The EMPF recently conducted a Lead-Free Design Conversion study in order to proactively identify the potential challenges of converting an existing printed circuit board (PCB) design from lead to lead-free using commercial off-the-shelf (COTS) components. While the manufacturing processes to produce new lead-free soldered hardware are well documented, converting an existing tin-lead design to lead-free presents new challenges. From a logistics perspective, these challenges must be identified, beginning with a thorough review of the components and assemblies.

For high reliability military applications, the designer and manufacturer must identify the available lead-free finishes. Of special concern is pure tin (Sn), which represents a reliability risk due to tin whisker growth, which occurs over a number of years.

The traditional tin-lead finishes on COTS components are already being transitioned to lead-free, but a new industry-standard finish has not been selected. Not all lead-free finishes are backwards compatible with tin-lead soldering processes, though many are.

At the EMPF, an existing PCB designed for the use on a Navy survival system was converted to lead-free for evaluation purposes. The chosen assembly charges AA batteries in the field. This PCB consists of 80 components with 31 unique part numbers. The components were made by 11 different manufacturers and were purchased from two different distributors. Four of the components were through-hole mounted, and the remaining were surface-mounted.

The initial task of the study was to identify the part numbers for the lead-free versions of each component. Only one manufacturer reviewed in this study (which produced two of the components) had assigned entirely new part numbers to the lead-free versions. Of the remaining 10 manufacturers, half assigned a suffix or prefix to the part number to denote the lead-free version. The other half made no changes to the part numbers. Furthermore, those manufacturers that added suffixes and prefixes to the part numbers noted that the
tactic is temporary; they plan to drop these tags as soon as lead-free finishes become standard.

The second task of the EMPF Lead-Free Design Conversion study was to identify bulk packages of lead-free components (Figure 1-1). One-third of the manufacturers did not provide information about lead-free identification of bulk packages. Among the remaining manufacturers, three different labeling strategies were used to identify bulk component packages as lead-free: 1) Standard industry labels; 2) Part number suffix and/or prefix; or 3) Lot number. In addition, one manufacturer used no labeling method, citing that they produce lead-free parts only.

In the next phase of the study, the EMPF identified the specific lead-free finish used on each component (Figure 1-2) necessary to build the new lead-free PCB. In all, there were seven different finishes on the component leads – matte tin, SnBi, SnAgCu, NiPdAu, SnCu, NiAu, and SnPbCuSbCr. In general, the component manufacturers avoided lead-free finishes which were not backwards compatible with tin-lead solders. Approximately two out of three manufacturers used matte tin. Existing equipment used to create tin-lead finishes can be easily converted to create matte tin finishes, which are compatible with both tin-lead and lead-free solder alloys.

For these pure tin finishes, the favored tin whisker mitigation strategy was to apply a matte finish instead of shiny. Additional tin whisker abatement methods used were the application of a nickel underplating and annealing of the component. However, a matte tin finish was the sole whisker mitigation method used for 75% of these components.

Views on the risk posed by tin whiskers varies among manufacturers. Manufacturers are mainly concerned with providing a level of reliability necessary for the few years consumer electronics are usually in service. Two of the reviewed component manufacturers had conducted an in-depth study of whisker growth and abatement and made a decision to use a matte tin finish based on the ease of implementation. Two other companies conducted studies which showed no whisker growth greater than 50 microns and concluded that whisker growth on their components was not a concern. Two manufacturers decided to use a matte tin finish based on a review of existing literature. The final manufacturer provided no information about tin whisker risk with their components.

All of the component manufacturers in this study stated that their lead-free finishes were compatible with most lead-free solders. Two identified SnAg and SnCu as the optimal lead-free solders for their components. The remaining did not identify an optimal lead-free solder.

The results of this EMPF Lead-Free Design Conversion study show that many individual part numbers are not being changed to reflect new lead-free finishes and that bulk packaged components often do not have finish identification. This lack of new, unique part numbers and clear identification of many lead-free components creates a logistics challenge. Electronics manufacturers will have to take additional steps to determine whether stock is tin-lead or lead-free based on X-ray fluorescence, visual analysis, or other criteria. Otherwise, because distributors may not be aware of component finishes, tin-finished components may inadvertently be used in high reliability applications. This is a critical manufacturing issue which affects the tracking and identification of finishes applied to COTS components used in military electronics.

Views on the risk posed by tin whiskers varies among manufacturers. Manufacturers are mainly concerned with providing a level of reliability necessary for the few years consumer electronics are usually in service. Two of the reviewed component manufacturers had conducted an in-depth study of whisker growth and abatement and made a decision to use a matte tin finish based on the ease of implementation. Two other companies conducted studies which showed no whisker growth greater than 50 microns and concluded that whisker growth on their components was not a concern. Two manufacturers decided to use a matte tin finish based on a review of existing literature. The final manufacturer provided no information about tin whisker risk with their components.

All of the component manufacturers in this study stated that their lead-free finishes were compatible with most lead-free solders. Two identified SnAg and SnCu as the optimal lead-free solders for their components. The remaining did not identify an optimal lead-free solder.

The results of this EMPF Lead-Free Design Conversion study show that many individual part numbers are not being changed to reflect new lead-free finishes and that bulk packaged components often do not have finish identification. This lack of new, unique part numbers and clear identification of many lead-free components creates a logistics challenge. Electronics manufacturers will have to take additional steps to determine whether stock is tin-lead or lead-free based on X-ray fluorescence, visual analysis, or other criteria. Otherwise, because distributors may not be aware of component finishes, tin-finished components may inadvertently be used in high reliability applications. This is a critical manufacturing issue which affects the tracking and identification of finishes applied to COTS components used in military electronics.

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