Nov/Dec, 2022
www.us-tech.com
Page 53 SAC305 Interconnects Using Supercooling Continued from previous page
BGA169 was analyzed by SAFI- Tech and by an external test company. All daisy-chain pairs in the assembly were measured for electrical connections. All 84 daisy-chain pairs were electrical- ly conductive. Sectioning and polishing of the solder joints revealed all solder joints in the line of 13 solder balls to be con- tinuous. The section assembly has the BGA package on top with the original SAC305 solder ball clearly distinguishable in shape. The supercooled SAC-based
microcapsule paste clearly coa- lesced into a continuous struc- ture at the assembly tempera- ture. The paste metal wet the solder ball. The final shape of the solder joint was very similar to the shape of a hybrid solder joint between a SAC305 and BiSn- based solder paste. They look similar since the SAC305 solder ball does not melt. The tin-rich solder ball and
paste show a continuous piece of metal between the ball and the paste. A closer look at the inter- face at the FR4 side of the solder joint shows the formation of IMC even at 356°F (180°C) with a supercooled microcapsule-based paste.
According to the elemental
mapping with an EDS detector there is no detectable silver line from ImmAg coating at the inter- face between the solder and the pad. The average thickness of the IMC measured from this SEM image was 2.5µ. The IMC is a mixture of tin
and copper and in the bulk solder joint there are regions high in silver. This is the typical struc- ture of a SAC305 solder joint and a copper pad. The atomic per- centages of tin and copper in the
IMC correspond to the Cu6Sn5 intermetallic compound. None of
the copper-rich Cu3Sn inter- metallic compound were detect- ed. This is interpreted as mean- ing the diffusion of copper was limited in the process, presum- ably because of the lower tem- perature of the process. The assembly was then sub-
jected to thermal shock testing from –40°F (–40°C) for 30 min- utes then 30 minutes in 230°F (110°C) for 1,000 cycles. After thermal cycling the daisy-chains were measured and then the assembly was sectioned for SEM analysis. The sample still passed electrical testing. In the SEM, voiding was observed in the sol- der and some of the voids started to fracture, but no full fractures were observed. SAFI-Tech’s SAC305 prod-
uct represents progress towards a drop-in replacement SAC sol- der paste that can be used at an LTS temperatures. The super- cooled liquid microcapsule tech-
nology avoids thermal damage to components and materials, or quality issues caused by CTE mismatch. This solder joint has the three hallmarks of a solder joint by demonstrating wetting, continuous structure, and IMC formation. Contact: SAFI-Tech, Inc.,
BGA assembly with supercooled SAC-based microcapsules.
1575 Food Sciences Building, 536 Farmhouse Lane, Ames, IA 50011 % 815-326-2902 E-mail:
ian@safi-tech.com Web:
www.safi-tech.com r
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