Thermal Capabilities of Solder Masks: How High Can We Go?


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Abstract

This article focuses on three different coating material groups that were formulated to operate under high thermal stress and are applied at the printed circuit board manufacturing level. While used for principally different applications, these coatings have in common that they can be key to a successful thermal management concept especially in e-mobility and lighting applications. The coatings looked at consist of: Specialty (green transparent) liquid photoimageable solder masks (LPiSM) are compatible with long-term thermal storage/stress in excess of 150°C. Combined with the appropriate high-temperature base material, and along with a suitable copper pre-treatment, these solder resists are capable of fulfilling higher thermal demands placed on them. In this context, long-term storage tests as well as temperature cycling tests were carried out. Moreover, the effect of various copper pre-treatment methods on the adhesion of the solder masks was examined at 150, 175 and 200°C over the aging process. For this purpose, test panels were stored for 2000 hours at the respective temperatures and were submitted to a cross-cut test every 500 hours. Within this test set-up it was found that a multi-level chemical pre-treatment gives significantly better adhesion results, in particular at 175°C and 200°C, as compared to a pretreatment by brush or pumice brush. Breakdown voltage as well as tracking resistance were also studied.

For an application in LED technology, the light reflectivity and white colour stability of the printed circuit board are of major importance, especially when high-power LED are used which can generate larger amounts of heat. For this reason, a very high coverage power and an intense white colour with high reflectivity values are essential for white solder masks. These “ultra-white” and largely nonyellowing LPiSM need to be able to withstand specific thermal loads, especially in combination with high-power LED lighting applications.

Introduction

The demands and loads placed on photoimageable solder masks rise to unknown new levels. Today’s solder masks are already exposed to a considerable level of thermal stress, high humidity and/or condensation. Especially in automotive electronics, the level specified for thermal resistance and thermal cycling resistance is constantly increasing. Due to higher currents and applications mounted in the engine compartment/gear box or close to the exhaust system, common photoimageable solder masks reach their performance limits. The thermal stress resulting from higher operating temperatures triggered the development of new solder masks or further development of existing solder masks. Here the level demanded by the industry in terms of permanent high temperature loads is typically 175°C.

To read the full version of this article which originally appeared in the July 2018 issue of PCB007 Magazine, click here.

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