There is a complex chemistry and metallurgy involved in the joining of metals through the tin/lead soldering process. Although many factors affect the final process and product, it is clear that the solderability of the metal surfaces to be joined plays a significant role in the quality of the final solder joint.
The surface finish of the component termination is critical to both the finished solder joint, and the solderability of the component after periods of extended storage. The success of the final soldering process is affected by the solderability of the surfaces to be joined.
When inadequate or marginal component termination surface finish is the starting point in the soldering process, a series of extraordinary and costly process control parameter modifications are necessary to obtain reliable solder joints.
A major impact of marginal solderability is excessive touch-up or rework which is detrimental to the soldered product.
A large portion of the measures taken to insure solderability of component terminations are remedial in nature, since these measures are designed to correct problems with either the basic materials or the processes that have been used for the termination, or its surface finish.
Existing Military specifications and standards are inconsistent in defining surface finishes, and most commonly describe the test for solderability that must be passed rather than the finish itself.
A fused tin-lead solder coating provides a very dense coating which enhances solderability retention in storage. Factors such as the termination basis metal selected, material requirements of the device, mechanical parameters, the need for a barrier layer, and the surface coating are all considerations in termination finish selection.
(1) The preferred termination finish is tin-lead applied by dipping.
(2) Plated coatings should be fused or reflowed and have a thickness of not less than 60 micro inches.
(3) Components which employ lead frame or header materials such as Kovar or Alloy 42 should have a barrier layer and be tin-lead applied by dipping after final burn-in operations.
(4) Date of termination finish application should be available.
(5) Finishes other than tin-lead that are considered "non-standard" should be identified by the manufacturer as such.
1.4 Future Investigations
Recommended areas for investigation include:
(1) Alternate basis metals that meet mechanical requirements and also are compatible with solderable finishes. The basis metals now used in many component terminations and frames are selected more by thermal coefficient of expansion and economics, than by the need to provide a good surface for soldering.
(2) Barrier layers that enhance solderability and avoid tin reactions with the basis metal.
(3) Solderable finishes other than fused tin-lead which can endure long storage.
(4) Protective coatings to preserve long-term solderability.