SMTA International Conference Proceedings


Authors: Scott J. Anson, Ph.D., PE, and Jacob G. Slezak
Company: Rochester Institute of Technology
Date Published: 10/11/2007   Conference: SMTA International

Seika Machinery, Inc.

Abstract: Printed Circuit Board (PCB) solderability is essential for effective electronics assembly. Historically, PCB surface finishes were Hot Air Solder Leveled (HASL), composed of 63Sn/37Pb. This surface finish was effective and commonly used. With current sanctioning involving the July 1, 2006 Reduction of Hazardous Substances (RoHS) compliance for lead free electronics in the European Union, the need for PCB finish alternatives has become much more urgent. OSP has the advantage as an economical surface finish that does not introduce any new metallurgical compositions to the solder joint while also providing flat pads. OSP has been somewhat prematurely dismissed as a lead free surface finish due to solderability concerns involving the need for elevated reflow temperature and multiple reflows. It has also been said that circuit board cleaning solvents, which are used for cleaning paste misprints, have unacceptable negative effects on OSP finishes.

There is a need to assess a modern OSP surface finish for lead free temperature and multiple reflow compatibility. This study will use a quantitative classical Design of Experiments (DOE) approach to investigate wetting using Sn96.5/Ag3.0/Cu0.5 (SAC305) solder paste on an industry relevant OSP surface finish. The PCB’s will be exposed to multiple reflows and common PCB paste misprint cleaning techniques. The resultant will be a better understanding of OSP finishes while attaining knowledge of the effects higher temperatures associated with lead free reflow, multiple reflow cycles, and paste misprint cleaning.

The results from this investigation allow for better process parameter knowledge when pertaining to SAC solder wetting on OSP substrates. These results are enough to further investigate OSP surface finishes as a cost effective alternative to other common industry relevant surface finishes. This experiment has identified several findings: 1) Solder paste wetting on OSP substrates is highly dependant on flux chemistry; 2) All four solder pastes in this study have increased wetted area with a peak temperature of 232.7°C versus a peak temperature of 258.4°C. This reduced temperature supports enhanced yield of moisture sensitive components, while also providing energy savings; 3) PCB cleaning (paste misprints) prior to soldering inhibits solder wetting; 4) Solder wetting is greater on OSP surfaces that have experienced multiple pre-solder reflow.

Key words: Cleaning, DOE, OSP, Organic Solderability Preservative, Reflow, SAC, SAC305

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