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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.

No abstract

Despite the tackiness offered by ball attach fluxes, they have failed to prevent ball movement on Solder on Pad (SoP) finishes when the solder protrudes above the solder mask. Dippable solder paste provides the tackiness needed for these adverse situations as well as small solder particles that inhibit ball rolling due to mechanical or convective forces downstream. Also, other pad finishes like OSP has issues with oxidation with partial pad wetting as a common defect. The use of dippable paste has also demonstrated relief with these wetting related issues. The most current application of dippable pastes is in the assembly of stacked BGA's also known as Package on Package (PoP). Although water clean formulations are typical in BGA assembly, PoP assembly typically utilizes no clean formulations. Although tacky flux has been successful in many PoP applications, issues such as coplanarity and package tolerances that require increasing the sphere size during assembly have found benefit from dippable solder paste.