SMT007 Magazine

SMT007-Jan2023

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74 SMT007 MAGAZINE I JANUARY 2023 and sent out to plating to protect the copper with a nickel-gold (Ni+Au) alloy. Nickel acts as a diffusion barrier, with gold protecting the circuit from elements. Various die-attach epoxies were then eval- uated. ese included Ablebond 84-1lmi and H70E, both electrically conductive epoxies with silver particles. In both cases, epoxy bleed- out later created delamination between encap- sulation and surface of the package, with parts failing solder stress tests as detected by con- focal scanning acoustic microscopy (CSAM) imaging. As the design comprised multiple down- bonds, it was essential that the die attach pad be free of delamination from solder stress. Experi- ments were performed that demonstrated that the die attach pad with its native copper and silver spot plating yielded better solder stress performance compared to those with nickel- gold plating. us, the extra plating step was eliminated. Next, another design-of-experiments (DOE) set was started, working with unplated OCPP packages. Various epoxies were evalu- ated, and a newly formulated product, Able- bond QMI529HT, was selected. is epoxy is designed specifically for copper, features min- imal bleed-out, and doesn't affect the adhe- sion of encapsulation to the copper, as CSAM indicates. e epoxy is also compliant with the Restriction of the Use of Certain Hazard- ous Substances in Electrical and Electronic Equipment directive (RoHS). In addition to EU RoHS guidelines, QP Tech adheres to Cal- ifornia's own RoHS laws governing electronic devices sold in the state. Once the optimal epoxy and OCPP type were determined, QP Technologies pursued multiple iterations to develop the ideal rec- ipe for building packages that would pass all- important MSL testing. e packages passed MSL-3 / 235°C solder stress tests with no evi- dence of delamination, with results equivalent to factory-fresh parts. To ensure repeatability of the process, a traveler was created to document the recipe, including the exact steps and order in which they must be performed—epoxy type, how the package is opened, the wire bonding process, encapsulation, material, etc. First, a lot of 50 packages was developed, on which DEI performed pre-compliance (con- fidence) reliability testing. Aer passing this testing, several hundred packages were assem- bled as a qualification lot. ese were produc- Figure 2: Qualification test plan for 64 QFP OCPP.

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