IWLPC (Wafer-Level Packaging) Conference Proceedings


LEAD-FREE IS NOT ALPHA-FREE - WHY WLCSPS SHOULD CARE ABOUT ALPHA PARTICLES

Authors: Glenn A. Rinne and Krishna K. Nair
Company: Unitive Electronics (Amkor)
Date Published: 10/10/2004   Conference: IWLPC (Wafer-Level Packaging)


Abstract: The human use of lead dates back at least 8,500 years as evidenced by lead statues found in Turkey. Additional evidence indicates the use of Galena (PbS) as eye makeup by the Egyptians at least 6,000 years ago. For centuries, lead compounds were also used medicinally to cure a variety of ailments.

Lead occurs naturally in four stable isotopes and as many as 20 different radioisotopes have been identified. Over the past 20 years, there has been growing pressure to eliminate a particular isotope of lead from electronic solders: the radioisotope 210Pb. While IBM gave us the flip-chip technology, it was also the first to warn us about the elusive issues that came with it, namely, the problem of alpha particle emission from the solder. Soft errors result when an alpha particle bombards and penetrates a semiconductor device, creating a trail of electron-hole pairs. While this process seldom causes permanent damage in the device (also known as hard errors), it changes the state of the device, causing a "soft error" or "single-event upset (SEU)".

Alpha particles are most often produced by the radioactive decay of various atomic species in the solder. Conventional thinking would lead one to believe lead (Pb) is the source of the problem (and hence the evolution of lead-free solders), but the problem is multi-faceted. Accordingly, an effective low-alpha strategy should include more than a single approach. Effective alpha particle shields have been designed at Amkor’s Unitive Electronics subsidiary to control the soft-error rate.

Keywords: Alpha particles, single event upset, soft errors, shielding.



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