Determination of the solderability of surfaces to be joined is typically assessed on a sample basis by dipping parts into a bath of 63 Sn/37 Pb or 60 Sn/40 Pb solder alloy. The degree of solderability is determined either by using visual criteria or a mechanized approach that measures wetting time and forces. Subsequent success in the assembly process using parts tested in the above manner may not be achieved if the solder alloy used in the joining process is different from the alloy used during the solderability testing. The major impacts of the solder alloy on the success of the soldering process are discussed below.
Electronic hardware designs intended for soldering process assembly should be designed to minimize the loadcarrying requirement of the solder connection. This can readily be achieved by using good design practices in assuring thermal expansion compatibility between the members being joined or providing compliance such as strain accommodation in one or both members. Also, steady state solder joint stress loading should not exceed approximately 400 psi. The use of a variety of solder alloys among solderability testing, tinning, and final assembly processes can complicate the situation and requires sophisticated understanding of the soldering process to assure adequate control and produce consistent quality and reliability in the final product, or process output.
Repeatability of the soldering process requires control of the process variables using statistical techniques for the critical parameters. Variation from the solder alloy of 63 Sn/ 37 Pd or 60 Sn/40 Pd adversely impacts the ability to control the following parameters:
(1) The SnPb eutectic solder allows a minimum optimum reflow temperature which minimizes the effects of excessive heating on the parts to be joined.
(2) Visual inspection results are relatively well defined for the bright, shiny SnPb eutectic solder joint. This statement is not true for high tin or lead alloys.
(3) The use of other than eutectic SnPb or 60/40 alloys may necessitate different fluxes and cleaners, and thereby increase the number of process variables requiring control.
(4) The metallurgical phenomena associated with the reaction between eutectic solder and common part materials and their finishes are fairly well understood. Joining alloy proliferation would demand increased process knowledge and should only be done when absolutely required.
Generally, adherence to use of 63/37 or 60/40 throughout the process, for solderability testing, for surface finishes, and for joining alloy will minimize the population of process variables. This will maximize the ability to control the process to consistently produce high quality and reliable output.
When the soldering process is used to connect electrical components to printed wiring boards or other equivalent substrates the connection should be accomplished using either standard 63 Sn/37 Pb or 60 Sn/40 Pb.
2.4 Future Investigations
Recommended areas for investigation include:
(1) Tensile, shear and cyclic fatigue of the 63/37 and 60/40 alloys over the anticipated range of application temperatures. While a large knowledge base exists for the mechanical and metallurgical properties of the 63/37 and 60/40 alloys, some deficiencies do exist. More reliable and manufacturable electronic hardware designs could be achieved if more data was available.
(2) The effects of minor alloy variations due to component materials and finishes.