|Ask the Experts|
December 5, 2019
Glued SMT Components Falling Off
We are having issues with glued SMD capacitors falling off after going through the oven curing process. We are using well known adhesive from a quality supplier.
The issue is erratic and we have no problem with SMD resistors. Why does this problem happen only with capacitors and not resistors. The components are very similar in size?
|Expert Panel Responses|
This could be a function of your glue placement....
Cap's 'pop' off the boards generally because of voids in glue. The glue is semi cured, and volatiles trapped mechanically pop. You mentioned this is happening in the curing oven, not the wave... which is another similar soldering issue.
The erratic pattern you've described should be tracked. Tantalum Capacitors, and"similar" package sizes of Ceramic Capacitors can have different composition. When soldering, we found, that testing could not be done until 12 hrs had passed on some packages, due to heat retention altering the value of capacitance. This consideration, might be the difference in your "similar" packages, and could be coupled with glue voids, which are pushing your parts from the assembly.
To minimize this, you may look at alternative method of glue... minimizing the profile to least possible compression of glue dot. Less is more, when it comes to SMD glue. For this reason, we used a stencil printer to improve performance of our SMD / Type II assemblies. The profile allowed us to reduce profiles, create different shapes of glue patterns and eliminate the 'popping'in the wave soldering process. I'm sure this would apply in your glue/curing process as well.
Capital Equipment Operations Manager
Specialty Coating Systems
First, some key assumptions:
If the failure is adhesive in nature and the adhesive remains on the board, look at the following potential root causes:
Suggest you check the thickness of the stencil and the stand-off distance of the Capacitors vs. the resistors. Maybe the Caps have bigger stand-off.
Vice President Technology
It would be nice to know if the glue is still adhering to the resist on the board or if it is only sticking to the component body. If it sticks to the resist then could it be down to a Mold Release contamination of the Component.
If stuck to the component then could be due to surface contamination or more likely under-cured solder resist leaching out and not allowing the adhesive to bind properly. This would be shown worse in heavily grounded areas of the PCB.
Technical Sales Manager
BLT Circuit Services Ltd
I would be suspicious of the capacitors' surface properties, i.e. contamination. Has your vendor changed ANYTHING in his manufacturing process? Has his supplier of raw materials used in the cap manufacturing changed anything?
The surface of the caps has changed which is why they do not adhere to the adhesive, if, in fact, there have been no changes of any kind in the adhesive.
There are a number of reasons why components might fall off, but if this is happening after cure but not with any outside force applied such as wave soldering then the reason is most likely one of the following.
The adhesive is either not biting into the surface of these particular capacitors due to a smoother finish for some reason and this is sometimes seen on glass diodes or the supplier has used a mould release agent in the manufacture of the component and this is causing the adhesive not to adhere properly and this causes the component to fail.
If the components are falling off in a wave solder process then there are a huge number of other reasons that might be the cause.
Global Product Champion
First check the cleaning process. There could be mold release left on the capacitors.
The issue is all of the above, plus a few more factors, including a combination of the material properties (CTE) of the capacitors, the CTE of the PWB, and the CTE of the chipbonder you are using. Cleanliness also plays a role.
Most chipbonder adhesives are very hard materials when cured, with a Shore D hardness in the range of 80 or higher. They have little modulus of elasticity. If there is a major difference between the CTE of the board and the component, in this case the leadless caps, then the stress is placed on the chipbonder.
The typical chipbonder such as Loctite 3609 or 3621 does not adhere well, it will work just fine to hold the parts long enough for reflow to take place, but if the ramp rate is too fast, the chipbonder will snap off due to the major difference in expansion rates between the FR-4 and the ceramic chip caps.
Try slowing down the ramp rate slightly and see if the issue is reduced. In cases like this, it is usually not any one issue, but a perfect storm of the adhesive, the cleanliness of the boards and parts, the reflow ramp rate, and perhaps other factors as well.
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