|Ask the Experts|
June 16, 2020
Selective Soldering Frame Causing Cold Solder Joints
We have continual problems with our wave soldering system, especially cold solder joints. Could the cold solder joints be caused by using a selective soldering frame?
|Expert Panel Responses|
The selective soldering frame could be contributing to cold solder joints in your wave solder process. Frames act as heat sinks pulling heat away from the circuit board. This cooling affect could result in cold solder joints.
I suggest running a temperature profile on the wave solder process on a board with a selective soldering frame on it. Check to be sure that the proper top side temperatures are being reached to enable good soldering. If not, then the preheat, conveyor speed, and solder pot temperature settings may have to be adjusted to correct this.
I also recommend having the solder analyzed to be sure it is within recommend specifications. When elements within the solder go out of specification, this can cause a dull/grainy appearance which may look like cold solder. High levels of elemental contamination, like copper, can cause a shift in melting point of the solder increasing the temperature required for good soldering. It is best to analyze the solder composition on a regular schedule and make adjustments to keep it within specification.
It is possible given that the frame can act as a heat sink and "wick" heat away from the areas of the board near the frame.
Technical Support Engineer
Yes, absolutely. Most wave soldering systems are a little challenged with getting adequate preheat and maintaining it across the gap between the preheaters and the wave. The selective frame will cause areas of the board "shaded" by the frame to remain cooler, and those areas will rapidly wick heat away from the areas to be soldered.
In addition, the frame, no matter how well designed, will affect the flow of the wave, and may result in some areas not seeing as much contact time. The combination of preheat challenges and flow variation can certainly cause issues.
Now, all that said, the first thing I would expect to see if the frame is causing issues is lack of complete hole fill near the edges of the openings in the frame. If you are really seeing classic "cold" solder joints, i.e. coarse, grainy joints, then perhaps there is another cause, specifically the components getting jostled just as the joint is solidifying. What you have then is actually a disturbed joint, and the reliability of such a joint is terrible. If that is the case, look for sources of mechanical energy that may be moving components after the exit from the wave.
"Selective Soldering Frames" can "pull" large amounts of heat from the solder wave at the point of contact, effectively lowering the temperature of the solder at the solder joint. You should confirm the temperature of the solder joints by running a thermal profile, a time verses temperature measurement of the solder process.
Once the solder joint temperatures are measured and understood, you can determine if temperature is one of the causes of the cold solder joints. You may find you will need to reduce the thickness of, or the materials that make up the selective solder frame to reduce the amount of heat it steals from the wave solder and thus the solder joints.
Senior Project Engineer
Electronic Controls Design Inc
In my experience, cold solder joints were manually created, due to insufficient flux, insufficient heat and insufficient time to create the solder joint.
If this is happening in the selective solder system something must be happening to that particular joint is that particular location, so it reflects all the conditions previously stated.
I would check for clearances in the selective solder frame fixture for those particular solder joints and this has to also include the thickness of the fixture. If the fixture is too thick the solder will not be able to make full contact with the solder joint and the results could end up being a bad solder joint.
So, again check for flux coverage of the area to be soldered and make sure the flux penetrates the plated through holes. Check the dwell time of the solder flow to make sure the flow is long enough to create the solder joint, and this also includes the preheating of the product prior to soldering.
Vice President, Technical Director
Hot nitrogen improves heat transfer to the board mass and also inerts the soldering process. Less dross at the end.Luiz Felipe Rodrigues, Air Liquide
The selective soldering frame is one of possible root causes for cold solder joints. Other root causes might be insufficient pot temperature, insufficient flux activity, flux burn off due to excessive pre-heating, solder alloy contaminations and vibrations during the cool down period.
Director New Product Development
Metallic Resources, Inc
Ensure the Solder pot is heated to at least 275C so the Selective pallet is suitably warmed to allow heat transfer to occur and the solder will be more mobile at this Temperature. If the pallet is particularly thick you may need to increase the pot temperature to 280-285C to get sufficient heat transfer through the thick pallet. Top of the PCB should be around 100-110C just prior to the wave IF you can get a measurement on the PCB.
You may also need to increase the flux deposition a little to accommodate the additional heat. If the solder joint is right next to the pallet and you have low heat then the capillary/wicking will also be affected.
It may be be a good idea to leave the pallets on top of the preheat cover prior and during use to increase the heat contained to make this process easier.
Technical Sales Manager
BLT Circuit Services Ltd
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