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August 2007

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Tue, 7 Aug 2007 12:59:19 -0500
Content-Type:
text/plain
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text/plain (249 lines)
I thought so. How could something like that be applied, as Bev correctly
pointed out. There are too many variables that would determine the
number of "safe" reworks.
I prefer the term "Tg excursions" when a rework limit is set on
agreement with customer and supplier. And a careful qualification is
needed, as Bev pointed out, and it is usually a worthwhile effort to do
so. Solder touchup and hand-soldering are usually excluded from this
number, as this type of rework seldom leads to reliability issues. But
again, there are exceptions.
In addition to having "seen thin boards delaminated after a single, poor
attempt at rework" I have also seen any given size and thickness and
material type delaminated in spite of very careful qualification of the
rework process. Also, I have seen Z-axis expansion as a result of too
large of a delta T between the localized rework site and the rest of the
assembly cause damage to vias and PTH barrels that went totally
undetected by a careful inspection and X-ray and electrical test,
including stress screening, only to fail later.
This is why the hard limitation of the number of reworks was deleted
when the standards changed from WS6536E, to J-STD-2000, to today's
J-STD-001D and IPC-A-610D. There is no number of Tg excursions that is
"safe" for every application.

  

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Tuesday, August 07, 2007 12:09 PM
To: [log in to unmask]
Subject: Re: [TN] Rework question

Same here.
Bev
RIM

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke
Sent: Tuesday, August 07, 2007 12:55 PM
To: [log in to unmask]
Subject: Re: [TN] Rework question

Just looked in 7711 - didn't see a reference to it...

John

 
 
John Burke
 
(408) 515 4992

-----Original Message-----
From: Guy Ramsey [mailto:[log in to unmask]]
Sent: Tuesday, August 07, 2007 9:14 AM
To: 'TechNet E-Mail Forum'; 'John Burke'
Subject: RE: [TN] Rework question

I don't think it is worded as a hard limit. Might be IPC-7711.  

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke
Sent: Tuesday, August 07, 2007 12:04 PM
To: [log in to unmask]
Subject: Re: [TN] Rework question

The question actually came up because there is stated (somewhere?) in an
IPC document the number of reflow cycles you are allowed to put the
board through.

Taking that limit into account (first side reflow, second side etc)
allowed them to remove and replace the same component 3 times in a worst
case scenario.

And I agree with Bev the whole thing is open to interpretation as to
what represents a reflow cycle - pad dressing removal etc.


 
 
John Burke
 
(408) 515 4992
-----Original Message-----
From: Bev Christian [mailto:[log in to unmask]]
Sent: Tuesday, August 07, 2007 8:53 AM
To: TechNet E-Mail Forum; John Burke
Subject: RE: [TN] Rework question

John,
I am having people look, but if there is a limit I do not remember.
However, from a practical point of view, the answer is Doug Pauls'
patented (Or is that copyrighted or trade marked? Being a lawyer, I
don't
know.) phrase, "It all depends".  It depends on the method of rework,
(reflow, wave solder, selective soldering, soldering iron, hot air gun
focused IR, flame), temperatures reached, ramp rates, dwells, size of
board, thickness of board, type of component, heat sinks, skill of
operator. I could go on.  Make yourself a fishbone diagram.

And then are we talking about say 3X of the same component, or 5X
side-by-side or 7x but each in a thermally separated area from the other
six?

Here's a scenario for you: double sided reflow and then required rework
under cans on both sides of the board. So this means two more for can
removal, dressing the can pads, removing the components, dressing the
pads, part deposition, can replacement.  And hope everything works!

So you need visual inspection, x-ray inspection, electrical test and if
you really want to do it right, x-sectioning and a whole battery of
reliability testing.

I have seen good work and I have seen thin boards delaminated after a
single, poor attempt at rework.

Bev
RIM 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke
Sent: Tuesday, August 07, 2007 10:29 AM
To: [log in to unmask]
Subject: [TN] Rework question

Hi,

 

I had a question yesterday which was "has the number of re-work cycles
allowed by IPC changed for lead free?"

 

And I confess I don't know but I bet someone on this link does.

 

If you could let me know number of thermal cycles allowed and the spec
reference I would be grateful.

 

John

 

 

 

John Burke

 

(408) 515 4992

 


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