MXPA06008091A - Improved populated printed wiring board and method of manufacture - Google Patents

Abstract

A populated printed wiring board (PWB) (100) and method of manufacturing the populated PWB are taught. The populated PWB is manufactured by fabricating a PWB (102, 402) with exposed copper pads (302), coating the copper pads with an organic solderability preservative (OSP) (404), depositing a solder paste that includes lead-free solder on the OSP covered copper pads (406), placing components (408) and heating the PWB above a liquidous temperature of the lead-free solder in an air atmosphere (410). The process allows very close spacing of components and component leads while forming reliable solder joints to components that are mechanically stressed and components that have non-negligible planarity or coplanarity tolerances.

Description

PRINTED CIRCUIT BOARD IMPROVED POPULATION AND MANUFACTURING METHOD FIELD OF THE INVENTION The present invention relates generally to the manufacture of populated printed circuit boards. Most particularly, the present invention relates to printed circuit boards that include very short distance adapters for components with a small gap to reduce the size of the product and also include welded joints for components that are mechanically stressed and / or components that have tolerances of non-negligible planarity or coplanarity.

BACKGROUND OF THE INVENTION As microelectronic technology has advanced, the degree of chip integration has increased to the point that complete electronic devices, such as cell phones, cameras and laptops, typically comprise a relatively small number of microchips, along with some discrete components (eg, resistors, capacitors) connected together on a printed circuit board (PWB). There are two commonly known types of PWB technologies: the old technology where the conductor cables of the components are welded in holes drilled through the PWB, and the currently favored technology where the conductor cables of the components are welded to surface adapters of the PCB. The latter is referred to as a surface mount technology (SMT). Highly integrated microchips often require a large number of external connections. The external connections to printed circuit boards are made through packet cables that enclose the microchips. Each connection between a conductor cable and the PWB involves a certain amount of space that is determined by a transverse dimension of the conductor cable (corresponding to the transverse dimension of the PWB adapter), and a minimum spacing between adapters. The minimum separation between adapters is determined by the need to avoid short circuits between adapters caused by welding bridges. The number of external connections required for a given microchip may be too large to determine a lower limit on the packet size for the microchip. That is, to accommodate the number of conductor cables while maintaining the separation of the same, the chip package must have a certain size. Therefore, although the microchip itself is highly miniaturized, a large package is needed, thus negating, in some aspects, the large expense incurred in the miniaturization of the microchip. Typically, the circuits include a number of discrete components (eg, resistors, capacitors). In fact, for many circuits, the discrete components comprise the majority of the surface area of the PWB. The area covered by the discrete components is, of course, determined by the size of the discrete components and the required separation between adapters. As in the case of packaged microchips, the required separation between adapters is determined by the need to avoid short welding. Therefore, it is generally desirable to be able to reduce the spacing between adapters. In manual devices, such as laptops, cameras and cell phones, it is particularly important to control the space occupied by the circuitry so that the overall size of the handheld device is not excessive and / or so that more functionality can be included in the handheld device. For manual devices, a single PWB can be used to mechanically support and electrically connect a wide variety of parts including microphones, discrete components, electrical connectors, and EMI / RFI protections. (One use of EMI / RFI protections is to protect the RF components mounted on the printed circuit boards of cell phones). In the interest of cost reduction, which is a particular concern for mass produced consumer devices, such as cell phones, it is desirable to secure all components using solder applied in a single welding procedure. However, the inclusion of different types of devices involves conflicting demands on the welding process through which the components are secured to the PWB. In particular, for microchips and discrete components (eg, capacitors, resistors) which are not subject to stress, it is desirable to be able to reduce the spacing between lead wires as much as possible. In the past, to reduce the spacing between conductor cables, without incurring electrical shorts due to welding bridges, the thickness of the solder paste applied to the PWB was reduced. However, if components, such as electrical connectors, which are mechanically stressed, will be included in a PWB, it is desirable to secure them with high strength welded joints. In the past, engineers resorted to step-type assembly to mechanically secure stressed components, such as electrical connectors. However, in the interest of more cost-effective manufacturing, it is desirable to use a single SMT welding procedure to secure all components. Conventional wisdom dictates that a thicker layer of solder paste should be applied to secure mechanically stressed components using SMT; however, this contrasts with the aforementioned goal of reducing adapter size and spacing between adapters for microchips and discrete components. The RFI / EMI protections present their own challenges to the welding process. A common type of RFI / EMI protection that is used in handheld devices, including cell phones, is die formed from a piece of flat metal in a rectangular or irregularly shaped hollow stack. The edge of the stack is welded to a closed curved trace congruently configured on the PWB. For the protection to work effectively, it is important that a continuous weld joint is formed all around, between the edge and the trace on the PWB. In the production of the real world, there is often some appreciable tolerance in the flatness (planarity) of the edge of protection. The correction of the imperfections in the flatness of the protection requires the use of a thicker solder paste; however, this again conflicts with the desire to reduce the separation between adapters for microchips and discrete components. For large chip packages with multiple lead wires, there may also be a substantial tolerance in the coplanarity of the many lead wires. The correction of tolerance to coplanarity also requires the use of coarser solder paste, in conflict with the desire to reduce the spacing between adapters. What is needed is a SMT welding procedure that can achieve reduced spacing between adapters for microchips and discrete components, which can secure large components that are mechanically stressed such as electrical connectors, which can also form continuous welded joints for components imperfectly flat, and do all that on a single board with a single welding procedure.

BRIEF DESCRIPTION OF THE FIGURES The present invention will be described by way of exemplary embodiments, but not limitations, which are illustrated in the accompanying drawings in which similar references denote similar elements, and wherein: Figure 1 is a top view of one embodiment of a printed circuit board populated; Figure 2 is a side view of the populated printed circuit board shown in Figure 1; Figure 3 is a fragmentary top view of the printed circuit board shown in Figure 1, with components removed to show a grouping of adapters; Fig. 4 is a flowchart of one embodiment of a method for manufacturing a populated printed circuit board; Figure 5 is a top view of a populated printed circuit board that is used to evaluate a comparative exemplary welding procedure; Figure 6 is an enlarged view of the populated printed circuit board shown in Figure 5; Figure 7 is a top view of a populated printed circuit board that is used to evaluate the process illustrated in Figure 4; Figure 8 is an enlarged view of the populated printed circuit board shown in Figure 7; Figure 9 is a side sectional view of a cellular telephone incorporating the populated printed circuit board shown in Figures 1-3; Figure 10 is a flowchart of a method of another method for manufacturing a populated printed circuit board; Figure 11 is a fragmentary top view of a second printed circuit board in a first stage of the process shown in Figure 10; Figure 12 is a fragmentary top view of the second circuit board printed in a second stage of the process shown in Figure 10; Figure 13 is a fragmentary side view corresponding to the top view shown in Figure 12; and Figure 14 is a fragmentary side view of the second printed circuit board after two connectors have been mounted.

DETAILED DESCRIPTION OF THE INVENTION As required, the detailed embodiments of the present invention are described below; however, it will be understood that the embodiments described are merely examples of the invention, which may be incorporated in various forms. Therefore, specific functional and structural details disclosed herein will not be construed as a limitation, but only as a basis for the claims and as a representative basis for showing those skilled in the art to variously employ the present invention in almost any appropriately detailed structure. In addition, the terms and phrases employed herein are not intended to be a limitation, rather, they are intended to provide an understandable description of the invention. The terms "one" or "one", as used in the present invention, are defined as one or more than one. The term plurality, as used in the present invention, is defined as two or more than two. The term "other", as used in the present invention, is defined as at least one second or more. The terms that it includes and / or that it has, as used in the present invention, are defined as comprising (ie, open language). The term "coupled", as used in the present invention, is defined as connected, although not necessarily directly, and not necessarily mechanically. Fig. 1 is a top view of a mode of a populated printed circuit board (PWB) 100 and Fig. 2 is a side view of the populated PWB 100 shown in Fig. 1. The populated PWB 100 includes a suitable PWB 102 , to which many components can be attached. In Figure 1 a main surface 103 of the PWB 102 is visible. The components fixed to the main surface 103 of the PWB 102 are of many different types, and as discussed above, involve different requirements regarding the welding process through which they are attached to the printed circuit board 102. Components mounted on a first major surface 103 of the PWB include, for example, an RFI / EMI protection 104 , a four-part microchip package 106, a plurality of discrete components 108 (eg, resistors or capacitors), and a connector 110. The shield 104 includes a lower edge 202 that is welded to a congruently configured trace (which is not sample) on the PWB 100 by means of a weld bead 204. The microchip package 106 includes a plurality of lead wires 112 which are individually fixed through welded joints 206 (one for each lead wire 112) to the adapters (not shown). visible in Figures 1-2) of the PWB 102 that are located below the distal ends of the lead wires 112. Similarly, each terminal of the components Discrete members 108 is fixed to an adapter (not visible in Figures 1-2) of the PWB 102 by means of a welded joint 208 (one for each terminal of each discrete component 108). Additionally, the connector 110 includes lead wires 114 that are secured to the adapters (not visible in Figures 1-2) of the PWB by means of welded seams 210. Figure 3 is a fragmentary top view of the printed circuit board 102 which it is shown in figure 1, with the components removed to show a grouping of adapters 302. The adapters 302 are separated by a very small spacing between adapters. The separation between adapters, in some cases, is below 0.25mm and can be as low as 0.05mm. The adapters 302 can be used for a packet of microchips having lead wires that are separated by the same spacing between adapters, or for discrete components whose spacing is very short. Although not all adapters in PWB 102 will necessarily be spaced as close as the minimum adaptable spacing between adapters, the ability to separate adapters by less than 0.25mm allows populated PWB 100 to be reduced in size, and / or that more circuits are included in populated PWB 100. The ability to closely separate adapters also allows conductive trajectories to be reduced thus improving electrical performance. All of the aforementioned components 104, 106, 108, 110 are secured to PWB 102 using a welding method with surface mounting technology (SMT). In said SMT welding process, it is convenient and effective in terms of cost to apply solder paste to the entire PCB in one operation. The use of a template to apply the solder paste is a convenient way to apply solder paste in one operation. However, as discussed above in the field section of the invention, different weld thicknesses are preferred for different types of components. For components with closely spaced lead wires or for components that are closely spaced, the accepted practice is to reduce the thickness of the solder paste. On the other hand, for components with important planarity or coplanarity tolerance, such as protection 104, or some large microchip packages, or for components that are subject to stress such as connector 110, the accepted practice is to increase the thickness of the solder paste. The so-called "reduction" templates allow the application of different thicknesses of solder paste to different areas of printed circuit boards but, unfortunately, said templates are full of problems of dirt with dry solder. Figure 4 is a flowchart of one embodiment of a method for manufacturing a populated PWB that allows the targets to be met to weld a variety of component types discussed above. Referring to figure 4, in block 402, a PWB is fabricated having one or more copper adapters that are separated by a spacing between adapters of less than 0.25mm. Apart from the separation between adapters, the manufacture of the PCB can be achieved through known techniques. In a subtractive process in which a copper continuous film is selectively etched to form copper adapters, the reduction in the spacing between adapters is conveniently effected by reducing the separation of the corresponding accessories in the graphic material (photolithographic masks) They use to configure a tough layer that is used in continuous film engraving to form copper adapters. In block 404, the copper adapters are coated with an organic weldability preservative (OSP). The coating can be easily achieved by dip coating on an OSP solution. Organic weldability preservatives suitable for use in the process include benzimidazole, benzotriazoles and imidazole. An OSP that is a form of substituted benzimidazole is sold under the trade name Entek by Cookson Electronics of West Haven, Connecticut. In block 406, a solder paste that includes a lead-free solder is deposited on the OSP in the copper adapters. According to some embodiments, the lead-free solder paste comprises from 95.1 to 95.9 percent tin, and from 3.6 to 4. 0 percent silver. In addition to tin and silver, in some embodiments, lead-free solder also includes copper and copper is present in an amount up to 0.9 percent. According to some embodiments, lead-free solder consists essentially of one or more materials selected from the group consisting of tin, silver and copper. The combinations of these materials are effective in combination with other materials and processing steps described in the present invention to achieve the desirable attributes described in another section of this document. All the percentages provided above are in terms of weight. The solder paste that is used in block 406 conveniently also comprises a lightly Resinous Active solder flux (RMA). In block 408 the electrical components are placed in the PWB 202 so that the contact areas of the electrical components (for example, the lower edge 202 of the RFI / EMI protection 104, the lead wires 112 of the microchip pack 106, the terminals of the discrete components 108, and the lead wires 114 of the connector 110) are in contact with the solder paste placed in the OSP on the adapters. The components are conveniently placed by means of an automated lifting and positioning machine. In block 410, PWB 102 is heated in an air atmosphere at a temperature above a solder liquefaction temperature included in the solder paste. It is desirable to maintain the temperature of PWB 102 above the liquefaction temperature for at least 40 seconds to achieve good welded joints. As mentioned above, it is quick and effective in terms of depositing the solder paste for the entire PCB 102 using a non-gradual template; however, this produces a uniform solder paste thickness for all components. Notwithstanding the use of the method shown in Figure 4, a template can be chosen to deposit a relatively thick solder paste to securely secure the components that are mechanically stressed, and the components that have a high tolerance to the coplanarity or planarity, and even so, the coarse solder paste does not lead to shorts in the welded bridges even in adapters with a relatively short separation. Alternatively, in block 406 the solder paste is deposited by means of supply, for example, automated supply. By using the procedure shown in Figure 4, printed circuit boards with adapter spacing as narrow as 0. lmm can be made using a 0.15mm thick deposit of solder paste. The procedure achieves a high performance use without suffering significant amounts of electrical shorts due to welding bridges, and in the same procedure on the same PWB creates strong welded joints with the components that are mechanically stressed and complement the welded joints with the components that have an important tolerance to coplanarity or planarity, for example O.lm. You can easily use these processing ratios from deposit thickness of solder paste to separation between adapters in excess of 0.5. This allows boards that have a mixture of components, including components with relatively small conductor spacing, mechanically stressed components, and components that have high tolerances to coplanarity or planarity to be welded to a PWB using a single welding procedure, ie, a application of solder paste and a reflow of solder. Although a range of thickness of deposited solder paste can be used in the process, the procedure allows coarse deposits of solder paste to be used in excess of 0.127mm without incurring welding bridges for solder adapters or deposits with a separation close to each other, for example, separated by less than 0.25mm.s.

COMPARATIVE EXAMPLE A first test PWB 502 was made and shown in Figures 5-6. The first test PWB included an arrangement of convenient adapters to fix discrete components. The adapters of the first test PWB were coated with OSP Entek. In one area of the first test PCB, the adapters for the adjacent devices were separated by 0.20mm, and in a second area, the adapters for the adjacent devices were separated by 0.15mm. A thick 0.127mm template was used to apply a solder paste that included a solder with 36% lead, 64% tin and 2% silver, and an RMA solder flux to the adapters of the first test PWB. After the application of the solder paste, the resistors mounted on the surface were placed on the first test PWB. The first PWB was then heated in an air atmosphere, at a temperature of 220 C. During heating, a temperature above 183 C was maintained for 60 seconds. The first test PWB was then allowed to cool and the state of the welded joints and the discrete components were examined. As shown in Figures 5-6, numerous welding bridges were formed, and many of the resistors mounted on the surface changed position. In general, the result was unacceptable.

EXAMPLE A second test circuit board 602 was made and shown in Figures 7-8. The second test PWB included the same adapter arrangements as the first circuit board. The adapters of the second test PWB were coated with OSP Entek. A 0.127mm thick template was used to apply a solder paste that included a solder with 95.5% tin, 3.8% silver and 0.7% copper, and an RMA solder flux for the adapters of the second test PWB. After application of the solder paste, resistors mounted on the surface were placed on the second test PWB. The second PWB was then heated in an air atmosphere to a temperature of 235 C. During heating, a temperature above 217 C was maintained for 70 seconds. The second test PWB was then allowed to cool and the state of the welded joints and the discrete components was examined. As shown in Figures 7-8, no welding bridges were formed and the surface mount resistor remained in position. Figure 9 is a side sectional view of a cell phone 900 incorporating the populated PWB 100 shown in Figures 1-3. The cell phone 900 is built around the populated PCB 100. The cell phone 900 includes a housing 902 that supports and encloses the populated PWB 100 and a number of other parts, including an antenna 904, a headset speaker 906, a screen 908, a keyboard 910, a microphone 912 and a battery 914. In the cell phone 900, the spacing between reduced adapters, of the adapters that are in the populated PCB 100, allows more circuits to be included, thus providing greater functionality to the cell phone 900. Increased functionality may include, for example, video recording and playback, sophisticated audio processing, Global Positioning, etc. Furthermore, in the main surface 103 of the PWB many components can be fixed in a single welding procedure, the 900 cell phone can be produced at an effective cost, which is a key concern related to consumer products. In addition, the welding procedures described above achieve mechanically robust welded joints that can withstand stresses such as the accidental fall that incurs in the daily use of the cellular phone. Additionally, the false radio emissions of cell phone 900 components are controlled protections that are adequately joined by means of full welded joints. Referring to figures 10-14, an additional modality is described. Fig. 10 is a flowchart of the additional mode of a method for manufacturing a populated printed circuit board. Referring to Fig. 10, a PWB with exposed copper adapters was manufactured in block 1002. Figure 11 is a fragmentary top view of a second PWB 1100 in a first stage of the process shown in Figure 10. As shown in Figure 11, the second PWB 1100 comprises a first copper adapter 1102, a second copper adapter 1104, a third copper adapter 1106, and a fourth copper adapter placed on a surface 1110 of the second PWB 1100. Referring again to FIG. 10 in block 1004, the copper adapters are coated with an OSP. The comments made above in the context of Figure 4 regarding the choice of an OSP apply in the same way to the procedure shown in Figure 10. In block 1006, extra large patches of lead-free solder paste are deposited on one or more of the copper adapters coated with OSP so that two or more patches of lead-free solder paste deposited are less than 0.25 millimeters apart. The solder paste used in the process shown in Fig. 10 is of the same description as that in the context of Fig. 4. Fig. 12 is a fragmentary top view of the second printed circuit board after it has been shown. deposited the extra large welding patches. In Figure 11, copper adapters 1102-1108 are shown in dotted outline. The coverage of the first to the fourth copper adapter 1102-1108 is a first 1202, a second 1204, a third 1206 and a fourth 1208 extra large solder patch respectively. The first 1102 and the second 1104 copper adapters are to be used to mount a first connector 1402 (figure 14), and the third 1106, and the fourth 1108 copper adapters are to be used to mount a second connector 1404 (figure 14). ). It can be seen that the second oversize lead-free solder patch 1204, and the third oversize lead-free solder patch 1206 are separated from each other by a very small distance of less than 0.25mm indicated as "S" in Figure 12 Despite the small spacing S, by using the procedure illustrated in Fig. 10, electrical shorts due to welding bridges are minimized. Fig. 13 is a fragmentary side view corresponding to the top view shown in Fig. 12. (Copper adapters 1102-1106, which are typically much thinner than solder paste, are not shown in Fig. 13 ). Referring again to Figure 10, in block 1008, the electrical components are placed in the PWB so that the contact areas of the electrical components are in contact with the solder paste coating the copper adapters. In block 1010, the PWB is heated in an air atmosphere at a temperature above a solder liquefaction temperature included in the solder paste deposited in block 1006. The comments made in the context of Figure 14 with respect to the heating temperature and time, also apply to the process illustrated in Figure 10. As a result of the procedure of Figure 10, the solder paste deposited in the extra-large patches will be fused over the copper adapters forming together large welds Joints soldered with a large amount of solder are useful for securing components that are mechanically stressed and for securing components that have significant tolerances to planarity or coplanarity, for example, RFI / EMI protections. By allowing the deposited weld patches 1102-1108 to be so close (for example, less than 0.2mm), the procedure illustrated in Figure 10 allows extra-large solder patches to be used to obtain soldered joints with a large amount of solder. welding, without increasing the thickness of the deposited weld, or without substantially increasing the area of the PWB. Accordingly, the weld can be deposited over a whole PWB using a technique that places a uniform thickness of weld, such as marking with a non-gradual template. The use of a non-gradual template is a reliable and economical way to apply solder. A relatively small thickness can be chosen to make it possible to weld lead wires of components and / or components to a very narrow separation for microchips and discrete components, and even through the provisioning of extra large patches, mechanically stressed components or components with high tolerances to planarity or coplanarity. Fig. 14 is a fragmentary side view of the second printed circuit board 1100 after the two connectors 1402, 1404 have been placed, and the solder paste 1202-1208 has been heated above a liquefaction temperature and has allowed to cool to form four welded joints 1406 with the lead wires 1408 of the connectors 1402, 1404. As used in the present description, the term "lead-free" solder includes a solder that does not contain lead, and a solder that It is essentially devoid of lead, including a solder that has indications of inevitable or unintentional lead. Although the preferred embodiments of the invention as well as others have been described and illustrated, it will be clear that the invention is not limited to the foregoing. Those skilled in the art will appreciate that numerous modifications, changes, variations, substitutions, and equivalents can be made without departing from the spirit and scope of the present invention, as defined in the following claims.

Claims (10)

NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following is claimed as a priority: CLAIMS

1. - A populated printed circuit board comprising: a printed circuit board comprising: a main surface; a plurality of copper adapters on the main surface; a plurality of components including: one or more components selected from the group consisting of microchips and discrete components; and one or more components selected from the group consisting of electrical connectors and protections; and a plurality of welded joints between said plurality of copper adapters, and said plurality of components, said plurality of welded joints comprises a lead-free solder, wherein said plurality of welded joints is formed by coating said copper adapters with an organic preservative. of weldability, depositing a solder paste including said lead-free solder on said organic weldability conservator, placing the contact areas of the plurality of components in contact with said solder paste and heating said printed circuit board in an atmosphere of air.

2. The populated printed circuit board according to claim 1, characterized in that: at least one sub-assembly of said plurality of copper adapters is separated by a spacing between adapters of less than 0.25 millimeters.

3. The populated printed circuit board according to claim 1, characterized in that: said welding consists essentially of one or more materials selected from the group including silver, tin and copper.

4. The populated printed circuit board according to claim 1, characterized in that: said solder paste comprises Lightly Resinous Active flow.

5. The populated printed circuit board according to claim 1, characterized in that: said solder paste has a thickness of at least 0.127 millimeters when applied to said copper adapters coated with said organic weldability preservative.

6. A method for manufacturing a populated printed circuit board comprising: manufacturing a printed circuit board comprising a plurality of copper adapters including exposed copper surfaces; coating said copper adapters with an organic weldability preservative; depositing a solder paste that includes a lead-free solder in the copper adapters coated with organic solderability preservative; placing a plurality of circuit components on said printed circuit board, so that the contact areas of the plurality of components are in contact with the solder paste; and heating the printed circuit board to a temperature above a liquefaction temperature of said lead-free solder in an air atmosphere.

7. The method for manufacturing a populated printed circuit board according to claim 6, characterized in that: the manufacture of the printed circuit board comprising the plurality of copper adapters includes the manufacture of a printed circuit board where one or more of the adapters have a spacing between adapters of less than 0.25 mm.

8. The method for manufacturing a populated printed circuit board according to claim 7, characterized in that: the coating of said copper adapters with an organic weldability conservator comprises coating said copper adapters with an organic weldability conservator selected from the A group consisting of benzimidazole, benzotriazoles and imidazole.

9. - The method for manufacturing a populated printed circuit board according to claim 7, characterized in that: the coating of said copper adapters with an organic weldability preservative comprises coating said copper adapters with substituted benzimidazole.

10. The method for manufacturing a populated printed circuit board in accordance with claim 7, characterized in that: the heating of the circuit board printed at a temperature above a liquefaction temperature, comprises heating the printed circuit board to a temperature above the liquefaction temperature for at least 40 seconds.