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April 2002

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Subject:
From:
tony steinke <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 4 Apr 2002 18:00:01 -0800
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Bogert,

The white staining with my experience is caused by moisture absorption
during
wave solder/cleaning. You are probably using a "matte" finish LPI which is
much more likely to occur than using a semi-gloss or glossy finish. The
reason the matte
is more likely to have white residue is that  it is very porous and has
microscopic air pockets in the ink which gives it the matte finish, when it
is passed over
the wave solder these pores open/expand due to the heat and when it is
placed in
cleaning solution or flux remover it is then absorbed into the pores.  As
far as
the tack cure and final cure it would be good to verify, but most board
shops
have dual processing soldermask applicators, which the ink is applied to
both sides at the
 same time and also cured at the same time. My suggestion would be to
evaluate a semi-gloss or glossy
LPI. it is much more resilient.
Tony Steinke
----- Original Message -----
From: <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, April 02, 2002 4:44 PM
Subject: Re: [TN] White residue on LPI Solder Mask coated (and incompletely
cured) PWAs, Is it a technical concern?


Bogert,
   White  staining  on  soldermask is usually caused by moisture absorption

during  post-processing  of  the PWB, for example HASL or Ni/ Au finishing.

You  may  be seeing it one one side only for several reasons, some of which

are  to do with the way the soldermask was applied & subsequently processed

by  your  supplier.  During  soldermask  processing the main steps that can

affect  white  staining  would  be  tack dry, exposure level and cure (post

bake).  As  you only see white staining on one side of the panels you could

ask  your  board  supplier how the panels are processed and see whether the

component  side  had less tack dry or cure time as I would assume both side

will have similar exposure levels.

   However  one process that is side specific is the wave solder process as

poor  rinsing  of the flux can also cause white stains. Also if a hot panel

hits cold rinse water, the thermal shock can contribute to this effect.

   Depending  on  the  severity of the white staining, re-baking the panels

may have some effect - what conditions did you use?

   My  feelings  would  be  that  if  the  white  stain  is due to moisture

absorption  there  will  always  be  a site for future breakdown/ dendritic

growth. For high performance/ MIL spec applications I would get your panels

redone  and  get  your  OEM to check rinsing conditions on the solder line.

For  the  board  supplier, you could ask him to confirm with his soldermask

supplier the optimum processing conditions for the mask that he is using.

   Just my 0.02 Euros worth...

   David Albin

   Coates Circuit Products (UK)



   -----Original Message-----

   From: Bogert [mailto:[log in to unmask]]

   Sent: 30 March 2002 22:51

   To: [log in to unmask]

   Subject:  [TN] White residue on LPI Solder Mask coated (and incompletely

cured) PWAs, Is it a technical concern?



   3/30/002

   Folks,  we  have  an OEM who purchased FR-4 printed wiring boards from a

MIL-P-55110  qualified  board  manufacturer. The OEM wave soldered the PWAs

using HF1189A water washable, organic flux and cleaned the assemblies using

DI  water  in an in-line cleaning process. Subsequent Omegameter 600 ionic

cleanliness  testing  passed. However, almost all of the solder side of the

PWAs  exhibited  white residue. Basically it looks like a water mark. While

you  can remove the residue using abrasion, such as by a pencil erasure, it

comes back, even after several subsequent baking operatioins and additional

in-line  cleaning. The OEM sent out samples for outside chemical evaluation

and found minute traces of carbon. The OEM traced the problem to incomplete

curing of the LPI solder mask. The white residue only appears on the solder

side  of the PWA. There is no evidence of the residue on the component side

of the assembly.

   The OEM proposes that a sample assembly be sent out for SIR testing, and

that  if  the  test passes, they will request us to accept the condition as

is. My concern is that over time in a humid environment, we may experience

leakage  current paths that cound cause operasting failures. The OEM has 20

PWAs   that   are  part  of  the  lot. However,  only  6  experience  this

condition. It  appears that I should reject all PWAs that show evidence of

the  white  residue. I  am  concerned with the presence of carbon, even in

trace  amounts. The  OEM  noted that almost everything will show traces of

carbon, so not to worry.

   Bottom  line  is, is there any technical concern if the white residue is

present? Any input on this topic wo\uld be appreciated.

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