead-free solder to which at least one of In and Bi is added, and electronic components are surface-connected and mounted by reflow soldering, and Sn-Ag-Cu 3 A hybrid mounting structure characterized by connecting and mounting other electronic components by flow soldering using lead-free solder composed of the original system. 2. The lead-free solder according to claim 1, wherein the lead-free solder composed of a ternary system of Sn-Ag-Cu is formed of Sn- (0 to 3.5 wt%) Ag- (0.2 to 0.8 w
(t%) A mixed mounting structure characterized by being composed of Cu 2. 3. The method according to claim 1, wherein Sn—Ag—Cu or one of In and Bi is added thereto.
The lead-free solder to which at least one kind is added is made of Sn- (1.5 to 3.5 wt%) Ag- (0.2 to 0.
8 wt%) Cu- (0-4 wt%) In- (0-2 wt%)
%) A mixed mounting structure characterized by being composed of Bi. 4. An electronic device comprising the hybrid mounting structure according to claim 1, 2, or 3. 5. The method according to claim 1, wherein one surface of the wiring substrate is formed of Sn-Ag-C.
u or a lead-free solder to which one or more of In and Bi is added, and the electronic component is surface-connected and mounted by reflow soldering. Then, the Sn-Ag-Cu And mounting another electronic component by flow soldering using a lead-free solder composed of a ternary system. 6. The method according to claim 5, wherein S is used for connection mounting by flow soldering.
The lead-free solder composed of a ternary system of n-Ag-Cu is composed of Sn- (0 to 3.5 wt%) Ag- (0.2 to 0.8 w
(t%) A mixed mounting method characterized by being a solder having a composition of Cu 2. 7. The Sn-Ag-Cu used for connection mounting by reflow soldering, or one of In and Bi, according to claim 5 or 6,
The lead-free solder to which at least one kind is added is Sn- (1.5 to 3.5 wt%) Ag- (0.2 to 0.
8 wt%) Cu- (0-4 wt%) In- (0-2 wt%)
%) A mixed mounting method characterized by being a solder having a composition of Bi.