Ultrasonic solder writing, the next level die attach process

Soft Solder die attach process has been used for more than 20 years. The conventional method evolved from the normal dot dispensing, soft solder dispenser (SSD) [1], Motorized Pre-Press Module (MPPM) to Programmable Pre-Press Module (PPPM) [2] and multi dot dispensing. The most commonly used process is the MPPM/PPPM method also known as spanking process. This technology is reaching its limits in terms of package design rules (die size vs. pad size) caused by lack of flexibility in solder pattern size and bond line thickness given by the mechanical spanker tool dimensions. A novel solder pattern distribution method named Programmable Ultrasonic Module (PUM) [3] forms the pre-dispensed solder into any versatile pattern geometry without die size dedicated tooling. Detailed studies using PUM on Esec 2009 SSI
^E
die bonder is being done to develop process applications which can address and overcome the current power package die attach limitations. The outcome of this study achieved promising results as followed: • Enabled full solder coverage for a solder with poor wetting ability which is not possible by using MPPM/PPPM. • Reduction of solder voiding for poor wetting solder from more than 40% voiding to less than 10% voiding. • Controlled solder bond line thickness and tilt. • Eliminated solder creepage to the sidewall of the chip for poor wetting solder. • Increased maximum chip size possible in TO Leadless (TOLL) package [4] by 33% from 30mm
²
to 40mm
²
using Pb-based solder. This paper focuses on the evaluation result done with PUM and the benefits of using PUM.