WO2002062520A1

WO2002062520A1 - A method of soldering and a preform therefor

Info

Publication number: WO2002062520A1
Application number: PCT/GB2002/000461
Authority: WO
Inventors: Kevin Joseph Lodge; Giles Humpston
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-02-03
Filing date: 2002-02-04
Publication date: 2002-08-15
Anticipated expiration: 2003-08-03
Also published as: GB0102771D0; GB2372005A

Abstract

A solder preform (20) is placed between two surfaces to be joined together. The preform has at least three solder wire limbs (21) which radiate outwardly from a centre (22). A cruciform shape is particularly advantageous. When the solder is heated, molten solder flows and expels trapped air from between the surfaces, thus reducing the void area in the joint which forms when the solder solidifies. The method and preform are particularly applicable to manufacture of opto-electronic modules.

Description

A METHOD OF SOLDERING AND A PREFORM THEREFORE Field This invention relates to a method of soldering two planar surfaces and more specifically to providing solder joints between solderable surfaces of microelectronic components.

Background of the Invention Recent advances in the functionality of micro-electronic components frequently require that such components are assembled using solder without flux. The problem is most acute in the manufacture of opto-electronic modules where many of the components have dual functionality. An example is a laser diode which must be robustly attached to a heat sink but leave the adjacent emitting facet free from contaminants. Further more for devices such as laser diodes it is essential that the joint is both thin and void free to ensure optimum thermal conductivity.

It is known to solder such devices using a solder preform or body in the form of a thin foil ^' having substantially the thickness of the required joint and the required planar shape of the joint. The foil preform typically has a thickness of about 25-75 microns and may be placed on a substrate with the component rested on top of the preform and the assembly heated in a controlled atmosphere to a temperature above the melting point of the solder which melts and wets the surfaces to be bonded. The assembly is then cooled allowing the solder to solidify and form a bond. This process generally results in a bond which contains between 30-50% voids by plan area. The presence of voids in a joint may not in itself be harmful, it is the location of the voids relative to the respective components that may give rise to problems. The location of potential voids is unpredictable and therefore it is desirable to minimise the presence of voids.

Voids may result when the components to be bonded are placed in contact with the foil preform and surface irregularities trap small pockets of air, which on melting of the solder, coalesce to form bubbles which cannot escape due to surface tension and cause voids on cooling. Voids may also be caused by impurities on the surface of the solder preform or component surfaces which prevent the solder from wetting areas to be joined. Common impurities are the products of ambient corrosion and organic compounds like greases.

US 3786 556 proposes the use of thin solder preforms (discs) which are placed adjacent the component to be bonded and it is suggested that the melted solder will flow between the surfaces due to capillary action expelling air from between the surfaces to be joined. Such solder preforms are difficult to clean prior to use.

US 3786 556 also proposes the use of two preforms and disadvantages of using two preforms of discussed in detail in US4709 849. In US 4709 849 there is disclosed a planar solder preform which is cruciform in shape. The preform is cut to the desired shape from foil and is therefore not simple to produce, cannot be easily cleaned since it is essentially thin foil, and may still trap air in surface irregularities in its planar surfaces.

The present invention seeks to provide a method of soldering and a solder preform that reduces the voids formed in a fluxless soldering process.

Statements of Invention

According to the present invention there is provided a planar solder preform comprising at least three wire limbs radiating outwardly from a centre.

The radiating limbs are angularly spaced so as to allow for the molten solder to flow to expel trapped air from between the surfaces being joined and to provide a stable support for any component resting on the preform during jigging for soldering.

Preferably the outer surface of the wire has a convex curved cross-section, and more preferably the wire has a round or elliptical cross-section. Typical solder wire will have a diameter of between 0.3 - 1.00 mm. The preform preferably comprises four limbs arranged to form a cruciform.

The preform may comprise lengths of wire joined by cold forming. Preferably, the preform comprises two lengths of wire joined at their approximate mid lengths.

Also according to the invention there is provided a method of fluxless soldering two planar surfaces in which method a preform, according to the present invention, is disposed between said surfaces with the surfaces in contact therewith, the preform being subject to heat to melt the solder and cause the solder to flow, and then cooling the flowed solder to form a soldered joint.

Another aspect of the invention provides a method of making a solder preform for fluxless soldering two planar surfaces wherein the solder preform is made from round cross-section wire which is surface cleaned, cut into required lengths, the wire then being joined to form a planar preform having at least three limbs radiating from a central point. The solder wire may be cleaned by gentle abrasion, for example, using a rag or tissue impregnated in a solvent, preferably an aliphatic alcohol such as ethanol or isopropyl alcohol (EPA), before the wire is cut to length.

The preform is preferably made from round wire which has several advantages. The round wire has a low surface area to volume ratio reducing the likelihood of surface oxide contamination, and since the wire is relatively thick it can be easily cleaned. Wire having a curved or round cross-section has only small areas of contact with the surfaces to be joined precluding trapped gases in surface irregularities. Solder wire is also relatively cheap compared with preforms cut from foil.

The solder wire is relatively thick compared with the final soldered joint, some 10-20 times thicker, and therefore the solder flows when molten ensuring that gases are expelled by the advancing solder front.

Description of Drawings The invention will be described by way of example only and with reference to the accompanying drawings in which :

Fig. 1 shows a method of making a solder preform according to the present invention, Fig.2 shows a solder preform according to the present invention, Fig.3 is schematic representation of the solder flow,

Fig.4 shows a solder preform located between surfaces to be joined Fig.5 shows a typical prior art joint with voids and Fig. 6 shows a joint according to the present invention.

Detailed Description of the Invention

The invention will be demonstrated by reference to examples in which two components are assembled together. In a first example and with reference now to Fig 4, one component 10 is an opto-electronic sub-assembly measuring about 5mm square. The component 10 has a surface-to-be-joined 11 which has been metallised with an inner layer of nickel 3-5 microns in thickness, and an outer layer of gold 3 microns in thickness. The corner to centre bow of the face 11 is less than 15 microns and the surface roughness was below 3 microns Ra.

The other component 12 , the mating heat sink, has similar physical characteristics except that its surface-to-be-joined

13 has an outer layer of gold which is an immersion coating having a thickness of 0.1 microns.

The sub-assembly 10 had itself been assembled using a number of solder joints including at least one lead-tin eutectic solder joint having a melting point of 183 degrees Celsius. The maximum excursion temperature for the sub-assembly was 150 degree Celsius.

For the purposes of comparison only, the two components 10 & 12 were assembled together utilising a 50 microns thick solder foil preform 5mm square made from Ih-48Sn (% weight) which melts at 120 degrees Celsius. The preform was sandwiched between the two components 10 & 11 and the entire assembly heated at a rate of 20 degrees per minute from room temperature to 140 degree Celsius and then back to room temperature, in a controlled nitrogen atmosphere. Under these conditions the assembly remained above the melting point of the solder for about twenty minutes. The solder joint was measured for shear strength and void/fill ratio and the results are given in table 1 below. A solder preform 20 as shown in Fig 2, in accordance with the invention, comprises at least three limbs 21, and preferably four limbs, radiating outwardly from a body centre 22. The plurality of spaced limbs provides stability on jigging the preform 20 between the components. The limbs 21 may each have any desired length as is required for the particular joint, and for this example are of all of equal length. The limbs maybe angularly spaced as is required for the particular joint and are, preferably, equiangularly spaced to form in this example a cruciform shape.

With reference to Fig 1., the preform is made from two lengths of solder wire 21 A cut to length from a continuous feed wire.

The feed wire was cleaned prior cutting using paper tissue impregnated with isopropyl alcohol and wiping the wire for at least 20 times to remove oxides and other impurities. The two lengths of wire were then assembled together at their approximate mid-points and cold welded together by the application of sufficient pressure to decrease the thickness of the body centre to be approximately equal to the wire diameter and form a single piece.

The assembled cruciform preform 20 was then jigged between the two components with the limbs extending towards the corners of the components.

hi order to produce a joint using the same volume of solder as prior art foil preform, the cruciform preform was made from frι-48Sn solder round wire 0.35mm in diameter cut to 4mm lengths. The assembled components and preform were processed as described for the prior art foil preform. The shear strength and void/fill ratio were measures and the results are given in table 1 below.

Table 1

Foil preform Cruciform Shear strength 15 kg 33kg

% M 45 95

The shear strength was measured using a standard method in compliance with Mil. Std 883C method 2019.5 and the % fill was measured from X ray imaging.

The solder flow in the joint is shown schematically in Fig.3.

In a second example and with reference again to Fig.4, the first component 10 is an electronic component measuring about 10 mm square and the outer surface 11 is coated in a 3 micron layer of silver. The mating part 12 is a ceramic tile metallised with a 3 micron thick silver layer overlaid with an immersion coating of gold, as previously described.

The standard foil preform was 10mm square 96Sn-4Ag (% by weight) foil having a thickness of 25 microns which melts at 221 degree Celsius. The soldering cycle involved heating the assembly in a nitrogen atmosphere at 50 degrees per minute up to 250 degree Celcius and holding at this temperature for 1 minute and then cooling at the same rate. When the components were assembled using a foil preform the % Fill was measured as 67. Fig. 5 shows a typical X ray image through a solder joint made in this manner and which has a % Fill of 72 by plan area (the dark areas being voids).

The components were also assembled using a cruciform preform made from 0.36 mm diameter wire wiped clean using IPA impregnated tissue (5 wipes). This produced a % fill in excess of 95.

Fig 6 shows a typical X ray image through a solder joint having a 10mm square joint area and made using a preform made from 0.3mm solder wire. The length of the limbs was calculated to provide to give 25 micron thick joint one the solder had flowed during processing. The void area is less than 5% of the plan area and the outline of the original preform is clearly visible.

Claims

1. A planar solder preform comprising at least three wire limbs radiating outwardly from a centre.

2. A solder preform as claimed in Claim 1 the outer surface of the wire having a convex curved shape.

3. A solder preform as claimed in claim 2 wherein the wire has a round cross-section.

4. A solder preform as claimed in any one of claims 1 to 3 wherein the preform comprises four limbs arranged to form a cruciform.

5. A solder preform as claimed in any one of claims 1 to 4 wherein the preform comprises lengths of wire joined by cold forming.

6 A solder preform as claimed in Claim 5 and which comprises two lengths of wire joined at their approximate mid lengths.

7. A method of fluxless soldering two planar surfaces in which method a preform according to any one of claims 1 to 6 is disposed between said surfaces with the surfaces in contact therewith, the preform being subject to heat to melt the solder and cause the solder to flow, and then cooling the flowed solder to form a soldered joint.

8. A method as claimed in Claim 7 wherein the wire is cleaned by mechanical abrasion of its surface prior to forming the preform.

9. A method of making a solder preform for fluxless soldering two planar surfaces wherein the solder preform is made from round cross-section wire which is surface cleaned, cut into required lengths, the wire then being joined to form a planar preform having at least three limbs radiating from a central point.

10. A method as claimed in any one of claims 7 or 8 wherein the preform is 10-20 times thicker than the soldered joint.