Graded PB for C4 pump technology

A solder bump connecting a semiconductor chip to one of a substrate, chip carrier and circuit board, said semiconductor chip including a device susceptible to a radiation-induced soft error in close proximity to said solder bump, said solder bump comprising:

• plural thin Pb layers, each less than about 0.5-0.2 .mu.m in thickness, said plural thin Pb layers electrically coupled to a circuit of said semiconductor chip, said plural thin Pb layers being substantially free of .alpha.-particle emissive isotopes and being substantially opaque to .alpha.-particles, each thin Pb layer being separated from an adjacent thin Pb layer by a layer interface;
• a Pb-bearing solder portion electrically coupled between said plural thin Pb layers and said substrate, said Pb-bearing solder portion comprising the substantial proportion of said solder bump;
• and
• a transition region extending into less than all of said thin Pb layers, said transition region comprising a mixture of material from said Pb-bearing solder portion and from said thin Pb layers.