In a message dated 3/6/99 8:15:38 AM Pacific Standard Time, [log in to unmask]
writes:
<< Based on Mil-STD-2000A, you are required to reflow the PTH location after
cutting the components leads. Is there any similar requirement in IPC class
3 assembly?
Thank you very much for all your help.
joyce koo
e-mail: [log in to unmask] >>
Hello Joyce,
What IPC 610B says about it is as follows:
4.2.1 Plated Through Hole Mounted Components - Exposed Basis Metal
Acceptable - Class 1,2,3
* Exposed copper on vertical conductor edges
* Exposed basis metal at ends of component leads
Acceptable - Class 1
Nonconforming Process Indicator - Class 2,3
* Exposed basis metal, (non-ferrous), on component lead and land surface
(except end) from nicks, scratches, or other defects, and do not comply with
requirements of figure 3-36.
NOTE: Surface mount IC's, organic coated PWB's, leaded components, sides of
land patterns, path traces, and use of liquid photoimagable solder mask, can
have exposed basis metal (non ferrous) per original designs.
Okay, in reading the above, looks like it's acceptable. For your info, figure
3-36 is just a picture that shows what a forming defect or nick in the lead
might look like that reduces the cross section more than 10%, which if is the
case, is nonconforming.
I built boards quite a while back for a company called Ampex that had that
requirement, we had to go back and touch-up every stinkin' lead that got
trimmed (what a pain!), and they stuck by that requirement. But I gotta tell
ya', I really question the practice. Personally, I think it's a waste of
time...all it does in my opinion is make the ends nice and shiny, and give you
a real good opportunity to increase the intermetallic layer thickness so the
joint fails sooner than it would've had you left it alone.
If one is fearing that the joints are being fractured when trimming, all one
needs to do to prevent that, is ensure that there are nice sharp tools on the
line and that the people know where to trim the leads...ie: not in the fillet!
If the assembly is such that the leads need to be trimmed so close that you
have to cut into the fillets (I've seen a few of those), then that really
sounds like a product design issue.
Bottom line, I feel that if you have good tools and the leads are trimmed in
such a way that you're not into the fillets, then you're not adding any value
or reliability by going back and touching-up the lead ends.
Just my opinion...
-Steve Gregory-
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