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Insufficient Wetting to Lands



Insufficient Wetting to Lands | 21 November, 2002

We produce several boards with reflowed components on top and bottom. We are finding the solder has wetted to the leads but did not flow completely to the end of the lands. Several factors/causes?: 1) Board warping but paste deposits look good. 2) Mole profile showed time above reference almost 60 seconds. Discovered problem with convection oven exhaust which is being fixed. 3) Solder stencils made to deposit 6 mil paste thickness (no fine pitch components). Is this standard thickness - smallest components are 0603. 4)Paste is Kester 256 no clean - anyone have issues with this? 5) HASL plated boards. Can thin plating along with excessive heating time, cause the solder to not properly wet to the entire land? 6) Is it standard to run mole profiles after every changeover to different products during the production day? I am looking for any and all information if anyone else had had this issue. Any other things I should be looking at? I am in supplier quality but I am very familiar with the soldering process and IPC standards. Of course - it is always the suppliers fault until proven otherwise so any help from all of you would be greatly appreciated!!

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Insufficient Wetting to Lands | 24 November, 2002

On a HASL board that has poor wetting on the pad, consider looking further downstream in the process than your assembly shop. For instance, take a look at the bare boards. Look for solder mask bleed onto the pads. This will show-up on the outside edges of the pads. Don't expect the bleeding to be the same color as the solder mask. Expect to see incomplete solder coverage on the pads after running the board over the wave solder machine.

Q1: 6 thou stencil acceptable? A1: Sure that's fine

Q2: Issues with Kester 256 no clean? A2: None. Although if you're leaning towards chasing solder paste issues, handling and storage of the paste might be a more fruitful direction to head rather than Kester.

Q3: Can thin plating on HASL boards cause dewetting? A3: Probably not, but other fabrication issues could. Think of thin HASL as a symptom. What do you mean by "thin"?

Q4: Is it standard to run mole profiles after every changeover to different products during the production day? Q4: Sounds reasonable to expect something like that. While not "standard", it IS good practice. Since not all products have the same profile, checking that the oven is at temperature before running product seems like a good thing.

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Insufficient Wetting to Lands | 24 November, 2002

Thanks for your response Dave. I did look at the bare boards after wave solder and they seem to solder fine in that process. I do not see any soldermask bleed on bare boards. Regarding "thin" plating...I have had issues in the past when the board supplier had the air knife turned up too high during HASL which left very little plating. The supplier was trying to keep the flatness but it caused definite dewetting issues when pasted and reflowed. The exhaust on our oven was fixed Friday so I will have to monitor the assemblies to see if there is any difference. At least that will rule in or rule out the excessive heat factor during reflow. Thanks again- Kelly

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Insufficient Wetting to Lands | 25 November, 2002

You're correct excessive temperature can cause dewetting, but that usually means realeeee hot. Excessive time at above liquidous will cause dewetting also.

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