Packaging According to Lead-Free Mandates

Although contrary and cautionary voices exist, it appears the industry drive toward lead-free packaging has gathered sufficient momentum to propel implementation. While initial proposals emanated from the United States, today lead-free initiatives are largely being driven by Japanese and European mandates. Practically speaking, adoption of lead-free packaging technology and materials by large Japanese corporations spearheads the movement and pushes global suppliers and manufacturing partners to climb on board.

The implications for IC packaging companies involve the elimination of lead from solder balls, lead finishes, die bonding materials and flip-chip interconnections. Other banned substances have been identified by legislation, but toxicity levels are not yet fully defined. Moreover, these other banned substances present greater challenges for downstream PCB assembly and PCB fabrication processes and have a lesser impact on semiconductor packaging itself.

The area of critical concern to lead-free packages, solder materials and all other components is the PCB reflow process. Eutectic tin/lead materials reach their melting point at 183°C. As such, a reflow process incorporating temperatures of 200-235°C is commonly assigned and, if carefully monitored, produces low defects with high yields. Relatively speaking, this provides a wide process window. In contrast, the most commonly accepted lead-free alloy, tin/silver/copper, has a melting point of ~217°C, depending on which precise formulation is used. If attempting to use a traditional reflow profile, there is a much narrower temperature range, and process control becomes a highly critical criterion for successful solder operations.

One solution to the shrinking process window is to set higher temperature reflow profiles, but some components cannot withstand the elevated temperatures. Also, additional tools and equipment will likely be necessary. In fact, to ensure complete liquidus and proper solder joint formation, many reflow profiles require temperatures in the 240-260°C range. Above 260°C, substrate deformation and other problems emerge, so industry does not foresee the need to explore higher temperatures. To promote lead-free reflow success, the National Electronics Manufacturing Initiative (NEMI) has provided industry with a reference tool for reflow profiles (Table ). Still, the design, assembly and reliability challenge for all lead-free packaging centers on the ability of the package structure to withstand the higher-temperature profile during reflow.

(Source: NEMI)

In addition to narrowing reflow process windows, higher temperatures introduce added reliability concerns such as package delamination or outright explosion (popcorning effect) if moisture is present. Further, package degradation as a result of lead-free processing stresses is not fully understood and long-term field reliability data are not available. The bottom line is that IC packaging operations must reevaluate materials for substrates, mold compounds and die attach, as well as the lead-free materials discussed above, to ensure that the entire package structure is capable of surviving downstream manufacturing processes.

Another area for concern is the terminations on leadframe packages. Changing from a tin/lead plating process to a tin plating process can be accomplished using standard equipment, but reliability issues linger. Electroplated tin can produce tin whiskers — electrically conductive, single-crystal structures that grow from plated surfaces, sometimes causing short circuits.

For array packages, tin whiskers do not present the same level of concern since the solder spheres can be made from a number of acceptable no-lead alloy formulations, primarily the tin/silver/copper variations. Still, the overall package reliability issues relative to thermal/mechanical stresses must be closely monitored.

Clearly, the lead-free mandates that loom dictate major technology shifts for IC packagers. Although traditional eutectic tin/lead formulations at 63Sn/37Pb served the electronics industry perfectly well for the entire 20th century, the past few years have seen extensive research with lead-free alloys along with the formation of various consortia to address implementation of lead-free processing steps. Most formulations have settled on higher tin content with 3-5% silver and small amounts of copper. These Sn/Ag/Cu formulations satisfy the no-lead requirement and IC packagers have incorporated them very successfully into their assembly processes.

However, the cost of lead-free implementation will almost certainly be at 2× or above compared with traditional 63Sn/37Pb eutectic. Couple this with the electronics manufacturing downturn, and some strong arguments for postponement or repeal of lead-free initiatives challenge the wisdom of implementation in 2006.

For additional information on assembly and packaging, go to www.semiconductor.net/assembly .