Greg-
I'm assuming that:
-the die are attached face up (active side up), and not flip chip attached.
-the PWB traces running beneath the die are covered with a layer of solder
mask, hopefully pinhole free, and that these underlying signals are, by and
large, NOT power or ground connections.
-there is a backside die potential, such as Vcc, which can be confirmed by
the die data sheet, or directly from the manufacturer.
Could you try some of the following, to screen out shorted assemblies, or at
least establish confidence in the isolation between backside potential and
the underlying signal traces:
1. Devise and implement a screening test to look for shorting on the suspect
assemblies. Temperature cycling, perhaps with moisture exposure, could
elicit shorting failures if the mask material is inadequate or has pin
holes. Testing at a high and/or cold temperature could increase the
confidence in your test results (?).
2. If available, visually inspect unpopulated boards (perhaps scrap or
excess stock at your board supplier). Look for solder mask pin holes (qty,
size of defect, distribution of location on PWB) and see if there are tests
for quality or degree of cure of the mask. Defective or incompletely cured
mask could provide inadequate insulation, even if pin hole free, and
especially if layer is deemed "too thin" (whatever that means!).
Also, check for thickness and thickness variation of the mask layer,
especially over the traces of interest.
3. Ask the assembly circuit designer and die manufacturer if die backside
must be isolated from all other signals, power planes, and grounds. Maybe
you luck out, and this particular part doesn't rely on electrical isolation.
Good luck.
Michael Alderete
Brett & Greg wrote...
------------------------------
Date: Fri, 26 Feb 1999 09:10:05 -0000
From: Brett Goldstein <[log in to unmask]>
Subject: Re: Help!! Big problem (solder resist vs die attach)
Greg,
I've seen this type of thing happen, where a silver filled die
attach adhesive (probably through silver migration) shorted to
traces below the die. We mostly make ceramic hybrids, so our
solution was to use a couple of layers of glass encapsulant
(designed for encapsulating fired resistors, and not containing any
pin holes) as a die pad. The glass encapsulant does have to be
fired at a high temperature (800 C or so), so it is only a solution for
ceramic substrates.
Brett Goldstein
EVI, Inc.
> Hi Designers,
>
> I'm in need of help! I have completed a design with some L157
> IC's
> on the Board. And Manufacturing came back to me after I finished the
> design and was about to ship it and said that they use a conductive epoxy
> to adhere the die to the Board and they're worried that if the Solder
> Resist has pin holes in it that it could leak a signal into the runners
> below those Die. They said they could use Pre-Forms but that would kill
> their yeild so they want me to reroute the traces below those die.
> Unfortuatley that would be a nightmare and add at least a week to my time.
> So can anyone tell me where I can get info to help prove that this won't
> be a problem or are they right and there's a good possiblitiy that this
> could happen.
>
> Thank you,
>
> Greg Bodi
>
> [log in to unmask]
>
------------------------------
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