I agree with Jack, but at the same time I wish to point out that
J-STD-001D requires you to document all rework. J-STD-001D also requires
you fully inspect (inspect with a microscope or endoscope, X-ray) a
defect and attempt to document the root cause of the problem BEFORE you
perform the rework, as well as inspect the rework afterwards, and
instigate corrective action. So, for every type of rework (whether
solder touchup or Tg excursion) there should be a report somewhere,
whether it is in paper form such as a test failure report, a rework
traveler, or a computerized record, whatever, it must be documented. The
next step from there is to make this historical database easy to access
and track by serial number so that the second time the board is tested
if it fails again for the same or similar reason some type of review is
required before you remove/replace/reflow that particualar component
again.
If the boards are not serialized, or there is absolutely no easy way to
track the rework, then go to the paint dot system, provided your
customer will allow it and it is not space product (outgassing). If the
test tech gets an assembly that fails for a given component, and that
component has a paint dot on it, it raises the red flag that says "This
component has been reworked before, therefore check a little more
closely to determine what is going on". Else a board with an internal
open or short will generate a lot of wasted troubleshooting time, rework
time, time spent ordering replacement parts, time spent re-purchasing
replacement parts, time spent re-stocking replacement parts, etc.
It is true that IPC cannot nor should not attempt to set a limit on the
number of reworks allowed, but it is therefore very important that the
test/rework/retest loop is controlled to prevent repeated, unecessary
rework/retest/troubleshoot-again type of activity. Coming up with a
system that works for any given company is NOT that difficult.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Crawford
Sent: Wednesday, August 08, 2007 10:30 AM
To: [log in to unmask]
Subject: Re: [TN] Rework question--No IPC limitations
Bev, thanks for your detail on this. One additional consideration is
cost and availability of a replacement assembly. Bottom line--every
soldering temperature excursion has a detrimental effect on the
assembly. Maybe a little or maybe a lot (see Bev's list of variables).
There was an Air Force spec that limited the number of rework/repair
operations, but unless every assembly has a unique serial number and
there is a complex, globally accessible database, how will a person know
if a particular component has already been removed/replaced three times?
Yes, depending on the skill of the technician, there may be evidence of
a rework but three times will look the same as one time.
Any limitations have to be established by a product owner based on info
that only they know, it isn't possible for the IPC committees to set
those limits.
Cordially,
Jack
Jack Crawford, IOM
IPC Director Certification and Assembly Technology [log in to unmask]
847-597-2893
FAX 847-615-5693
3000 Lakeside Drive, Suite 309 S
Bannockburn, IL, 60015
-----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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