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December 2003

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Subject:
From:
Kathy Kuhlow <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 30 Dec 2003 13:51:28 -0600
Content-Type:
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Oh well, but the question was would you expect a class 3 in the assembly.  I wouldn't expect anything I had not made known in some method of communication.  As someone living in the contract world I wouldn't take anything for granted.  If I went back to the OEM world I would ask for a class 2 PCB and a class 3 assembly based on some of the exact class requirement differences you mention.  I have a hard time accepting the class 2 minimum solder joint especially through hole with blow holes and voids as only being process indicators.  I know of a few OEM's that have been burned by the minimum class 2 solder joint for industrial products.  To remedy this they put a little more money into the product and ask for a class 3 assembly but leave the PCB at a class 2.  Class 2 is what they really want but to make sure they don't hit that bottom line of minimal they ask for a class 3.  I think there are more class 3 products being built only because of the class specifications not the end use of the product.  

Kat

>>> [log in to unmask] 12/30/03 01:13PM >>>
Very rarely do I disagree with Kathy. But, this time I think I do,
perhaps, a little.

Yes, you can get Class 3 solder results from boards that were fabricated
under contract to Class 2 requirements.
But, at least one variation allowed by IPC-A-600 and IPC-6012A for class
2 can result in material conditions that may not yield Class 3 results
at the assembly level.

Plating voids in PTH are not allowed for Class 3 but a limited number
are allowed in Class 2 products. Plating voids can produce blow holes in
solder joints. These are process indicators for Class 2&3. We have,
however, observed blow holes that exceeded the 30 degrees
circumferential fillet variation allowed for Class 3. The product would
have passed Class 2 acceptance criteria, with its 90 degree requirement.

In all fairness to Kathy. We get see this weird stuff because we are an
independent lab. In fact, there is very little difference between
IPC-A-600 Class 2 and 3 requirements. Generally the differences would
not be observed at the solder connection.

Your final question is the real question. Why Class 3.
1. Mission Critical Applications. Class 3 allows less variation in
process output. Less variation means better control. Better control
means less Murphy.
2. Harsh environments. The thicker solder fillets may withstand the
rigors of harsh environments. (Assembly)
3. When your product lives in a glass house and you cannot tolerate
cosmetic variation. (Assembly)

BTW if you want Class 3 better start with IPC-2221, D-279 and SM-782
(Design).


> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Dobbs, Marie
> Sent: Tuesday, December 30, 2003 9:51 AM
> To: [log in to unmask]
> Subject: [TN] IPC Board Quality
>
>
> I am looking for your opinions and experience.
> IPC has the 3 levels of classification for the Acceptability
> pf Electronic Assemblies, and IPC also offers 3 levels of
> classification for the fabrication of Printed Circuit boards.
>
> My question is this- Can I purchase a Class 2 Fabricated
> Printed Circuit Card, and Expect to get Class 3 Workmanship
> Solder Joints from Manufacturing Wave Soldering and Employee
> Hand Soldering?
>
>
> One additional question, Other than the Military and NASA,
> what kinds of products do you feel require the guidelines of
> a Class 3 Board and or Assembly.
>
>
> Any input is greatly appreciated.
>
> Marie A. Dobbs
> Training and Development Specialist
> 100 Technology Drive
> Westminster, MA 01473
> Phone:  1-978-731-6827
> Fax:  1-978-731-6950

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