TABLE OF CONTENTS Solders are the electrical and mechanical glue of high-density interconnects. Low-cost tin-lead solders have been used as joining materials in the electronics industry for many years. The unique physical and mechanical properties of tin-lead solders have facilitated PCB assembly choices that have fueled creative advances in packaging developments, such as solder-bumped flip chip, BGA packages, and WLCSP.
In the past few years, different bills have been introduced in the U.S. Congress to ban lead from a wide variety of uses, including solders. The reasons are, among others: (1) lead and its compounds are ranked as one of the top 10 hazardous materials, and (2) lead is the number one environmental threat to children. Since then, many major electronics companies, national laboratories, universities, research organizations, and solder vendors worldwide have responded by initiating research programs to eliminate lead from solders. Lead-free solders will be discussed in Sec. 9.4.
The trend toward sub-0.18- m IC feature size and the continuous reliance on lead-bearing solder alloys bring up a new issue soft error due to alpha particle emission of lead-bearing solders. These alpha particles do not cause permanent physical damage to the chip but are sufficiently energetic to switch the capacitor from 0 to 1 or vice versa, and resulted in so called soft error. Alpha particles and related issues will be discussed in Sec. 9.3. Because most of the solder alloys are used at high homologous temperatures, creep deformation plays a very impor- tant role in almost all aspects of their mechanical...
