Many concerns come to mind with this lead length criteria.  I assume that 
you are talking about components that will be hand inserted into a plated 
thru hole.  The accepted norm in the past for all my applications has been 
a max of 0.100"

Longer lead lengths mean that:
--Spacing between the leads will have to be increased because there will be 
a greater tendency for the leads to short together during the wave solder 
process.
--Your solder pot clearance will have to be adjusted for the lead length, 
and as a result will have to throw a higher wave to contact the PCB, 
possibly resulting in wave solder defects(skips) if not maintained 
properly.  If you are not using a nitrogen system, this will result in an 
increased amount of dross.
--There is a greater tendency for the leads to be bent in normal handling 
and could short to each other if they get bent.
--Consider the product design and how it fits in the package.

I recommend that you set a criteria of 0.060" +/- 0.030" for lead length.  
This is an accepted standard and has been successful for years.

E. Holton
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-------------
Original Text
From: C=US/A=INTERNET/DDA=ID/TechNet-request(a)ipc.ipc.org, on 11/15/96 
5:59 PM:
Greetings Technetters-

In the interest of streamlining a process, we're experimenting
with straight-thru assembly of leaded components in mixed-tech products.
The proposal on the table is to allow protrusion of up to 3.6mm (0.15").
This doesn't conform to existing IPC requirements (we're building Class 2
products).

What should be our concerns, other than potential shorts?
Why is Class 3 so stringent at 1.5mm max?


Thanks for your help.
 

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