Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Paul Horan

#14783

reliability of epoxy | 5 August, 1998

I am looking for advice or information from anyone about the possible reliability problems associated with epoxy on solder pads of discrete devices. Will the epoxy expand and lift over time, Will there be adverse reaction between the solder and the epoxy over time.

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P.L. Sorenson - Technical Consultant

#14786

Re: reliability of epoxy | 6 August, 1998

| I am looking for advice or information from anyone about the possible reliability problems associated with epoxy on solder pads of discrete devices. Will the epoxy expand and lift over time, Will there be adverse reaction between the solder and the epoxy over time. Dear Mr. Horan, You could run into these types of problems. It is possible, with proper materials and procedures to encapsulate printed wiring boards in epoxy such that they meet the most severe environmental test conditions and operate reliably in the field. Much more information would be necessary to provide more than this general comment.

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Mike

#14787

Re: reliability of epoxy | 6 August, 1998

| | I am looking for advice or information from anyone about the possible reliability problems associated with epoxy on solder pads of discrete devices. Will the epoxy expand and lift over time, Will there be adverse reaction between the solder and the epoxy over time. | Dear Mr. Horan, | You could run into these types of problems. | It is possible, with proper materials and procedures | to encapsulate printed wiring boards in epoxy | such that they meet the most severe environmental | test conditions and operate reliably in the field. | Much more information would be necessary to provide | more than this general comment. Hay PL I think you misunderstood the question. I have had this problem a couple of times. If the Adhesive on the pads did not violate IPC specs we would typically leave the adhesive and correct the process issue. I have not heard of any reliablity problems associated with adhesive intruding into the solder joint. Good luck Mike

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justin medernach

#14784

Re: reliability of epoxy | 6 August, 1998

| I am looking for advice or information from anyone about the possible reliability problems associated with epoxy on solder pads of discrete devices. Will the epoxy expand and lift over time, Will there be adverse reaction between the solder and the epoxy over time. Paul, You most certainly can have huge reliability problems. Are you running a water soluble operation? Have you qualified an epoxy? Do you plan on running a qualification? Are you aware of the hydroscopic nature of epoxies? Let me just give you a worst case scenario and we can go from there. Running a water soluble process, single sided surface mount, secondary side population done with adhesive dispense, cure, then wave. Surface mount adhesives tend to draw a ton of moisture from the ambient in a short time. The more moisture in the adhesive, the better the curing profile HAS to be. This is due to the outgassing of the moisture. If the moisture doesn't outgas properly, there will be voids and blow holes in the adhesive deposition. Now you go to the wave soldering process and you pass the assembly over a fluxer of some sort and an extremely active water soluble flux gets trapped in some of the voids and blow holes. That entrapped flux WILL NOT BE REMOVED IN THE WASH AND IT'S VERY LIKELY THAT IONIC CONTAMINATION TESTING WON'T EVEN PICK IT UP!!! Now that assembly with the trapped flux makes it into the field and fails out there because the OA ate through the chips. Talk to Federal Express about it. They felt the pain of this problem a couple of years ago right before Christmas. You need to run a qualification on your adhesive. I don't want to endorse or crap on any suppliers on the web so e-mail me and we'll chat about qualification tests and who's hot / who's not. Regards, Justin Medernach Flextronics International

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zeek - just wanna to get a word in here

#14785

Re: reliability of epoxy | 7 August, 1998

| | I am looking for advice or information from anyone about the possible reliability problems associated with epoxy on solder pads of discrete devices. Will the epoxy expand and lift over time, Will there be adverse reaction between the solder and the epoxy over time. | Paul, | You most certainly can have huge reliability problems. Are you running a water soluble operation? Have you qualified an epoxy? Do you plan on running a qualification? Are you aware of the hydroscopic nature of epoxies? Let me just give you a worst case scenario and we can go from there. Running a water soluble process, single sided surface mount, secondary side population done with adhesive dispense, cure, then wave. Surface mount adhesives tend to draw a ton of moisture from the ambient in a short time. The more moisture in the adhesive, the better the curing profile HAS to be. This is due to the outgassing of the moisture. If the moisture doesn't outgas properly, there will be voids and blow holes in the adhesive deposition. Now you go to the wave soldering process and you pass the assembly over a fluxer of some sort and an extremely active water soluble flux gets trapped in some of the voids and blow holes. That entrapped flux WILL NOT BE REMOVED IN THE WASH AND IT'S VERY LIKELY THAT IONIC | CONTAMINATION TESTING WON'T EVEN PICK IT UP!!! Now that assembly with the trapped flux makes it into the field and fails out there because the OA ate through the chips. Talk to Federal Express about it. They felt the pain of this problem a couple of years ago right before Christmas. You need to run a qualification on your adhesive. I don't want to endorse or crap on any suppliers on the web so e-mail me and we'll chat about qualification tests and who's hot / who's not. | Regards, | Justin Medernach | Flextronics International Paul and others interested, Justin is right,..... almost. We have had a very serious issue with this over the last 8 months and we feel we know this problem all to well. I am not sure from your post that the issue you descibe is what Justin is talking about. To elaborate, the root cause is partially due to the hydroscopic nature of the adhesive but there may be a better root cause. Think about this, the PCB is kinda like a sponge and where would the moisture come from and what would it do? Take a look under some components at the formation of the cured adhesive. Keeping the PCB "sponge" in consideration, where did those voids come from? Use glass plates on top of the adhesive dots and look under a microscope after they are cured. We saw it between the pads and the solder mask. From here I will give you one last clue. Change the cure pofile. O.K., did not want to give it away... but change the slope to 1.5 degrees C/second. Hope your customer is not as concerned as ours, DWZ

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P.L. Sorenson - Technical Consultant

#14790

Re: reliability of epoxy | 7 August, 1998

| | | I am looking for advice or information from anyone about the possible reliability problems associated with epoxy on solder pads of discrete devices. Will the epoxy expand and lift over time, Will there be adverse reaction between the solder and the epoxy over time. | | Dear Mr. Horan, | | You could run into these types of problems. | | It is possible, with proper materials and procedures | | to encapsulate printed wiring boards in epoxy | | such that they meet the most severe environmental | | test conditions and operate reliably in the field. | | Much more information would be necessary to provide | | more than this general comment. | Hay PL | I think you misunderstood the question. | I have had this problem a couple of times. If the Adhesive on the pads did not violate IPC specs we would typically leave the adhesive and correct the process issue. I have not heard of any reliablity problems associated with adhesive intruding into the solder joint. | Good luck | Mike You might be right Mike. It really isn't clear if the problem is stray adhesive or intentional application of epoxy as a CC. Epoxy is widely used as an approved touchup for rework of parylene CC, for example. Pete Sorenson

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Paul Horan

#14789

Re: reliability of epoxy | 24 August, 1998

| | | I am looking for advice or information from anyone about the possible reliability problems associated with epoxy on solder pads of discrete devices. Will the epoxy expand and lift over time, Will there be adverse reaction between the solder and the epoxy over time. | | Dear Mr. Horan, | | You could run into these types of problems. | | It is possible, with proper materials and procedures | | to encapsulate printed wiring boards in epoxy | | such that they meet the most severe environmental | | test conditions and operate reliably in the field. | | Much more information would be necessary to provide | | more than this general comment. | Hay PL | I think you misunderstood the question. | I have had this problem a couple of times. If the Adhesive on the pads did not violate IPC specs we would typically leave the adhesive and correct the process issue. I have not heard of any reliablity problems associated with adhesive intruding into the solder joint. | Good luck | Mike

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Paul Horan

#14788

Re: reliability of epoxy | 24 August, 1998

| | | I am looking for advice or information from anyone about the possible reliability problems associated with epoxy on solder pads of discrete devices. Will the epoxy expand and lift over time, Will there be adverse reaction between the solder and the epoxy over time. | | Dear Mr. Horan, | | You could run into these types of problems. | | It is possible, with proper materials and procedures | | to encapsulate printed wiring boards in epoxy | | such that they meet the most severe environmental | | test conditions and operate reliably in the field. | | Much more information would be necessary to provide | | more than this general comment. | Hay PL | I think you misunderstood the question. | I have had this problem a couple of times. If the Adhesive on the pads did not violate IPC specs we would typically leave the adhesive and correct the process issue. I have not heard of any reliablity problems associated with adhesive intruding into the solder joint. | Good luck | Mike Mike, Yes my question was regarding stray epoxy on pads. Where the epoxy deposits were minimal we touched up. Can residues be left behind? Will microscopic quantities of epoxy weaken joints Thanks Paul

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