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Solderability of Vapor vs. Convection Reflow
Materials Tech |
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Authored By:Theron Lewis IBM Corporation, Rochester, MN USA Brian Chapman IBM Corporation, Poughkeepsie, NY USA SummaryTo help address the environmental requirements driven by the European Union RoHS Directive, consumer applications have changed the solder alloys for the manufacturing of printed circuit board assemblies (PCBAs) by removing Pb from solder. Based on the anticipated end to various exemptions and other market forces, high end server applications are now following suit. In addition, as the server/computer industry evolves, the requirements for speed and memory storage continue to increase, causing a need for higher levels of signal integrity along with greater density/mass of components and wiring within PCBA's. This change to more dense/higher thermal mass components on PCBA's and going to a Pb Free solder at higher melting temperature than SnPb Eutectic Solder will aggravate the temperature gradients that occur during reflow, causing major limitations when using standard IR/Convection reflow. Excessive temperature gradients can damage less massive components and less dense laminate areas of the PCBA's. Consequently, other techniques need to be investigated, and the leading alternative is Vapor Phase Reflow. Vapor Phase Reflow is a legacy soldering method that was popular before the 1990's. Vapor Phase Reflow has a processing advantage: its thermal blanket possesses a much greater heat density than convection or IR heating. This reduces the temperature gradients across the board assembly, preventing sensitive components from exceeding maximum temperature limitations. One of the many concerns for implementing Pb Free Vapor Phase Reflow is the effect on solderability. The objective of this publication is to compare the solder wetting between Vapor Phase Reflow and Convection Reflow using a specific Pb Free (SnAgCu) SAC solder paste. This study will compare the amount of area the solder wetted, solder heights, wetting angles, and voiding. ConclusionsIn this study, the goal was to control as many factors that affect solder paste solderability so that the effect of the reflow process itself could be observed. The factors that were controlled were: 1. Solderable Cu Block surface was the same and recently cleaned for each reflow process. 2. The reflow processes had Nitrogen (Inert) atmospheres. 3. The same Pb Free (SAC305) Solder Paste was used. 4. The solder paste deposition procedure was the same for each Cu Block. The amount of material was equivalent within the process capability of the screening process. 5. The actual thermal profile, maximum temperatures, heating rates, cooling rates, and time above liquidous were similar, but not exactly the same for each reflow process. With these controlled factors, it was discovered that Vapor Phase Reflow caused the solder to have greater wetting of the Cu metal surface than Convection Reflow. This conclusion was reached through the examination of the wetting angles of the solder rather than the amount of wetting area observed. The solder bumps from the Vapor Phase samples had a wetting angle average of 28° versus the Convection sample with a wetting angle average of 34°. As the wetting angle approaches 0° or total wetting, the greater the solderability. The Sn haloing around the solder bumps on the Vapor Phase samples showed that melted solder during Vapor Phase Reflow covered a greater area and then was pulled back during solidification. Also the type of reflow did not affect the amount of voiding observed within the solder bumps. The Vapor Phase Reflow did appear to cause a greater amount of tarnishing on the Cu Block and left a greater amount of no clean residues. The demonstration of good wetting in a Vapor Phase reflow process as shown in this study should address one of the concerns related to implementing Vapor Phase reflow for the Pb free soldering of high thermal mass PCBAs. From a wetting perspective, Vapor Phase reflow has been shown to be a viable candidate for Pb free soldering as compared to Convection reflow. The results of this study correlate well with past studies where Vapor Phase Reflow had been investigated as an option for Pb Free Soldering for printed circuit board assembly. This study's contribution was investigating further into specific wetting effects caused by the type of reflow.Most of the past studies showed improvements in the actual application of Pb Free soldering between SMT components to PCB's (Samat, 2009; Sequeira, 2007). These past studies provided a broader comparison between Vapor Phase Reflow and Convection Reflow with respect Pb Free Soldering, while the current study analyzed the wetting phenomenon in depth. Initially Published in the IPC Proceedings |
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