Was this a cohesive failure (cracks in the adhesive) or an adhesive failure (separation of the adhesive from the substrate)

For cohesive failures, you may want to look at the curing method to ensure all of the material is fully cured, especially with UV curable adhesives.  You also may want to evaluate the ductility of the cured adhesive.  For example, elastomeric silicones (RTV's) may be a better option than some epoxies which could be more brittle.  Reference IPC-A-610 rev C (new rev), section 4.4.2 and 4.4.3 for acceptance criteria for adhesive bonding.  Depending on whether the components are mounted above the board, 20% to 25% of the circumference needs to be bonded, and either 50% of the component height needs to be bonded, or the bottom and sides (w/ no height requirement).

Just some ideas to look for.

For adhesive failures, the root of the problem is probably cleanliness or unsuitable adhesive material.

Also, make sure this adhesive material is intended for electronic applications.

Hope this helps.

Mike Aldrich
AMETEK Aerospace


______________________________ Reply Separator _________________________________
Subject: [TN] Electrolytic Capacitor Staking Requirements
Author:  MIME:[log in to unmask] at INTERNET
Date:    3/20/2000 5:06 PM


OK, I know, I know. Why is anyone still using thru hole parts which have to be bonded to the board......!!

Well. all I can say is that we are and the question I have for this group is what are the requirements for how much and how high.

Here's the background info:

An automotive electronics circuit card has about seven very tall electrolytic caps on it which need to be secured to the board so that the vibration does not sever the two leads on these parts.  Historically I have always asked that these be bonded 360 degrees around the perimeter and a minimum of 50%, up to a maximum of 100% of the height of the part using a thixotropic UV cure adhesive.  This way the part could not bend or move at all.

Using this criteria, I have never seen a failure as a result of the bonding.

Here's the problem:

A new engineer decides to minimize the adhesive amount (due to cycle time of course) so that just it connects the board and the bottom of the part only (maybe as high as .15" total which only allows about .10" of contact to the body of the part.  These parts are .75" tall.  The new engineer feels justified because the products which were bonded this way made it thru a HALT test exposure without electrical failures.

I look at the assemblies which are supposedly OK and find that the adhesive has broken loose from the pwb on at least five out of the seven parts and what is remaining on the parts have fracture lines as well.

I consider this condition unacceptable and feel that after environmental testing, adhesive bonding should remain intact 100%.

Am I right or should I yield since the assemblies made it thru the HALT testing?  What bonding requirements do you impose for these type of parts

All comments are welcome.


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