If the epoxy is cured, you can not remove moisture that easily by baking
below 100 C. What is the problem? Why you sure that voids are caused
by moisture? Is the board dry when you apply the epoxy? Is your
assembly room RH controlled? Is the epoxy degassed prior to package
into the tube? Is the mixing of two part done by you? Is the mixing
ratio correct? Well, you do not have reply any of the questions... Just
start looking and solution will come to you soon.
jk
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Peter L.
>Sent: Thursday, September 11, 2003 2:51 PM
>To: [log in to unmask]
>Subject: Re: [TN] Moisture under chip capacitors
>
>
>Hi Technet,
>
>I didn't receive many responses when I asked if anyone has
>come across seeing the moisture-within-the-glue defect
>underneath SMT components.
>
>In order to remove the mositure off the assembled units, I am
>experimenting with various low temp. baking cycles. I like to
>ask if anyone have experience removing moisture that was
>trapped within the glue underneath SMT components (0805s)?
>
>Due to other thermal sensitive T/H components on the boards I
>am limited to 100C baking or less.
>
>Rgds,
>Peter
>
>
>
>Date: Thu, 28 Aug 2003 13:29:00 -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 L." <[log in to unmask]>
>In-Reply-To: <[log in to unmask]>
>Content-Type: text/plain; charset="iso-8859-1"
>
>It is epoxy based material. If you see voids, it got be there
>before curing....
> jk
>
>>-----Original Message-----
>>From: TechNet [mailto:[log in to unmask]] On Behalf Of Peter L.
>>Sent: Thursday, August 28, 2003 12:26 PM
>>To: [log in to unmask]
>>Subject: Re: [TN] Moisture under chip capacitors
>>
>>
>>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,
>
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