Lessons Learned Converting A Class i Medical Flex Circuit to Rohs Compliance

ABSTRACT There has been enormous worldwide effort to eliminate hazardous substances from consumer products over the past few years. Much of this effort has been driven by the European Union directive on the Reduction of Hazardous Substances (RoHS) scheduled to take effect on July 1, 2006. There are a number of exceptions to this directive that have caused a reduced urgency in the medical electronics world. Nevertheless, some medical electronics is being transition to lead-free solder assembly. This paper will examine the lessons learned in converting a Class I medical flexible circuit micro-electronic assembly from a leaded solder SMT assembly to a lead-free solder SMT assembly. Various issues will be examined including: the due diligence process and lead-free conversion protocols; establishment of a special production line for RoHS to avoid cross- contamination issues; selection of the solder alloy; selection of the solder paste type and flux system; stencil design changes associated with lead-free processing; required changes to the reflow profile; changes to the inspection criteria; changes to the rework process; wetting properties of lead-free solder and defect control; changes to the water soluble flux removal process; special handling of solder pastes and stencil cleaning; and evaluation of the temperature stability of the flexible circuit. Results indicate that the manufacture of reliable and high quality medical electronics is feasible when it is designed and developed for RoHS compliance.