It was great to get few excellent responses to my e-mail on solder getting
between caps and epoxy and I thank those who responded for all their help
but I still have the following questions.
(1) What is the best practice in the industry - epoxy dispense through glue
dispenser machine or screening epoxy through stencil ?
(2) How to control the left over epoxy after use for stencil dispensed
epoxy? How it is being control in the industry? stored in the jar &
refrigerated/use small tube/etc...
(3) How can I confirm that entrapped flux is really a flux residue which is
difficult see using microscope, is there any test any lab can perform
keeping in mind that board was handled using bare hands? This test should
be done on localized area.
(4) Any other useful information I should be aware off.
re,
ken patel
At 08:00 AM 2/6/98 -0500, [log in to unmask] wrote:
>
>
>
>
>I too have seen this problem in the past, the voids are caused by
>outgassing of the material during cure or moisture within the adhesive or
>in the PCB. If you cure the glue with a fast high temperature profile, the
>problem is more likely to occur. For example, the adhesive can be cured
>for 1.5 minutes at 150?C or 2.5 minutes at 125?C. The lower temperature
>cure is better. Obviously you need to work with the thru-put of your
>process, but slowing down the cure time worked for us and did not affect
>thru-put.
>
>The voids in the adhesive create a path for the flux and solder (via
>capillary action), and i have seen shorts under components from one
>termination to the other of pure solder, or worse yet, solder under the
>component that did not create a short, but was close enough to the
>termination that eventually a dendritic growth occured and completed the
>short. HOWEVER, the main cause of the problem was that the adhesive dot
>under the component was TOO BIG!! After the component was placed, the
>adhesive dot covered the entire area under the component and onto the pads.
>When the component was wave soldered, the pad became liquid, and due to
>capillary action, solder was sucked into the adhesive voids, thus creating
>the short. If you make your adhesive dot such that it does not touch the
>pads or the end terminations then you have eliminated the path for the
>solder, and thus no more shorts. With this design, even if you have voids
>in the adhesive and solder is caught in the voids, it does not stretch from
>pad to pad. Obviously it is best to eliminate the voids altogether.
>
>Consider adhesive dot design:
>With a single dot, make sure that it does not touch the pads or
>terminations.
>With a dual dot design: make sure that the dots do not touch the pads or
>terminations. We also had another problem with dual dots, the gap between
>the dots also created a pathway for flux or solder. You will find in many
>books a suggestion to put a dummy trace between the pads. This acts as a
>dam.
>
>The adhesive could be a contributing factor. Hereaus had similiar problems
>(as was mentioned with Loctite), but they have new formulations(PD944) to
>help reduce the moisture and outgassing issues.
>
>Ed Holton
>Hella Electronics
>
>
>
>
>
>[log in to unmask] on 02/05/98 09:02:04 PM
>
>Please respond to [log in to unmask]; Please respond to [log in to unmask]
>
>To: [log in to unmask]
>cc: (bcc: Ed Holton/Hella North America Inc.)
>Subject: [TN] Most weird problem: Solder between epoxy & fab or epoxy &
> comp
>
>
>
>
>Dear Technetters,
>
>I have seen the most weird problem in my 7 years in PCBA manufacturing
>environment. Solder is getting between the epoxy and component and/or
>between fab and epoxy thus shorting it out across terminations of SMT caps.
>
>Does any one has seen similar problem? Any resolution? Our regular process
>is epoxy application thr' stencil, component loading, curing and then hand
>load & wave.
>
>Any help or leads will be highly appreciated.
>
>re,
>ken patel
>______________________________________________________
>Ken Patel Phone: (408) 490-6804
>1708 McCarthy Blvd. Fax: (408) 490-6859
>Milpitas, CA 95035 Beeper: (888) 769-1808
>
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______________________________________________________
Ken Patel Phone: (408) 490-6804
1708 McCarthy Blvd. Fax: (408) 490-6859
Milpitas, CA 95035 Beeper: (888) 769-1808
################################################################
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To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
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