Low and ultralow alpha-emitting semiconductor assembly materials are now essential for flip-chip packaging and are also becoming increasingly critical for power semiconductor assembly, as smaller active device sizes and thinner wafers increase the devices' sensitivity to ionizing radiation. 
An alpha particle is an ionized particle consisting of 2 neutrons and 2 protons emitted spontaneously from the nucleus of specific isotopes of certain high atomic weight elements. Of particular interest and concern to semiconductor fabricators and packaging houses are the elements uranium (U) and thorium (Th), isotopes of which decay to give stable isotopes of lead (Pb), but which, by decaying, give rise to alpha particles.
For over a year now, we have been supplying low alpha (LA) and ultralow alpha (ULA) emitting solder pastes to Asian customers. During this time, we have faced numerous challenges: most importantly:
- The absence of standards in the area of measurement and test methodology
- Metrology at the ultralow alpha level is hampered by a signal to noise ratio of about 1:1, where the “noise” is simple background radiation, present at around the 0.002cph/cm2 level.
Even the definitions of the different levels are not well defined, but we are operating under the working rule that:
- LA =< 0.020cph/cm2
- ULA =< 0.0020cph/cm2
I had the opportunity to discuss our work at the latest Second Annual IEEE-SCV Soft Error Rate (SER) Workshop in late October this year - please follow the link to learn more - and I’d like to thank Peng Su of Cisco for giving me the opportunity to speak at this meeting. Also look for the forthcoming (at time of writing) review paper “Challenges in Supply of Ultralow Alpha-Emitting Solder Materials” by me and Olivier Lauzeral of iROC Technologies in Chip Scale Review magazine for November/December 2010.
In the coming months, Indium Corporation will be providing LA and ULA solder alloy, preform, solder powder, solder paste and flux materials.
Cheers! Andy
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