|Ask the Experts|
March 22, 2018
Components Lifting During Wave Soldering
We are having a problem with a few components rising while going through the wave soldering process. We prefer to avoid lead clinching. What do you suggest?
|Expert Panel Responses|
This is always and interesting question.
Many of us believe that when we see a crescent wrench that was dropped into a molten bath of tin lead alloy float rather than sink, we become confused. This sight can make us think that surly if a wrench will float so won't or components
Actually, it's very rare that this is the cause. When component leads come in contact with the molten alloy they are actually pulled down to the alloy due to capillary attraction. This is the wetting force that pulls all like wettable surfaces in a positive direction.
Meaning, when like alloys meet in a liquidus environment, they "pull" towards each other, not away. You can search the web for "Solder Wetting Balance Test procedure" or search for Meniscograph testing f you need more details and understanding about wetting properties.
Most likely what you are seeing is caused during the infeed process at the entrance end of the conveyor by an operator. It could also be caused by a poor lubricated finger conveyor that is "jerking".
The force of a wave does have the strength to move components right at the entrance to the wave, but rarely does coming in contact with tin lead alloys and leads lift a component.
As a note for all you lead free tin based alloy users, that same crescent wrench that floats in tin lead, well in lead free alloys... It won't float, It will sink. Not a good situation if those solder pumps are on. Keep a close watch out for that!!
Take care I hope this was helpful.
This can be caused by buoyancy, the upward force that a fluid exerts on an object less dense than itself, of the component lifting during contact with the wave or outgassing of trapped flux. Clinching the leads, bends the leads to hold it in place while going over the wave, but this process can induce stress on the component or the PCB depending on how it is done. A quick method to prevent movement is to put a small dot of chip bonder under the component or use a peelable resist, manually applied, just before wave soldering. Both will have enough cure during preheat to hold the component in place and the peelable resist can be easily removed post soldering to leave a better cosmetic appearance.
The forces exerted by the surface tension of the molten solder is greater than the weight of the components and this force pushes the components out of the plated through holes or pushes the components up off the surface on the top side of the board. Since clinching is not an option, I would offer the following to keep the components down onto the board. One, use a fixture with a foam cover that folds over the top of the board and holds the components down onto the board. The foam has to be flexible enough to be able to hold both the highest component and the lowest component to be successful. Two, use a bean bag to place on the board to hold the component down. Third: is to make sure the component leads are long, don't trim the component prior to assembly, because you are looking for the solder to wet those leads and through capillary action pull the components down into the board. A caution must be taken however with the long leads and that is the depth of the solder wave, making sure the leads don't contact the top of the wave generator as this will cause the components to pop up and this could lead to components being up off the top side of the board. Hope this helps.
Vice President, Technical Director
Aside from component clinching we offer the following suggestions:
Components lifting during reflow could be the result of excess solder. Have your stencil manufacturer evaluate the component lead to pad compatibility to determine if this is the cause. Often, especially with leadless devices, there is a large ground under the part. If the pad is printed 1:1, or not properly reduced, the volume of solder under the part will cause lifting during reflow, resulting in what looks like an insufficient toe fillet. This may also be the case with large connectors that have oversized anchor pads.
Integrated Ideas & Technologies, Inc.
If you not utilizing a wave pallet they can be constructed with hold downs that will keep the components in place during the wave process. If you are using wave pallets make sure your supplier knows of the problems that you are having and they can usually be reworked to add the holddowns for troubling components. If you have any further questions please feel free to let me know.
Global PCB Sales Manager
InsulFab PCB Tooling
Is your wave set up properly? Preheats, fluxing, immersion depth, angle, solder bath temperature? Are your lead lengths per the IPC class that you're building to? Too short and they may float in the instant before they wet. Too long and they may be catching and being physically flicked out. Are the fingers on the conveyor clean and undamaged? Is the conveyor running smoothly? Are the parts that lift always the same? Are they solderable? If not, due to finish or lack of flux, they will tend to float. If all else fails, prep these components to hold them in the board.Rob Spoerri, Unlock Recreational Brands
I cannot find what they call beam bags these like little pillows that prevent components from lifting.Bismarck Miranda, Aimtron
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