Ken ,
on the dispense to screen dilemma , go with dispenser if you can ;
the control in shift sized syringes is a dream comparing with stencils
and associated consequences .
Just the skip of stencils alone , doubled material efficiency and ease
of maintenance would change your view of process .
It's incomparable .
The syringes are coming frozen, degassed, ready to use and discard .
On clean cycle you should have a pair of parts , one in use, one ready
or being cleaned .
Let me know if you go this way ; however most of the time the capital in
place dictates stencils .
Mind you for price of 100 stencils you can have a decent dispenser ;
like one of Camelot's
Paul Klasek; ResMed ; www.resmed.com
>----------
>From: Ken Patel[SMTP:[log in to unmask]]
>Sent: Tuesday, 10 February 1998 13:06
>To: [log in to unmask]
>Subject: [TN] Again-Most weird problem: Solder between epoxy & fab orepoxy
>&comp
>
>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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>information.
>For the technical support contact Dmitriy Sklyar at [log in to unmask] or
>847-509-9700 ext.311
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