|Ask the Experts|
September 4, 2020
Components Falling Off During Wave Soldering
We are facing a problem with components falling off, or being displaced, during PCB wave soldering for a particular board type. The board type with the problem has a lacquer coating. We glue the components in place prior to soldering.
Could the problem be caused by the lacquer coating on the PCBs?
|Expert Panel Responses|
This sounds like surface mount components which are secured with an adhesive are then sent over the wave solder system. The adhesive needs to be qualified to make sure it is compatible with any coatings or surfaces it is going to come in contact with during the manufacturing cycle.
The wave solder process will heat and soften many coatings and adhesives and the forces of the moving mass of solder wiping on the bottom of the board will overcome any of the adhesive forces securing the component in place and when this happens the components come off the board.
Also, check whether the components which are wiped off the board still have the adhesive stuck to them, if this is a yes answer, then focus on the adhesive and the coating, if no then focus on the component and adhesive compatibility.
Initially, I would suggest reviewing the adhesive and doing experiments regarding the coating on the boards to prove the lacquer coating is not the culprit in this case. Check the lacquer data sheet to see its temperature range for functionality and also check the materials in the adhesive to make sure they are compatible with the lacquer coating. If this proves compatibility then repeat the experiment with the adhesive and the compatibility with the components and the temperature of the wave.
It is a matter of compatibility between the adhesive, the board surface materials, the components and the wave temperature.
Vice President, Technical Director
Let me ask the question are you following the glue manufacturer's ramp time and rate of cure, or are you using the reflow process to cure the glue? If you are doing a standard reflow profile it is too hot too fast and will over cure the glue (chipbonder) and the parts will fall off in wave.
If you are using an appropriate profile and the PCB has a problem with the butter coat (top layer of the PCB) under the soldermask then this can separate. The more feasible answer is that the soldermask is porous and holding fabrication residues (such as HASL) that are leaving a barrier film between the chipbonder and the PCB / soldermask surface. Additional detail is needed to draw additional conclusions.
President/Senior Technical Consultant
I have to presume you are speaking of a buttercoating when you say the board type has a "lacquer" finish. FR-4 bare circuit boards are fabricated without soldermask on certain designs, and the board's final coating is (usually) a very thin hard sealant known as the buttercoat.
Solder mask is typically a matte finish, for maximum adherence of conformal coat and other adhesives such as chip bonder or potting encapsulants after assembly of all components. Bonding of components to soldermask is a standard practice and is usually problem-free with a few rare exceptions.
Attempting to bond components to buttercoat is a completely different story. The adhesion of the epoxy to buttercoat is not so durable to begin with, and with the differences of CTE of the bonded components, the PWB itself, and the CTE of the adhesive being used, quite often either the bond will break off from the buttercoat, or from the component, or some combination of both during reflow, when everything expands at its own rate of thermal expansion (CTE).
The CTE of the adhesive should closely match that of the PWB. You can check the Technical Data Sheet of the bonding adhesive to obtain the CTE of the cured material in the "cured material properties" section.
You can obtain the CTE of the PWB if you know which type of PWB material it is, as listed within the IPC 4101 slash sheet for that particular PWB.
Inspect the parts that have fallen off; are they all the same part number or part type, and is any of the bonding adhesive still stuck on the part? Or is it still stuck on the board? There are always a few exceptions.
Try using a different bonding adhesive (chipbonder) with a CTE that is close to whichever material it is separating from, the PWB or the component. You may have to settle for a chipbonder with a CTE that can accommodate both the PWB AND the components.
There are other process options, but without knowing more about your board design I cannot provide any further advice. I would need to know if the topside components are all SMT or if they are primarily PTH, etc.
But the fact that your problem child has a lacquer finish that is different from all of your other PWBs, and that is the only one you are having this problem with, I would start with that.
Good Luck and let us know how you fix this issue, which I am sure you will.
I'm not at all familiar with lacquer coating, but that should be of little importance. Here's what to look for: when a component falls off, look at what is left on the PWB:
It's possible that the lacquer could interfere with the glue's adhesion to the board surface. I suggest contacting the glue supplier for their input.
Technical Support Engineer
Short answer on this one. Yes. The lacquer coating may not be compatible with some adhesives, I would recommend you consult the manufacturer of the Chipbonder to determine if this is causing the issue.
I am assuming the Chipbonder here is adhering to the components and the PCB, if this is the case there would be no Chipbonder residual on the PCB. If the Chipbonder is still seen on the board, then the failure mode would be adhesion to the components, if this is the case there could be some residual mold release on the on the components that is not compatible with the Chipbonder.
Recommend you send sample PCB to your supplier to test for compatibility, or cure a sample of Chipbonder on the PCB and then try to push off with fingernail, if it is easy to remove after curing there is an adhesion issue on the PCB coating.
If it's a Lacquer coating then it could be the old type rosin Lacquer which would liquefy around 82C causing loss off adhesion, may be time to jump towards an OSP coating. Hope it helps.
Technical Sales Manager
BLT Circuit Services Ltd
The coating can be a problem but since you are using the adhesive to hold the components in place, this should not be an issue.
Try to run a test with adhesive only and understand if it has a good adhesion on the board. Another thing that might contribute to this is the cleanliness level of the board.
Engineering and Operations Management
I would recommend looking into wave pallet option as well if the cost justifies.Sundaram, EIT LLC
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