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Corrosion Resistance of Finishes in Harsh Environments
Analysis Lab |
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Authored By:Mustafa Ozkok, Guenter Heinz, Atotech Deutschland GmbH Joe McGurran, Hugh Roberts, Atotech USA Inc. Kenneth Lee, Atotech China TranscriptCorrosion resistance is becoming one of the most important topics in the electronics industry. Corrosion results in field failures and huge losses, which annually total several billion dollars. The actual extent of losses caused by corrosion is not well documented. As such, corrosion is currently one of the most challenging topics and is acquiring more attention as a result of increased product warranties, new materials, and process changes caused by recent legislation impacting the electronics industry. Depending on the final application and the environment to which the product is exposed, the requirements for corrosion resistance can be significantly different. Products used in military, automotive and medical applications typically demand higher corrosion resistance than products for lower performance or lifetime expectations, such as consumer electronics or similar products used in non-aggressive environments. As a result, to avoid corrosion on electronic products each industry sector has essentially adopted its own reliability testing procedures and standards. These facts all lead to the question, "What is the right corrosion resistance level of the surface finish for a particular product?" This paper investigates the performance of seven primary types of surface finishes using four different corrosion tests. The compiled data, findings and recommendations are offered as a guide to selecting the most suitable surface finish based on the end use application and required level of corrosion resistance. SummaryCorrosion resistance is becoming one of the most important topics in the electronics industry. Corrosion results in field failures and huge losses, which annually total several billion U.S. dollars. The actual extent of losses caused by corrosion is not well documented in the industry. As such, corrosion is currently one of the most challenging topics and is acquiring more attention as a result of increased product warranties, new materials and process changes caused by recent legislation impacting the electronics industry. Another factor is that the industry used in the past the lead containing surface finish "Hot Air Solder Leveling" (HASL) in very large volumes. This surface finish does have a superior corrosion resistance because of the Copper/Tin IMC and the corrosion resistance of the tin surface itself. Therefore corrosion resistance was for a long time no topic for the applications using HASL. But since the RoHS legislation came in effect in July 2006 and the use of lead containing HASL was restricted, the industry has looked into and qualified new alternative lead-free surface finishes. Furthermore the lead-free version of HASL shows some major disadvantages like uneven deposition thickness and as a higher working temperature is needed, a detrimental impact on the base material cannot be avoided. Companies do expect from these new alternative surface finishes to show the same corrosion resistance like HASL but many missed to investigate these alternatives concerning their corrosion resistance performance in combination with their applications. It only came to the attention of the electronics Industry as they were recently confronted with more and more field failures due to corrosion. Depending on the final application and the environment to which the product is exposed, the requirements for corrosion resistance can be significantly different. Products used in military, automotive and medical applications typically demand higher corrosion resistance than products for lower performance or lifetime expectations, such as consumer electronics or similar products used in non-aggressive environments. As a result, to avoid corrosion on electronic products each industry sector has essentially adopted its own reliability testing procedures and standards. These facts all lead to the question, "What is the right corrosion resistance level of the surface finish for a particular product?" One key function of surface finishes on printed wiring boards (PWB) is the protection of the underlying metal surface from environmental influences until assembly operations, such as soldering or wire-bonding, are performed. Also, after assembly there are areas on the PWB that are not covered by solder, including contact pads, test pads, heat seal and heat sink areas and the inside of through holes and vias. These areas are covered only by the surface finish and must still be resistant against any corrosive environment in the field. When corrosion occurs on a surface finish the metal decays and undefined corrosion products are created. The result of this process could be either an "open", caused by attack of the underlying copper or a "short", caused by creep from undefined corrosion products. This paper investigates the performance of seven primary types of surface finishes using four different corrosion tests. The compiled data, findings and recommendations are offered as a guide to selecting the most suitable surface finish based on the end use application and required level of corrosion resistance. ConclusionsAs shown in this evaluation, each surface finish exhibits different corrosion protection characteristics in the various harsh environments. Typically, products used in military, automotive and medical applications have higher corrosion resistance requirements than products with lower lifetime or performance expectations, such as consumer or office products. In nearly all segments of the electronics industry, the increasing variety of raw materials, manufacturing processes and performance specifications pose a wide range of factors influencing the requirements for corrosion resistance. Initially Published in the IPC Proceedings |
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