Assembly process development of 50um fine pitch wire bonded devices

This paper covers wire bonding and molding process development for 50 μm fine pitch wire bond plastic ball grid array packages (PBGA). Since a large percentage of the total bonding area was damaged during probe testing, the adhesive strength of the ball bond was reduced. Bond lift issues have been observed after destructive wire pull test. A comparative study of different probe mark size using 0.9 mil wire diameter was presented and analysis on bonded units concludes that the critical damage threshold was around 20% of bonded area. A possible solution with irregular elliptical ball bonding was proposed to improve the adhesive strength of ball bonds. On the other hand, reduced wire diameter has decreased the bending strength of bonded wires significantly, and thus causing serious wire sweep after transfer molding. Experiments were conducted to evaluate wire length with 0.9 mil diameter gold wire. The results show that the wire threshold length is about 4.1 mm. A novel solution was applied using a non-sweep method. The wire length could be extended to 5.0 mm within 1.0% wire sweep. Reliability tests conducted showed that all the units passed 1000 temperature cycles (-55°C∼125°C) with JEDEC moisture preconditioning at level 2a (60°C/60% relative humidity for 120 hours) and 3 times reflow (peak temperature at 220-225°C). It is believed that this solution can efficiently overcome the risk of wire shorting and improve the yield of ultra fine pitch wire bonds in high volume production.