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March 2004

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Subject:
From:
"Victor G. Hernandez" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 16 Mar 2004 06:37:21 -0600
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Collins,

   This is a GOOD topic for discussion.   I have notice that even the
PCB pad size does
not match the TO252 foot print.   The lead parameters are about the same
size as the PCB
pads.   My biggest concern is the wetting of the Thermal heat sink bar
with different PCB
foot print layout.   We conduct a lot of x-ray imaging to ensure
adequate wetting.

   I will keep out a look for info on this topic.

Victor,

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Collins Graham
Sent: Friday, March 12, 2004 2:28 PM
To: [log in to unmask]
Subject: [TN] solder fillet requirements for a TO-252 style part, class
3 assy.


Good day TechNet!

We are having a debate about the solder fillet requirements on TO-252
style transistors, in a class 3 SMT application.  I'm wondering where
the experts on TechNet sit on this one.  An example of the type of part
I'm talking about can be seen at this link:
http://www.onsemi.com/pub/Collateral/418B-04.PDF

There are two types of attachment on this part, the ground pad and the
leads.  We have treated the ground pad as per IPC-A-610 section 12.2.9,
"flat lug leads".  This part of the attachment is not controversial.

What is being debated is what rules should apply to the other leads.
Physically they are gull wing formed leads, but they are very thick.  So
- do the requirements of 12.2.5 "flat ribbon, L, and gull wing leads"
apply to these?  In particular, the heel fillet must be up at least the
equivalent of the lead thickness.  We've been meeting this requirement,
but not without pain, and I'm wondering if we need to meet this.  On a
lighter component (e.g. a SOIC, where the lead thickness may be say 10
thou) this height requirement is pretty trivial to meet, but on one
particular part I'm looking at the lead thickness is 18 - 25 thou.

So - specifically - would you agree the height requirement for this
particular style of component is that the heel fillet be at least 1 lead
thickness high (plus dimension G, the gap between bottom of lead and the
pad) per 12.2.5.6?  Am I missing something in the spec?

Have a good weekend!

regards,

Graham Collins
Process Engineer,
Northrop Grumman Canada Corporation
Halifax
(902) 873-2000 ext 6215

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