Soldering and Solder Joint Reliability
for Selected Surface Finishes with
LF SAC305 alloy
By George Milad & Don Gudeczauskas,
Uyemura International Corporation
The following board surface finishes: electroless nickel / immersion gold (ENIG),
Electroless nickel electroless palladium / immersion gold (ENEPIG), Immersion
Silver (IAg) and Direct Immersion Gold on copper (DIG) were evaluated for
wettability/solderability of the finishes using the lead free alloy SAC305
(Sn 96.5%,Ag 3%,Cu 0.5%). Wetting balance data on the as-received
and simulated aging conditions will be presented.
The reliability of the solder joint formed with eutectic Sn/Pb and SAC305 solders,
was also evaluated. Eutectic solder and SAC305 alloy balls were soldered
respectively, to a soldermask defined BGA pattern on copper laminate coated
with the respective surface finish. Simulated aging of the solder join formed
between the ball and the substrate was conducted by baking the parts in a
controlled temperature oven for up to 1000 hours at 155°C. Shear strength
and fracture analysis were done at various baking intervals for both the eutectic
and the SAC305 solder joints formed. The solder joints were also examined with
SEM and EDS to determine the extent and the nature of the intermetallic
formation. The propagation of the intermetallic layer vs. shear strength was
evaluated and documented.
The data will show that these surface finishes will transition readily to the
world of lead-free soldering. ENEPIG is the surprise surface finish, as the data
shows that it is ideally suited for SAC305 alloy and the solder joint formed is
actually superior to its Sn/Pb eutectic counterpart.