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SMT-Jan2017

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14 SMT Magazine • January 2017 are relative to Hot Air Solder Leveling (HASL), which makes it advantageous for smaller PB de- sign features. As electronic component packag- ing technology advances and parts get small- er, the PB must have finer features to attach these parts. This decrease in feature size typical- ly drives a decrease in solder paste stencil thick- ness for effective solder release, resulting in smaller solder joint volume. As the solder joint size gets smaller, the thickness of the surface finish on the PCB remains constant, so that the relative volume of gold and palladium in the solder joint increases. ENEPIG has been dem- onstrated to have acceptable performance and reliability for many current packages and sol- der joint configurations, but some studies raise concerns for use of ENEPIG at high concentra- tions or in very small solder joints 1 . Round robin testing for the development of the IPC-4556 specification included palladium thicknesses of up to 17.95 microinches (µin). A 0.005-inch-thick stencil was used with a 0.025- inch diameter SnPb solder sphere, resulting a solder joint containing approximately 0.17% Pd. Shear testing of the solder ball on pad re- sulted in both lifted pads and cohesive failure in the bulk solder. IPC-7095C Design and Assembly Process Im- plementation for BGAs warns of reliability im- pacts from excessive and non-uniform interme- tallic compound (IMC) layer growth. IPC-4556 mentions a 3% limit for gold and palladium, and other sources have identified 2% as a lim- it 2 , but these limits based on a percentage are misleading when applied to Pd. Gold can (and is desired to) disperse throughout a solder joint to minimize impact, but Pd tends to form a dis- tinct and concentrated IMC layer above the Ni. Palladium has a slower solubility rate in molten solder than other metals. During sol- dering, the tin-palladium intermetallic com- pounds (either PdSn3 or PdSn4) will rapidly grow in the form of a thick lamellar structure perpendicular to the original palladium surface. Solder, consisting of a lead-rich phase, will be present between the lamellae. Further aging of solder joint can result in the movement (spall- ing) of the tin-palladium layer into the bulk sol- der and likely leave tin-palladium crystals with- in the bulk solder. ENEPIG has been found to be prone to brittle fractures. The tin-palladium layers that form directly above the nickel plating has been found to be brittle, although some studies in- dicate that this may be based on weakness of a phosphorus rich nickel layer, not necessarily the Pd intermetallics 3 . The presence of tin-pal- ladium crystals within the bulk solder has not been well-documented in terms of the effect on solder joint integrity; however as a compar- ison, tin-gold intermetallic crystals in bulk sol- der have been shown to embrittle a solder joint, enabling fracture of the solder joint along the gold intermetallics. It has not been shown that Pd intermetallics can have a similar effect with- in the bulk solder, the typical failure is associat- ed with brittle fracture at the PB pad. Based on the initial formation of a thick la- mellar Pd intermetallic structure above the Ni layer, a limit on Pd thickness may be more ap- propriate to prevent excessive intermetallic lay- er thickness. A higher Pd thickness leads to an increased interfacial IMC thickness, specifical- ly when SnPb solder is used. A thin palladium layer should dissolve rapidly into molten sol- der and result in no detrimental effect on sol- der joint mechanical properties. A variety of Pd thickness percentages or thickness limits have been proposed, some of which are lower than EVALUATION OF THE USE OF ENEPIG IN SMALL SOLDER JOINTS Figure 1: PdSn intermetallic structure spalling off substrate surface.

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