Assemblies of substrates and electronic components

Abstract: An assembly of a substrate (10) and an electrical or electronic component (12), the component having electrically conductive leads (14) with surfaces of an alloy of tin and copper and the leads (14) being soldered to copper based terminals (16) of the substrate (10) by a solder alloy (18) of tin and copper. Lead solder is thus avoided. Preferably each conductive lead (14) has a surface layer (22) of the alloy of tin and copper and core (20) within the surface layer of another material which remains solid at the soldering temperature of the surface layer (22). Also included is a method of soldering a component (12) with electrically conductive leads (14) having surfaces of tin and copper alloy to copper based terminals (16) by a solder alloy (18) of tin and copper.

Description: This invention relates to assemblies of substrates and electronic components. In conventional assemblies of substrates and electronic components, electrical circuitries carried by the substrates have terminals for connection to electrical terminal leads of the components. For this purpose, a solder is used which has a lead constituent. These lead containing solders are known to be largely reliable for forming permanent electrically conductive solder joints between terminals and electrical conductive leads. However, lead, it is now realized, is an undesirable material to be used in industry. When used in solder, it is not a captive constituent of the solder material and is known to be environmentally unfriendly and is toxic. Conceivably it is dangerous for use both by the manufacturing worker and by the consumer. Lead presents health hazards in manufacturing, disposal, and use of assemblies, and problems arise in the safe disposal of scrap materials containing lead. Various assemblies have been made in which leadless solder has been used, of. J.H. Vincent et al.: "Lead-free solders for electronic assembly", GEC Journal of Research, vol. 11, No. 2, 1994, Great Baddow Chemsford GB, pages 76 - 89. However, problems are found during manufacturing where lead is not a component of solder as resultant solder joints are found to be unreliable as separation and cracking occurs. The present invention seeks to provide an assembly of substrate and electronic component which minimizes or overcomes the above lead free solder problems. Accordingly, the present invention provides an assembly of a substrate and an electrical component wherein the substrate has a plurality of copper based terminals, the component has a plurality of electrically conductive leads having surfaces comprising alloys of tin and copper, and the electrically conductive leads are soldered to the terminals by means of a solder alloy of tin and copper. In the assembly in a preferred and practical arrangement, each of the electrically conductive leads has a conductive body with a surface layer of the alloy of tin and copper and the conductive body necessarily remains solid, i.e., unsoftened, at the softening temperature of the coating. The solder must be such that during rise in temperature, it comprises a pasty constituent for soldering purposes at or around a specific temperature, and will solidify to provide a solder joint immediately upon removal of heat. A solder alloy having these requirements is provided by tin having a weight percent at or around 99.3 of the total weight of the solder and with the copper being at or around 0.7% of the total weight of the solder. It is also preferable that the alloy in the coating of each electrically conductive lead has approximately the percentage weights referred to above of the constituent parts are copper and tin. Hence, the coating and the solder are in the same physical pasty state at soldering temperature. This temperature is at approximately 227ºC. A flux to be used with the above solder according to the invention must be compatible with the solder. It must also be a flux which is operable at the desired soldering temperatures which are envisaged for the invention. An organic no-clean flux is considered to be most desirable and this flux is one which should be active at the temperature which the solder alloy has the pasty constituent, i.e. around 227ºC, and will successfully remove oxides of copper and tin. Such a flux suitable for this purpose is that referred to as X39 flux manufactured by Multicore Solders Inc. of Richardson, Texas. One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: - Figure 1 is a plan view of part of a printed circuit board having an electronic component mounted therein; Figure 2 is a cross-sectional view taken along line II-II in Figure 1 and to a larger scale of part of the printed circuit board and showing one of the leads of the component secured thereto; and Figure 3 is a cross-sectional view of the lead taken along line III-III in Figure 2 and to a larger scale. In the embodiment as shown by Figures 1, 2 and 3, a substrate comprises a printed circuit board 10 having mounted therein a plurality of surface components 12 only one of which is shown by the drawings. Each of the components 12 has leads 14 which are soldered to terminal pads 16 (Figure 2) of the printed circuit board 10. In the embodiment as in the invention, the structure avoids the use of lead materials in the solder to be used for attaching the terminals permanently to the terminal pads 16. Thus, in the embodiment solder 18 located between ends of the leads 14 and the terminal pads 16 is composed of a solder alloy of tin and copper. This alloy in this instance has a quantity of tin with a weight percent at or around 99.3% of the total weight of the solder with the remainder of the solder, i.e. 0.7% of the total weight, being copper. The solder is compatible with the copper terminal pads 16 so that a soldering operation will permanently attach the solder 18 to each of the terminal pads. It is also necessary for the solder to be compatible for soldering purposes to the surface material of each of the leads 14. With this object in mind, each of the leads is formed from a core 20 having a coating of a material which is compatible with the solder. This coating 22 in this particular case is provided by a tin copper alloy which is preferably of the same composition as the solder alloy 18. In this case, the coating 22 on each of the leads 14 is an alloy having tin with a weight percent of around 99.3 of the total weight of the coating with the remainder of the coating, i.e. 0.7% of the total weight, being formed from copper. It is essential that the core 20 of each of the leads should be formed from a material which not only is capable of supporting the coating 22 but also one which remains in an unsoftened or unmelted condition at soldering temperatures. Thus, in this particular embodiment, each of the cores 20 may be formed from an alloy of tin, silver and copper alloy or an alloy such as silver palladium alloy. During the soldering process, which may be by wave soldering, reflow soldering or wire bond soldering techniques, the solder alloy has a pasty constituent which rapidly forms at a specific temperature around 220ºC or between 227 and 240ºC and which upon removal of the heat immediately returns to a solid condition. This sudden return to a solid condition is particularly important in that accuracy in fixed location of each of the leads 14 of the electronic component upon the board is an absolute necessity when subsequent process steps are to be followed after the soldering operation. In addition, the flux which is to be used needs to be compatible with the solder, the terminal pad 16 and with the coating layers 22. Such a suitable flux is that known as flux X39 made by Multicore Solders Inc. of Richardson, Texas. This flux is known to be active at temperatures of around and slightly above 227ºC for removal of copper tin oxides from metal surfaces. It is found that after the soldering operation, the solder 18 efficiently solders each of the leads 14 to the terminal pads 16. As the material of the coating layers 22 is the same as the solder 18, then there is a fusion between the pads 18 and the coating layers 22 so that at their interfaces, the layers 22 and the solder 18 become metallurgically integral. Integrity of the solder also takes place between the solder 18 and the terminal pads 16 through the copper constituent of the solder. It is found that with the above construction, an efficient solder junction is provided between each of the leads 14 and its terminal pad 16. The solder junction in each case is reliable in manufacture and is not known to crack or break away. In addition to this of course the main object of the invention is realized in that the use of lead as a component of solder is avoided together with its attendent manufacturing use and scrap disposal problems.

Claims: An assembly of a substrate and an electrical component wherein the substrate has the plurality of copper based terminals, the component has a plurality of electrical conductive leads having surfaces comprising an alloy of tin and copper, and the electrically conductive leads are soldered to the terminals by means of a solder alloy of tin and copper. An assembly according to claim 1 wherein the solder alloy comprises tin having a weight at or around 99.3% of the total weight of the solder and copper having a weight at or around 0.7% of the total weight of the solder. An assembly according to claim 1 wherein each electrically conductive lead comprises a core of a material other than that of the alloy of tin and copper, the core being provided with a surface layer of the alloy of tin and copper, and the material of the core remaining solid at the soldering temperature of the layer of metal. An assembly according to claim 3 wherein the alloy of tin and copper of the layer of material on each electrically conductive lead comprises tin having a weight at or around 99.3% of the total weight of the layer and with the copper having a weight at or around 0.7% of the total weight of the layer. An assembly according to claim 1 wherein the solder alloy comprises tin having a weight at or around 99.3% of the total weight of the solder with the copper having a weight at or around 0.7% of the total weight of the solder and wherein each electrically conductive lead comprises a core having a surface layer formed of the alloy at the surface, the alloy of the surface layer comprising tin having a weight at or around 99.3% of the total weight of the layer with the copper having a weight at or around 0.7% of the total weight of the layer, and the core being a material which remains solid at the soldering temperature of the layer of the material. A method of soldering comprising:- providing an electronic component having a plurality of electrically conductive leads which have surfaces comprising an alloy of tin and copper; providing solder comprising an alloy of tin and copper; and soldering the leads of the electronic component onto copper based terminals by raising the temperature of the terminals and of the surfaces of the leads at the terminal positions to the soldering temperature of the solder and with use of a flux which removes copper oxide and tin oxide at the soldering temperatures.

Inventors: BILTON DAVID, HAMILTON MURRAY W, TRUMBLE WILLIAM P, BILTON, DAVID, HAMILTON, MURRAY W., TRUMBLE, WILLIAM P.
Applicants: NORTEL NETWORKS LTD, NORTEL NETWORKS LIMITED
Application year: 1997
Main classification: H05K3
IPC codes: H05K3, B23K35