SMT007 Magazine

SMT-Nov2016

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60 SMT Magazine • November 2016 by Yash Sutariya SATURN ELECTRONICS/SATURN FLEX I'll always remember the summer of 2004 as the "Summer of Lead-Free." Finally, Pb-free cir- cuit boards were going into standard produc- tion mode. Assemblers focused the majority of their efforts (often at my behest) on final finish and proper laminate selection. What none of us saw coming, however, was the rash of delami- nation that would burn the entire industry dur- ing that long, hot lead-free summer. After a good amount of research, Isola came up with a Lead-Free PCB Fabrication and Assem- bly Guideline that outlined various steps in the PCB fabrication and assembly processes that are critical to successful lead-free PCB assemblies. The most prominent process step that was add- ed in this guideline is baking—during fabrica- tion and, most critically, just prior to assembly. The goal of baking is simply to drive moisture out of the PCB. Moisture plays a critical role in lead-free PCB assembly. Therefore, it is important to dis- cern the specifications of your base laminate. One knee-jerk reaction of North American PCB users was to specify what they believed to be the highest end laminates, those that qualify under IPC 4101/126 and /129 slash sheets. These are typically phenolic-cured materials, which have much higher thermal properties in terms of glass transition temperature (Tg) and decompo- sition temperature (Td) that are required to sur- vive lead-free assembly without via failure. Unfortunately, other properties of the lam- inate and assembly temperatures create a per- fect storm for reliability failures. These include: moisture absorption, interlaminate adhesion strength, and water vapor pressure at lead-free assembly temperatures. Moisture Absorption Moisture absorption of phenolic materials is more than 2x as compared to traditional FR-4 materials that qualify under IPC 4101/21. In the early days, the technical data sheets used a 0.028" thickness core material for moisture ab- sorption measurement. Based on this test ve- hicle, phenolic laminates showed moisture ab- sorption of 0.45%; comparatively standard FR-4 laminates scored a 0.20% rate of moisture ab- sorption. Over time, I think someone got wise and changed the test vehicle to a 0.059" thick- ness core, thereby increasing the denominator TO BAKE OR NOT TO BAKE: Examining the Impact of Waiving PCB Pre-Baking Prior to Assembly FEATURE

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