The use of these alloys for repair, rework, and sustainment within the DoD (Department of Defense) will serve to increase confidence in the reliability of the repaired/reworked/modified assemblies. The most likely occurrence of lead-free technology where only the trusted tin-lead electronics has existed for the past 60 years will be in the area of surface finishes on components and sub-assemblies. These have been applied by the vendor to service the vast commercial electronics industry that is essentially all lead-free, but must also be used by military contractors, who do not represent enough market volume to entice the vendors to continue supplying the previous tin-lead components and sub-assemblies. When lead-free surface finished components are used, either out of necessity or out of error or mislabeling, in assemblies that are tin-lead soldered and contain other tin-lead surface finished components, serious reliability concerns have been reported.

In this section, we identify the materials most commonly in use for lead-free surface finishes and assemblies. This represents a benchmark of the current state-of-the-art for lead-free solder alloy use.

This alloy is chosen by the general commercial industry for surface mount application because:

• It is not proprietary and it is universally available.
• Government, commercial, and academic consortia are compiling large amounts of solder joint reliability data based upon this alloy of tin, silver, and copper.
• It is not particularly expensive.
• It has not shown particular tendency to support tin whisker growth

The other alloy for lead-free wave soldering is SAC 305 (same alloy as for surface mounting), which requires a higher temperature in the wave solder bath than the SNIC, but was chosen in the study defined here in order to limit the number of variables encompassed by the project, and potentially the number of alloys that would have to be maintained at the depot level.

The selected wave solder alloy of these Guidelines is therefore SAC 305. Since inspection rules have been written based on the less reflective SAC 305 solder joints, both commercial and military contractors will be able to use it.

T_d is defined either as delamination temperature (delamination in a certain time at that temperature) or more commonly, the decomposition temperature (a per cent weight loss of a sample of the material at that temperature). The most critical aspect for lead-free manufacturing is the delamination temperature (less commonly cited than T_g). There are several commercial grade FR-4 materials that are recommended for lead-free applications, and are readily available from the laminate vendors such as Oak Laminates and Polyclad division of Cookson Electronics.

High T_g (glass transition temperature) and more importantly high T_d (delamination temperature) are considered important to enable lead-free assemblies to withstand the higher processing temperatures needed to reflow, wave solder, or hand solder assemblies.

The most common T_g (glass transition temperature) FR-4 substrate material as of this writing is 170 ºC T_g. However, 140-155 ºC T_g material is also available. The lower T_g is usually not recommended for lead-free manufacturing because of the risk of delamination or warping or both when the substrate is subjected to the high temperatures of the lead-free processing.

This trait can be illustrated by a case study. Legacy FR-4 circuit boards of low (135 ºC) T_g that had been used in the past to manufacture tin-lead wave soldered assemblies were planned to be used to wave solder the same assemblies using SAC 305 rather than tin-lead eutectic solder. The boards were ordered correctly, but the supplier substituted a higher T_g material as a "better than" substitution allowed by the procurement contract. The higher T_g boards did not delaminate or warp in the higher temperature wave solder used for the Pb-free assembly. Duplicate boards were ordered specifying "no substitutions allowed," and the boards were assembled. No delamination or board warpage was noted during the hot SAC 305 wave solder operation. The final functional test results, relative to the normal tin-lead assembly or the high T_g FR-4 assembly, were adequate for the intended application. This shows that the legacy FR-4 assemblies did not warp or delaminate in the wave solder, and passed functional tests after assembly. The application, in this case can tolerate the low T_g FR-2 material, even in a Pb-free wave soldering process.

For the JCAA/JG-PP lead-free project, two different sizes of Brady 426 polyimide labels (0.375 inches by 0.375 inches and 1.5 inches by 0.25 inches) printed with a Brady Series R4000 ribbon were used. The assemblies were processed through the reflow and wave solder process. The results were label size dependent. Both size labels survived the lead-free reflow temperatures. The smaller ones (0.375 inches by 0.375 inches) did not withstand the additional heat from the lead-free wave solder process. After this secondary lead-free process, the smaller labels came off.

Labels currently used for SnPb processing may or may not withstand the higher lead-free processing temperatures. As a result of the JCAA/JG-PP experience and the Lead-Free Manufacturing for Navy Systems experience reported here, it is recommended that labels (as well as FR-4 substrate laminate) are validated for lead-free temperatures compatibility.

Recent developments in this area, which are impervious to the higher temperature processes of Pb-free electronics processing are represented by in-line laser marking, now available from some electronic assembly equipment vendors, which laser mark a bar code onto the circuit board as it traverses the processing machine. The laser mark is indelible in the high temperature Pb-free assembly process.

The data from numerous studies point out different causes of tin whiskers, but there is no scientific consensus on the mechanism of whisker formation and growth. The most commonly cited driving source for whisker formation is a buildup of compressive stresses in the plated tin layer. The compressive stresses originate from intermetallic growth, grain size, electroplating parameters, surface damage, environmental stresses, and many others. Based on this understanding, an assortment of whisker mitigation strategies is being implemented: nickel underplate, low stress tin-finish, solder dipping, and annealing. Although these mitigation strategies are fundamentally sound in the short term of a few years, there is little experimental proof that they are effective over a long period of time as required for long product lifetime military systems. In the absence of a fundamental growth mechanism for whiskers, there are no verified acceleration factors and therefore no acceleration of the whisker phenomenon is possible. From the past data, it is known that room temperature conditions are good conditions to grow tin whiskers. This means one can not accelerate the growth of whiskers in the laboratory and to test the parts for tin whisker growth for 25 year life, one will have to observe components for 25 years.

A vendor for tinning service can be selected from the following list: