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TECHNET  April 2007

TECHNET April 2007

Subject:

Re: Conformal Coating Voids

From:

BADRI <[log in to unmask]>

Reply-To:

TechNet E-Mail Forum <[log in to unmask]>, BADRI <[log in to unmask]>

Date:

Tue, 24 Apr 2007 18:45:21 +0530

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (218 lines)

I agree with Ramon. It is practically true.We solved the air bubble issue by
placing the PCBA's in a closed box for an hour. This eliminated bubbles.
Since our qty of assembly is low we use a syringe to dispense the coating
material. We also ensure that the syringe is full and has no entrapment.

Thanx
Badri

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Tuesday, April 24, 2007 6:23 PM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coating Voids

	Many of times bubbles are caused by the solvents warming up too
fast and turning into gas while still in the Coating. It could be too
fast air movement going over the coating while being coated or placing
the boards in the oven right away. In many instances the bubble are all
the way to the top of the surface of the coating. It is necessary to let
the board skin in a box for about 30 min prior to place them in an oven.
One way to fix the problem is to brush or spray the conformal coating
with one of the solvents in the coating and melting the bubbles out. Let
it skin and place the board in the oven. I would not fight the customer.
Experiment on one board with the advice from the coating manufacturer.
Show the way the board is and after the effort to the customer. Win the
battle and war and keep the customer happy. Good luck.
	Ramon	


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Tuesday, April 24, 2007 8:27 AM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coating Voids

Brian,

The day you agree blindly on what I say, I will get worried, Brian.
Theoretically, you are so right, sir. But I dare use the conclusions of
daily work and practical experience. We accept single bubbles, air
entrapments or outgassing entrapments, according to what I said, but if
the bubbles increase in size, and become real atmospheric or gas bridges
between hot lines or between package leads, then we pull the emergency
brake and call people to MBR or similar. Our failure report system, as
well as incoming inspection and other tools, have not indicted one
single failure caused by \single\ bubbles in the coatings the last 20
years. At the same time, I take the occasion to mention about
globtopping or passivation or coating semi chips. Here, bubbles can be
catastrophic. But that belongs more or less to IMAP members and like, I
think. Naked semi chips on PWBs still not a hit, which I regret. COB
would make daily life little more interesting....
Thanks for your correct adds.

By the way, can you still find useful Cu on your isle


Inge 

-----Original Message-----
From: Brian Ellis [mailto:[log in to unmask]]
Sent: den 24 april 2007 13:06
To: TechNet E-Mail Forum; Hernefjord Ingemar
Subject: Re: [TN] Conformal Coating Voids

I would disagree with your blind acceptation of isolated bubbles, Inge. 
I would insist on knowing the cause of the bubbles before accepting
them, and their location. It would not be oxygen that would worry me;
have you thought about water vapour that could condense? Even if there
is only one now, will there be more in a few months?

Now, let's look at modern high-speed/frequency electronics. An airfilled
microbubble between a transmission line pair will have little effect
because substituting a dielectric constant of 1 for, say, 3 over such a
tiny distance will hardly cause a visible blip on your TD reflectometer.

But are you sure that a dielectric constant of 72, instead of 3, would
have so little effect? Not to mention the capacitive effect from
neighbouring tracks causing crosstalk?

And one little thought for you to ponder: all vesication probably starts
with a single microbubble.

Sorry to upset your complacency.

Brian

Hernefjord Ingemar wrote:
> If you get bubbles that 'look like foam', there is something wrong in 
> the process. We use Humiseal acrylic coatings, and have tried Urethane

> before. Single microscopic bubbles are acceptable, which means in the 
> range 1 mil or less. We use to say, that the bubbles must not exceed 
> one third of the total thickness of the coating.
> 
> A bubble is entrapped atmosphere, and the oxygen content can 
> theoretically cause oxidation. The oxygen content in such little a 
> bubble is not enough to cause any oxidation. When you use the word 
> corrosion, one use to think of chemical reactions serious enough to 
> damage the whole solder joint, or make the lead fall off the package.
> I would like to see the microscopic bubble that could cause such
damage!
> Would be no less than a miracle! Single air bubbles is nothing to be 
> worried about, foam of bubbles is something you need overlook.
> 
> Inge
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Joel Alexander
> Sent: den 23 april 2007 21:47
> To: [log in to unmask]
> Subject: [TN] Conformal Coating Voids
> 
> We are using Urethane coating on an assembly that uses  63/37 solder. 
> We are using organic water soluble flux. We are Ionic testing after 
> wash and we are well within IPC cleanliness specifications. We 
> installed a selective coating machine and in our initial run of 
> product we had some small bubbles that look like foam. Our customer 
> does not like it but our stand is that it meets IPC-A-610 and
> J-STD-001 requirements since there in no obvious voids between 
> conductors. They were worried about corrosion if a bubble was against 
> a lead. The bubbles are fully encapsulated. Does anyone think this is
a valid concern?
> 
> 
> 
> Joel Alexander
> Quality Assurance Manager
> TT APSCO, INC. 
> 
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