Yellow solder: an assessment of the quality and reliability of Pb-free lead finishes and solder ball alloys exhibiting excessive Sn oxidation

As the microelectronics industry transitions to a Pb-free infrastructure, a variety of quality and reliability issues associated with Pb-free solder alloys need to be understood. One interesting byproduct of the utilization of highly Sn-rich (>95% Sn) alloys as alternatives to the standard Pb-Sn eutectic is their tendency to develop thick oxide layers under certain temperature, humidity, and time conditions. As these oxide layer thicknesses increase, a pronounced ´yellowing´ of these materials has been observed. This condition is of concern for a variety of reasons: 1) it can directly impact the solderability of the product; 2) it can result in ball and lead-scanning process equipment failures; 3) the cosmetic appearance is not in keeping with the end user´s expectation of solder to be ´bright and shiny´, and may result in rejected material during incoming quality assurance (IQA) inspection. This paper explores the chemical makeup of these oxide layers, the conditions under which they form, and their potential impact upon a finished product´s quality and reliability. Three Pb-free alloys are characterized, including Sn-3.5Ag and Sn-3.0Ag-0.5Cu (in the form of BGA solder spheres), and Sn-2.0Cu (in the form of external QFP lead plating)