ICSR (Soldering and Reliability) Conference Proceedings

SIR Measurements During Modified Automotive Damp Heat Cycling: Solder Paste and Conformal Coating Compatibility

Authors: Karen Tellefsen and Mitch Holtzer, Prof. Dr.-Ing. Marcus Reichenberger
Company: Alpha and Technische Hochschule Nürnberg Georg Simon Ohm
Date Published: 5/13/2014   Conference: ICSR (Soldering and Reliability)

Seika Machinery, Inc.

Abstract: Manufacturers of automotive electronics are very concerned with reliability, as the devices they assemble will be used in harsh environments and many must function reliably for the safe operation of the vehicles in which they are installed. For this reason, automotive circuits are often treated with conformal coating after assembly to protect against moisture and to reduce the growth of tin whiskers [1]. Sometimes the flux residues, even residues from "no-clean" materials, are cleaned from the circuit before coating, but often these coatings are applied directly over uncleaned reflowed solder paste. Therefore, it is important to provide reliability data on combinations of these materials.

In this study, Surface Insulation Resistance (SIR) coupons were reflowed with four lead-free solder pastes, and then coated with one of five conformal coatings from one manufacturer and four coatings from a second. These coatings included silicone, acrylic and urethane chemistries. These coupons were exposed to a modification of Automotive Damp Heat Cycling Test [2] with an applied bias of 50V and frequent SIR measurements at bias. After exposure, the coupons were examined for evidence of electrochemical migration, such as metallic dendrite formation. A water-based urethane coating was found to protect poorly in combination with all four solder pastes, while an acrylic coating from the same manufacturer provided good protection in combination with these same solder pastes. Other coatings worked well with some solder pastes, usually less activated ones, but not others.

Key Words: 

Solder Paste, flux residue, conformal coating, SIR

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