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Epoxy damaging SMT Components

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#61097

Epoxy damaging SMT Components | 10 February, 2010

We experiences some failures on PCB assemblies from the field when a methacrylate epoxy adhesive was used to secure some through-hole components to pcb board as reinforcement against vibration. During application, the epoxy covered some smaller SMT components. Failure analysis was difficult (because the components were encased in the epoxy) but it appeared in some cases that solder joints were cracked as were the actual components (0603 resistor, sot23 diodes). We are attribuiting this to improper use of this epoxy (Plexus MA310) in this application. Does this kind of damage seem likely from this adhesive? I have reearched other adhesives commonly used for bonding/staking of electronic components have similiar durometer. (EP1320, EP11HTFS)

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#61098

Epoxy damaging SMT Components | 10 February, 2010

"Because of the rapid curing features of this product, large amounts of heat are generated when large masses of material are mixed at one time."[ITW Plexus MA310 TDS]

Questions are: * Have you looked at the exotherm curve for this stuff? * Can the components that you're burying in this epoxy bear the heat with and margin of error?

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#61099

Epoxy damaging SMT Components | 10 February, 2010

Yes, I looked at the curve but it was high 250F max, but not higher than reflow oven temps..

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#61103

Epoxy damaging SMT Components | 11 February, 2010

Have you checked to make sure the mix ratio is correct? My experience with epoxies is that if the mix ratio is off with too much hardener you will get a much higher exotherm than expected. Modulus is high too, pretty stiff material.

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#61104

Epoxy damaging SMT Components | 11 February, 2010

One other thing, if you're using this for vibration dampening, would a silicone work? Much lower modulus.

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#61106

Epoxy damaging SMT Components | 11 February, 2010

We have temporarily switched to an RTV for the vibration reincforcement, but it is not compatible with acrylic conformal coating and we really do not want to switch conformal coatings to a silicone based.

Additionally we are testing the product in house after applying the epoxy and cure and did not see this fallout before shipping.

Products are used in extreme temperature variation environments so we also discussed CTE but dont really have a way to point at it for sure.

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