PCB007 Magazine

PCB007-July2018

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60 PCB007 MAGAZINE I JULY 2018 are all needed to create a coating which can be easy, fast and reproducibly be processed in the specified quality—the actual "backbone" of such a coating is made up of only three com- ponents: The resins, fillers and hardener used. These constituents ultimately determine the (di)electrical and physical/mechanical proper- ties. Table 1 and Table 2 give an overview of a principle LPiSM formulation composition. Aging Process of Polymers When looking at the most relevant factors involved in the general aging of polymers the following come to mind: First, a loss in vola- tile components (e.g., a loss of low-molecular components that were present from the begin- ning); second, oxidations that are often accom- panied by an increase in cross-linking and em- brittlement. The continuation of molecular po- lymerisation often initially leads to an increase in electrical and mechanical stability, but fi- nally ends in embrittlement. Additionally, the chemical separation of low-molecular compo- nents plays a role. This separation is a result of the aging process. These processes can act au- tocatalytically. Finally, also a hydrolysis of the polymer due to present moisture needs to be considered. General Procedure for Long-Term Temperature Storage Testing of LPiSM The approach for LPiSM testing consisted of the ap- plication (20 to 40 µm dry thickness) and processing of different coating mate- rials on various (high Tg, low CTE) substrates fol- lowed by a surface finish (ENIG, IMT or HASL). Pri- or to the actual testing, a pre-conditioning consisting of one wave soldering or two reflow soldering pro- cesses was carried out be- fore the actual temperature specific thermal loads, especially in combi- nation with high-power LED lighting applica- tions. Introduction The demands and loads placed on photo- imageable solder masks rise to unknown new levels. Today's solder masks are already ex- posed to a considerable level of thermal stress, high humidity and/or condensation. Especial- ly in automotive electronics, the level specified for thermal resistance and thermal cycling re- sistance is constantly increasing. Due to high- er currents and applications mounted in the engine compartment/gear box or close to the exhaust system, common photoimageable sol- der masks reach their performance limits. The thermal stress resulting from higher operat- ing temperatures triggered the development of new solder masks or further development of existing solder masks. Here the level demand- ed by the industry in terms of permanent high temperature loads is typically 175°C. Photoimageable Solder Masks While an LPiSM can consist of more than twenty different ingredients—which of course Table 1: Constituents of resin component in LPiSM. Table 2: Constituents of hardener component in LPiSM.

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