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July 2000

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Subject:
From:
Brent Stayer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 21 Jul 2000 14:15:00 -0400
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Bev,
Answer 1)

If the component does not have built in standoffs (preferred), we have used lead forming as the most common way of meeting this requirement.  Adding a spacer of some sort will work but adds a step that may be missed in your assembly process.  Therefore, additional inspection is needed on each assembly.  This relates to your "by chance" solution.  As for your last comment, "don't", is not acceptable...........you will have problems eventually.

Answer 2)

In cases were the component does not stand off the PCB, several things as you stated can happen (I give a little or detail to your reasons).  1)  Cannot inspect for complete solder fillet on top of component. 2)  Coating of component (plastic, ceramic, etc) will actually sit down in the PTH and prevent the lead from soldering.  3)  Heat dissipation can go both ways.  Mounting close to PCB can sink out heat.  Depending on the heat being dissipated, mounting close can also cause your PCB to fail prematurely or not meet CE requirements of maximum PCB temperature.  4)  Solder may also wick between leads due to the proximity of the component to the PCB (Cohesion).

Answer 3)

The easiest way to inspect for this is not to inspect.  Eliminate the chance of occurrence by using lead forming or standoffs.  Remember, if you use standoffs, you must inspect for the presence of the standoff. 

Hope this helps,
Brent

>>> [log in to unmask] 07/21/00 08:51AM >>>
Here's an easy one.  Page 5-12 of IPC-A-610C shows radial PTH components
with a standoff between the component base and the board.  1) How do you all
meet this requirement?  2) What is the purpose of it? 3) How do you
inspection for it?
My guesses:
1) lead forming, soluble plastic disks or tubing, friction fit, by chance,
don't
2) heat dissipation, inspection, strain on solder joint
3) eye, thickness gauge, don't

I await your sage comments, oh wise ones.

regards,

Bev Christian
XLTEK

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