Genny,
 
When I used deal with creapage and clearance requirements for Telecom
network voltage application.  I believe UL 1950 spec stated that when
you had an isolated conductor between two conductors that needed a
certain separation you added the spaces on each side of the isolated
conductor.  So in your case if the sum of the two spaces is greater than
.13mm so the minimum clearance is not violated.  The rational being that
arcing occurs based on the potential difference between the conductors
on each side of the gap.  Think of the two gaps as series resistors.
 
By having the solder ball in the middle you now have a greater chance of
something else coming along or growing off one pad and can now cause a
short faster since there is less gap to fill.  I guess it depends on
your area of concern and I don't know what it was for the IPC spec.
 
Jim Verrette
Senior Electrical Engineer
 
------------------------------

Date: Tue, 9 Sep 2008 09:13:42 -0600

From: Genny Gibbard <[log in to unmask]>

Subject: Solder balls and minimum electrical clearance

I posted a question a couple months ago about solderballs. The debate

is still happening here. I thought I would post a more specific

question regarding how to interpret the standards.

Consider a low voltage situation, where minimum electrical clearance is

0.13mm according to Table 6-1 in IPC-2221.=20

IPC-A-610 states for acceptable: Solderballs are entrapped/encapsulated

and do not violate minimum electrical clearance.

Components are so small now, that solder fines are starting to come into

question when they would probably have never been noticed a few years

ago.

If a solderball is entrapped under a component between two pads, and the

sum of the distance from pad to ball and ball to next pad is greater

than 0.13mm, but each individual distance (pad to ball) is less than

0.13mm - does this violate minimum electrical clearance?

The solderball is not connected to anything.

My thought this is a violation, because the solderball is sort of like a

stepping stone in the middle of the stream, that would make arcing that

much easier. But I am wondering if my logic is flawed? An arc still

needs to cover the whole span, and the solder ball isn't electrically

connected or contributing to the voltage difference

.

These are usually solder fines that aren't moving. If one were to

somehow get loose and move toward one pad, though, the remaining

distance would not be a failure. It is just when they are trapped in

the middle between two pads that each individual distance is a failure.

If the stepping stone idea is correct, we have a lot of solder fines to

rework. So I am hoping it is incorrect... But we will do what is

right.

Your assistance in interpreting the standards is much appreciated.

Genny=20


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