I addressed part of this in my recent article in the May Circuits Assembly Magazine.
Paraphrasing in answer to your questions:

When extracting solder from a PTH, it is important to get (nearly) all the solder out
and desirable to have lead movement during the freeze cycle so that a sweat joint
doesn't occur from the lead to the side of the barrel.  Capillary action of wicking
is NOT sufficient to overcome the same force holding solder between the edge of the
lead and the barrel.  You WILL have a sweat joint.  Pulse extractors may/may not (1)
introduce a pretty strong ESD pulse depending on (a) type (b) quality (c) condition
of the pulse extractor (2) remove all/most of solder depending on (a) cleanliness (b)
strength of spring (c) size of barrel (d) coverage of tip over hole (e)
cooling/freezing of solder in the "remove heat & quickly get extractor tip over and
release plunger" operation.  It's also easy to knock a pad off or break a knee in
that manual operation.  Because there isn't room to move the lead around there is a
good probability of forming a sweat joint to the side of the barrel.

The problem with the sweat joints is that they are prone to tearing away a chunk of
the barrel with the lead is forcibly removed.  It may or may not interupt an internal
layer connection.  It may affect the ability to meet barrel fill and wetting
requirements, depending on what spec you are working to (see IPC-A-610B paragraph 4.1
or J-STD-001B paragraph 9.2.5.1.

Continuous vacuum extractors permit applying the heated element over the lead and
nearly sealing the vacuum into the hole. Vacuum can run as long as the operator
wishes to assure all the solder is extracted, even from large pad areas that might
require several pulse extraction operations (each with a complete thermal cycle).
With the vacuum still running, the exractor tip can be raised slightly off the pad
and the lead rotated around inside the barrel ("stirred") while cool air is pulled in
from the back side freezing the barrel and preventing a sweat joint from forming.
All the molten solder is pulled well into the collection chamber and the tip is open
and ready for immediate use.  Pulse extractors have to be repeatedly cleaned.

There is no thermal "shock" to be concerned about in any of these operations; rapid
cooling has been shown to cause a finer grain structure which is NOT bad.  Multiple
and extended thermal cycles from pulse extractors and solder wick CAN introduce
failure mechanisms.  Jack


IPC/SMTA Electronics Assembly Expo
Technical Committee Meetings, Conference, Exhibits
Providence RI   October 24-29
More info at http://www.ipc.org

Jack Crawford, IPC Project Manager - Assembly
2215 Sanders Road, Northbrook IL  60062-6135
[log in to unmask]      847-509-9700 x 393     fax 847-509-9798

>>> <[log in to unmask]> 06/05/98 12:33PM >>>
All,

I'm Baaack....

Got another one for you all.  We occasionally have a need to
rework a solder connection or an assembly by removing solder
from the PTH's.  Our rework/repair operators are very
skilled in this type of work and use the following methods
to remove solder - Solder wick, one shot solder suckers, and
continuous vacuum solder suckers.  Our customer is concerned
about the use of the wick and the one shot solder suckers.
He states that at one time they were not allowed on the high
reliability standards (Mil-Std-2000) because of damage they
cause.

My question:  What kind of damage do they cause except in
the hands of an unskilled operator??  Our rework/repair
success rate is phenomenal here.  Our rework operators (the
only ones allowed to use the solder suckers) are trained and
skilled in what they do.  Our customer isn't telling us we
need to get rid of our solder wick and suckers, but I am
sure he expects us to answer his concerns.  I would consider
it sufficient that we simply don't have any failures
attributed to solder rework or repair, but would like a more
educated answer than that.

His concerns are based on a white paper he reviewed quite a
number of years ago.  He believes the solder sucker problems
come from a thermal shock problem when ambient air is forced
through a hot PTH.  Some merit there, but any real-world
problems?

Any Insight?

Thanks in advance!

Steve McBride
[log in to unmask]
(405) 624-5281

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