The adhesive was printed and cured at no more than 2deg/s and 150degC for
100secs.

The particular adhesive used was Epibond 7275. We suspected that it was a
material related issue - either the glue was mishandled and left too long
on the line, or old/ expired batch of glue was used.

Any suggestion on method to drive the moisture away under the components?
Baking conditions?


Rgds,
Peter




>Date:         Mon, 25 Aug 2003 07:47:30 -0400
>Reply-To:     [log in to unmask]
>Sender:       TechNet <[log in to unmask]>
>From:         joyce <[log in to unmask]>
>Organization: EXFO-TO
>Subject:      Re: Moisture under chip capacitors
>X-To:         "TechNet E-Mail Forum." <[log in to unmask]>,
              Peter Lee <[log in to unmask]>
>In-Reply-To:  <000001c36abe$e03472d0$7e63ad8e@homeg8br34o791>
>Content-Type: text/plain; charset="iso-8859-1"

>What is the adhesive and how was it cured?
                                            jk

>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Peter Lee
>Sent: Monday, August 25, 2003 12:11 AM
>To: [log in to unmask]
>Subject: [TN] Moisture under chip capacitors
>
>
>Technet,
>
>I have come across a rash of failed assemblies that have 0805
>capacitors and resistors, bottom side glued, wave soldered and
>washed. Trouble shooter reported touching up the solder joints
>on a few areas and the boards would pass test.
>
>I had a look at one board which was reported to have a 0805
>56K resistor with "drifting" values. I pulled the component
>off mechanically in order not to apply any heat and noticed
>several mini-voids within the adhesive body. I have seen
>solder bridge within the glue before but this was the first
>time I saw wet residue within the voids.
>
>You can view a sample image from Steve's web site:
>http://www.stevezeva.homestead.com/files/SMTAdhesive.jpg
>
>The
>image showed the bottom side of the cracked resistor after
>removal (white), with a portion of solder mask (green) and SMT
>adhesive (red).
>
>I used a 8mil stencil. The thermal profile was verified and
>conformed to the slope/ curing specs.
>
>Has anyone seen this before? Any explanation as to how the
>void was formed and how moisture got tapped in? Does anyone
>re-use their glue on the line or throw away any left-over to
>avoid moisture in the glue?
>
>
>
>Rgds,
>Peter
>
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