Challenges And Development For Lead-Free Wave Rework Soldering
Authors: Sunil Gopakumar, Francois Billaut, Eric Fremd, K.Y.Tsai, Chu Lin, and Jasbir Bath Company: Brocade Communications, Foxconn, Bath Technical Consultancy Date Published: 10/16/2011
Abstract: The increasing legislative pressures and the limited supply of tin-lead components is hastening the lead-free transition of many companies in the server and networking domain. Lead-free wave soldering has received considerable attention due to the challenges to achieve adequate solder barrel fill with OSP finish with board thickness of 93 mils [ 2.3mm] or greater, and PTH components with pins connected to multiple ground connections. Information on PTH rework development has not been as widely reported and it is an area that merits considerable attention. During lead-free wave soldering, copper dissolution from the barrels adds an additional constraint during the subsequent rework of PTH components. In addition, since the higher melting lead-free solder alloys require increased processing temperatures, the boards have to be pre-heated to a higher temperature to reduce the thermal shock on the boards during rework. The objective of this paper is to identify critical lead-free process parameters that drive successful rework of PTH components in terms of IPC hole fill requirements and minimal copper barrel knee dissolution. A representative product of high complexity with 125mils [3.2mm] board thickness with OSP finish was used as a product test board. The board featured different types of PTH components with varying pin counts and dimensions which were representative of components on a wide range of products. SAC305 lead-free alloy (Sn3Ag0.5Cu) was used for wave soldering, and SnCuNi alloy (Sn0.7Cu0.05Ni) was used in the mini-pot solder fountain machine for PTH rework. Three types of PTH component were selected for the rework study. Results from the rework process DOEs determined some of the key process parameters and optimum settings which included pre-heat temperature, pot temperature, contact time and flux type to ensure lead-free products met IPC 610 Class 2  standard hole fill requirements after rework as well as reducing copper barrel knee dissolution.