Whitepapers

Fine Powder Solder Pastes: Stencil Printing and Reflow in Lead-Free Assembly

Chris Anglin, Dr. Ronald C. Lasky, Ed Briggs, Timothy Jensen

4 Mar 2010

Summary: The explosive growth of personal electronic devices, such as mobile phones and personal music devices, have driven the need for smaller and smaller active and passive electrical components. Not too long ago, 0401 (40 x 10 mils) passives were seen as the ultimate in miniaturization, but recently 0201 and now 01005 passives have arrived, with rumors of even smaller sizes to come. For active components, the 30 mil CSP (a chip scale package with the solder balls on 30 mil (0.75mm) centers) has become virtually a requirement for enabling the many features in modern portable electronic devices. The more than 1 billion mobile phones assembled in 2008 will use the lion's share of the 12 billion or so CSPs concurrently manufactured. This miniaturization trend occurring at the same time as the conversion to RoHS compliant lead-free assembly has put a considerable strain on the electronic assembly industry. This paper will discuss some of these challenges and the work that has been performed to mitigate them. Among the challenges discussed are stencil printing the small features and obtaining consistent volume in the printed solder paste deposit, minimizing the oxidation of the solder powder in the small deposit during reflow, and assuring a good finished solder joint after the reflow process.