Overcoming Tin Whisker Problems on Lead-free Packaging
BY MELISSA GRUPEN-SHEMANSKY
The EU has set in place a Restriction of Hazardous Substances (RoHS) directive that states: "Member States shall ensure that from July 1, 2006, new electrical and electronic equipment put on the market does not contain lead (Pb), mercury, cadmium, hexavalent chromium, polybrominated biphenyls and polybrominated diphenyl ethers."
The electronics supply chain spans the globe and has no geographic boundaries. It would waste a lot of money and time to set up the materials and equipment to package lead-free products for Europe, and keep an alternate parallel track for products for the rest of the world. Frankly, knowing the end location of the billions of electronic components produced each year is not feasible.
Lead-frame-based packages are plated on the lead edges with tin-lead (Sn-Pb), most often to enable efficient and reliable solder attachment to system boards. The addition of lead to the plated alloy effectively retards tin-plated whisker growth, a stress-driven mechanism that results in long, tin "extrusions" that may reach sufficient length to short electrical connections and cause IC failure. Without lead present, the length and width of tin whiskers varies, depending on the deposition process and conditions to which the product is subjected. Some as long as 9 mm have been reported in literature, while lead-to-lead spacing within a lead-frame-based package typically is 200 µm. Tin whisker growth in the lead-free case can also be effectively retarded by depositing or plating a barrier metal, such as nickel, between the copper lead frame and the solderable matte-Sn finish.
The primary standard body setting the qualification criteria for IC components is JEDEC. Recently, JEDEC proposed a set of qualification and reliability tests for tin whiskers, entitled "Tin Whisker Growth Tests." The conditions specified are: 1) storage at 60°C, 93 percent RH for ≥10 weeks; 2) storage at room ambient (22° to 28°C, 40 to 70 percent RH uncontrolled) for up to 6 months; and 3) -55° to +85°C temperature cycling for up to 2,000 cycles (Figure 1). However, there is presently nothing in the testing procedure that calls attention to the need to subject the products to a board attachment process. Experimental evidence shows us that it is critical to subject IC products to such an attachment process, in addition to the accelerated JEDEC-defined environmental testing, to better determine susceptibility to tin whiskering.
Figure 1. Tin whiskers after 5 weeks at 60??C/93 percent RH.
On an experimental basis, Agere has subjected devices to simulated board attachment processes with peak reflow temperatures of 200°, 215°, 225°, 235° and 260°C. Components not subjected to any reflow pass inspection for whiskers under various conditions, after JEDEC testing. If the same components undergo board assembly, vastly different results are observed. Among other things, this set of tests discredits the popular notion that a 150°C annealed 15-µm-thick matte-tin finish on a copper lead frame is a robust lead-free alternative.
To evaluate whiskering and ensure backward compatibility (defined as lead-free IC components used in non-lead-free equipment), we are subjecting all lead-free lead frame components to three JEDEC tests using three types of components: as made; as made, followed by a 215°C peak board reflow process; as made, followed by a 260°C peak board reflow process. The 215°C reflow process is typical of a present Sn-Pb solder attachment process, and the 260°C conditions are typical of a lead-free attachment process.
Experimental findings indicate that current standard life testing may miss the issue. In evaluation, prospective lead-free finishes must be subjected to board-level reflow conditions as a preparatory step to accelerated life testing or reliability testing. To ensure backward compatibility, it is advised that reliability testing be conducted with both sets of solder preconditioning exposure. Thorough observations show that tin whiskers may grow, even in the Sn/Pb case under specific application conditions. Experimental evidence, however, indicates that a barrier metal such as nickel deposited between the copper lead frame and the matte-tin finish can significantly retard tin whisker formation — even eliminate it.
MELISSA GRUPEN-SHEMANSKY, director of Packaging and Interconnect Technology, may be contacted at Agere, 555 Union Blvd., Allentown, PA 18109; (610) 712-6184; e-mail: firstname.lastname@example.org.