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Wave Soldering Defects - Outgassing Outgassing is still a common problem associated with wave and hand soldering. Basically when a board is soldered any moisture in the board close to the hole is heated and turned to vapor. If there is thin plating or voids in the plating, gassing can come through the plated hole wall. If solder is present in the hole, this will produce voids in the solder as it solidifies. The voids may appear as small holes in the surface of the joint, as shown in Figure 1, or much larger cavities. Having the correct copper plating thickness in the through holes is the key. A minimum of 25µm of copper should be present on the surface of the hole walls.
We recently received a defect question related to outgassing: I'm getting solder balls on the top side of the board after wave solder (same thing also happens at rework on solder fountain). I'm using a SAC405 alloy. Have tried different fluxes, different preheats. Have run the boards in 2 different factories with same results. Electrovert wave solder being used. Getting some bottom-side solder balls as well, but that's manageable. Topside solder balls can be seen by prying off the topside connectors.
Can be either a single stray solder ball, or a small cluster of balls as shown in the photo (Figure 2). Get the same results when trying to rework a connector as well. Problem is not isolated to any one particular connector. Solder fountain used for rework is Airvac PCBRM14. Get the same results a rework. Have tried different flux applications, and using more/less flux without change in the process. Any ideas? Bob Willis answers: Based on the image (Figure 2) and your description of the trials you have run, the problem can only be outgassing from the through hole. This can be related to the flux outgassing as the board goes over the solder wave. From what you've said of your trials, this would be less likely to occur if there were no volatile material left in the through hole. Having experienced a similar problem working with the introduction of VOC material for a customer, I've found that this is possible. There is a lot of volatile material from these water based materials. If it is outgassing from the boards, this may be possible as you said you see this issue on the other wave soldering systems, fluxes and the rework stations that would not normally have pre-heat. Have you tested the boards with the outgassing test? Try this test with the procedure found here. If you look at the my video clips they show us testing both good and bad boards using the test method I have provided. Its simple, cheap and not destructive to test boards. www.bobwillis.co.uk/videoclips/outgassing01.wmv Most common reasons or solutions:
Wave Soldering Defects:
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Text and photos courtesy of Bob Willis