New Robust Process Improvement for BGA Solder Ball Attach First Pass Yield

Abstract

In BGA package assembly, the solder ball attach process is one of the yield off points that impacts the overall product yield and cycle time because of the need to rework. From the beginning of BGA product manufacturing the race to achieve high solder ball attach yields has spurned various ball attach technology from flux printing to flux pin transfer, a multitude of flux formulations by various flux suppliers, improvement in solder ball pad finishes by substrate suppliers, and many other process improvement iteration. Today, the most popular ball attach technology in the industry is the use of pin transfer using tacky flux on solder on BGA pad (SOPBGA), Imm Sn, CuOSP, and ENIG. Yet, the solder ball attach process continues to suffer missing balls, joined balls, and bridging balls, impacting the overall cycle time due to rework. This study shows the use of dippable paste, a novel ball attach material replacing flux that's compatible with pin transfer equipment technology and which pushes the first pass yield to 100%, virtually eliminating rework and could potentially reduce manufacturing cycle time.Dippable paste has been used mostly in package-onpackage technology but most formulations are no-clean. A new water-soluble formulation was successfully prepared and applied to normal pin transfer ball attach process for the first time. The solder powder in the dippable paste improved wettability onto the pad finish. Results showed 100% first pass yield on FCBGA samples with SOPBGA, Imm Sn, ENIG, and CuOSP pad finishes. It also showed comparable coplanarity, solder ball height, solder ball diameter, solder ball shear, solder ball pull, and x-y pitch values with flux process. The robustness of the process is confirmed with perfect first pass yield even on poor surface conditions.

Dippable Paste: A Robust Assembly Material and Enabling Technology

“…Recently, a breakthrough water soluble solder paste with very fine metal powder applied by pin transfer application methods, also known as a "dippable paste" was developed. The results of replacing tacky flux with this unique material was that no missing, bridging, or joined balls have been reported (2).…”

“…The results of replacing tacky flux with this unique material was that no missing, bridging, or joined balls have been reported (2).…”

“…Although the tackiness is lower than some of the tackiest fluxes available it has shown remarkable capability to prevent ball movement. In a pilot study of 100 units with comparing flux to dippable paste on flat Ceramic FCBGA, the dippable paste showed 100% yield while the flux only, 98% (2).…”