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April 2008

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TechNet E-Mail Forum <[log in to unmask]>, Hfjord <[log in to unmask]>
Date:
Tue, 15 Apr 2008 23:47:42 +0200
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Any flux residues are risky. Where we have rigorous MIL/SPACE requirements,
we perform high pressure liquid cleaning, "no" residues are allowed under
the BGAs or elsewhere. (Getting 100% cleanliness under a superBGA isn't
easy.) Where ionic residues are left, enclosed water droplets can start
parasitic leakage currents or even migration and corrosion. After the
cleaning and bake out, we apply conformal coating. Room conditions are not
quite safe either..rare but occasional, chemical processes can self-generate
microscopic amount of water. If mother nature is in her worst mode, she can
tunnel hydrogen just straight through 1 mm thick Kovar package walls
(hermetic packages)and, depending on what's on inside, create water there.
When we got such reports from NASA (think it was), they were classified and
locked in! Today, no problems, we have water absorbants screenprinted on the
inside of the lids.

Inge

-----Ursprungligt meddelande-----
Från: Steve Kelly [mailto:[log in to unmask]]
Skickat: den 15 april 2008 22:30
Till: 'TechNet E-Mail Forum'; 'Hfjord'
Ämne: RE: [TN] SV: [TN] Possible flux contamination issue

Hi Inge,
No you are correct- the bias voltage is after the damp heat test not during.
The problem is we are failing the test sometimes (high resistance short) -
and when we grind off the BGA connector we see flux residues on the
connector side but not on the flex. So my next question would be if the
connector BGA balls are already assembled with no-clean is there any concern
of assembling the connector onto the flex with no-clean - I cannot think of
any. At the end of the day this product sits in an air-conditioned room.
Thanks. Steve Kelly

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: April-15-08 2:13 PM
To: [log in to unmask]
Subject: [TN] SV: [TN] Possible flux contamination issue

Why this extreme requirement? BGA's use to be signal processing units, which
is normally maximum 48 VDC. If you mean 250 Volts AFTER the environmental
test and at room conditions, I think it's reasonable, but 250 Volts bias
under test condition is horrible, not to say madness.
/Inge

-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Steve Kelly
Skickat: den 15 april 2008 14:42
Till: [log in to unmask]
Ämne: Re: [TN] Possible flux contamination issue

Good Morning,
I need to ask some more questions on this thread.
One of the reasons our customer went to a water soluble paste was the high
voltage/damp heat test requirement on the finished assembly.
The test requirement is 1000 hours of damp heat test at 85C/85RH. The
voltage requirement is 250 volts. BGA pad spacing is .050. The customer was
concerned about passing this test.
Regards Steve Kelly

Steve Kelly
PFC Flexible Circuits Limited
PH: (416) 750-8433
Fax: (416) 750-0016
Cell: (416) 577-8433


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Kelly
Sent: Monday, April 14, 2008 1:58 PM
To: [log in to unmask]
Subject: Re: [TN] Possible flux contamination issue

Thanks everyone for the help. Regards Steve Kelly

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: April-11-08 11:50 AM
To: [log in to unmask]
Subject: Re: [TN] Possible flux contamination issue

24 years ago (yes 24!), I gave a paper at InterNepcon, in Brighton, UK,
entitled "Contamination on Components and Small Assemblies". I no
longer have a copy, but I'm fairly sure I evoked the need to have all
the components supplied clean to avoid just this kind of problem.

Brian

Steve Kelly wrote:
> Good Morning,
>
> We are assembling a BGA connector onto a flex circuit. The BGA has leaded
> balls and the BGA manufacturer uses a no-clean process to attach the balls
> to the connector and does not clean afterwards.
>
> When we assemble the BGA connector to the flex we use water soluble paste
> and then we clean. After damp heat cycling we have seen some shorts. When
we
> take off the connector we see flux residues on the underside of the
> connector but not on the flex. So my question is - what are the concerns
> about mixing no-clean and water soluble? Is the failure likely due to
> inadequate cleaning or is there something else I should be concerned
about.
> I realize this is probably not the way it should be done but some days you
> just do what the customer wants - good or bad. Thanks in advance. Steve
> Kelly
>
>
>
> Steve Kelly
>
> (416) 750-8433 (work)
>
> (416) 750-0016 (fax)
>
> (416) 577-8433 (cell)
>
>
>
>
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