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March 2012

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Subject:
From:
Dwight Mattix <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Dwight Mattix <[log in to unmask]>
Date:
Wed, 14 Mar 2012 06:35:26 -0700
Content-Type:
text/plain
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text/plain (133 lines)
we build boards with thermal via farms (usually under PA's,sometimes 
power devices) everyday. Rather than deal with the solderign issues 
we have them filled with non-conductive epoxy and planarized. 
Typically a Peters or Taiyo material and San-Ei if it needs to be 
overplated (bad idea but sometimes specified).

  At 05:47 PM 3/13/2012, Wenger, George M. wrote:
>Richard,
>
>As usual all good points.  When we design via farms under components 
>that require a heat transfer we design for the heat transfer to 
>occur through the plated area of the via barrel and we do not rely 
>on the vias being filled during a soldering process (wave or solder 
>paste on heat spreader pads beneath the components.  We also go to 
>great lengths to design the via diameter and stencil print pattern 
>so that solder doesn't flow into the vias because we don't want 
>solder bumps on the bottom of the board to prevent good contact to 
>the heat sink the board is mounted to.  We also don't try to fill 
>the vias with epoxy or solder mask or any non-conductive material 
>because we would probably get some on the top or bottom side land of 
>the via which would prevent good heat transfer into the plated copper barrel.
>
>Regards,
>George
>George M. Wenger
>Senior Principal Reliability / FMA Engineer
>Andrew Corporation - Wireless Network Solutions
>40 Technology Drive, Warren, NJ 07059
>(908) 546-4531 Office (732) 309-8964 Mobile
>E-mail: [log in to unmask]
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
>Sent: Tuesday, March 13, 2012 8:36 PM
>To: [log in to unmask]
>Subject: Re: [TN] Purpose of these plated vias?
>
>Hold on there, Dwight!
>Does the assembly drawing call out to epoxy fill those holes? Have 
>you considered the implications of what could go wrong if someone 
>were to follow your advice?
>1. Loss of thermal conductivity/change in thermal performance. It 
>could be that the unfilled holes are required to transfer heat to 
>the thermistor on the opposite side of the CCA, so the thermistor 
>can sense the heat and the circuit can react accordingly. What 
>happens if you fill those holes? You insulate the back (sensor 
>portion) of the thermistor.
>2. A change in dielectric characheristics could occur. We have no 
>idea how this circuit is used.
>3. If the wrong epoxy is used, it may get hot and even possibly 
>ignite.  Total catastrophic failure (automotive fire) could follow.
>
>Be careful what you say, the first rule is Do No Harm.
>
>Charlie,
>Discuss this with the customer's design engineer or contract 
>manager. They may wish to know about issues such as flux entrapment 
>driving up their costs, or convey certain cleanliness or hole-fill 
>requirements. Have you checked the assembly drawing for the CCA?
>
>
>________________________________________
>From: TechNet [[log in to unmask]] On Behalf Of Dwight Mattix 
>[[log in to unmask]]
>Sent: Tuesday, March 13, 2012 4:56 PM
>To: [log in to unmask]
>Subject: Re: [TN] Purpose of these plated vias?
>
>thermal via farm
>
>100% fill those vias with non-conductive epoxy will fix what ails you
>in this case
>
>
>At 12:42 PM 3/13/2012, Charlie Pitarys wrote:
> >I have these PCB's (mixed technology) in house, used for an automotive under
> >hood application. In several spots on the board are these array of holes. On
> >the A-side a smt component like a thyristor will cover up the array.
> >What are these there for?
> >The wave flux residue is proving tenacious to effectively clean out of every
> >via. Especially with the old school cleaning tool at this account.
> >
> >Some of the vias are clear but some have solder filling or partially filling
> >them. These are now also filled with wave flux.
> >Below is a link to a picture of the array.
> >http://i123.photobucket.com/albums/o319/dartfrogmts/Kyzen/A-Side00140xFCR145
> >F20min-1.jpg
> >
> >I'll get them clean but curious as to why these are on the board.
> >Thanks in advance
> >
> >Charlie
> >
> >
> >
> >
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