Well written, Terry! General areas of a board is seldom a contamination
problem, but ENTRAPMENTS, yes! And one can not be sure the ionic
contamination test tells you about them.
/Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Terry L. Munson
Sent: den 18 april 2008 22:35
To: [log in to unmask]
Subject: Re: [TN] Ionic Contamination Question (& added info).
Bill
As one of the active task group participants during the 90s when we were
asked to update the equivalence factors table and using low solids
cleanable and no clean formulations, we found it difficult to assess
these new types of low solids residues and if you look at the large
detailed report from the task group (IPC TR 583) we found it difficult
to correlate between static and dynamic systems. The use of higher
pass fail criteria were in place for automotive users (one in
particular used 28 ug/in2 instead of the 14 ug/in2 from
1978 to 1991 with no field performance problems related to flux
residues with mean levels at 21 ug/in2 while using an 18-24% solids RMA
flux) that did not see problems.
My concern with cleanliness is not the generalized cleanliness of a
total board but the amount of contamination present between pads, in
via's or thru-hole devices. The pocket of contamination below an 0805
capacitor has created drained batteries when the entire assembly tests
clean by ROSE and by bag extraction Ion Chromatography. It is only when
we look at the localized pocket of contamination do we see the direct
correlation to field and reliability performance. Many of the
localized areas we look at with the C3 tester are clean and show low
levels of contamination, but when we see the combined processing
effects of selective wave soldering using a pallet to isolate the area
we find that the residues that can be trapped between the pallet and
circuit board (low solids no clean VOC free) are also protected from
the heat but are very corrosive due to the water carrier and acidic pH
2.35. Dendrites are growing in these nearby areas, as well as stray
voltage problems and intermittent performance issues and No Trouble
Found (NTF) returns.
It is important to understand the cleanliness of an assembly but it is
more important to understand the cleanliness of the processing steps
such as 1st reflow, 2nd reflow, wave soldering bottom and top side, bare
board unsoldered areas, micro via's that have fabrication residues
trapped inside causing performance problems and the hand solder / touch
up residues. This type of cleanliness understanding comes from being
able to do localized non-destructive residue assessment and ion
chromatography analysis.
Cleanliness testing must predict field performance. Using localized
testing is the only way I am aware of understanding how much
contamination is present in the areas of critical circuitry that tends
to fail earliest.
Terry Munson
Foresite
765-457-8095
_www.Residues.com_ (http://www.Residues.com)
In a message dated 4/18/2008 1:54:54 P.M. Eastern Daylight Time,
[log in to unmask] writes:
As perhaps the only person still around that attended the meetings
resulting
in the equivalence factors and IPC Cleaning & Contamination Chair at
that time, perhaps a few points would facilitate the discussion:
1. The Navy set up the ionic testing development program to solve a
serious
failure problem in S.E. Asia.
It worked.
2. In the timeframe when the test was developed and put in place by
the
military, most of the rest of the electronics industry in the US used
the mil specs since they were free.
3. As the IPC set up and adopted Classes 1-3 (basically toys up to
military/high rel), I asked the committee if we used the mil test
result for Class 3, could we use 1.5x that limit for Class 2 and 2-3x
for Class 1?
The response was that with proper cleaning, the mil limit could
readily be
achieved while serving to monitor daily production. So the industry
continued to use the (free) mil spec test standard.
4. The ionic contamination test was a valuable monitoring tool, since
the SIR tests were done on coupons, not on actual assemblies, and took
1-2 weeks to complete. Needless to say, a high volume electronics
producer could turn out
a significant volume of PWAs during that time, often shipping them
into the field as soon as assembly was completed.
5. As noted in my SMT column (offered yesterday) T. O. Duyck of
Northern
Telecom was charged with implementing water soluble flux for NT
electronics
production. During that time he observed and reported the differences
in flux residue release rates, pointing out that rosin ca 90% of
rosin flux residues
release from the PWA surface during the 10-15 test time for ionic test
equipment, while water soluble flux residues may take up to 2 hrs. to
achieve the same
level of release. Thus the release rate should be checked to ensure
the flux used, time test time and the instrument employed provide
reliable results and guidance to the production engineer.
(See T. O. Duyck and M. Boulos, "Water Washes Reliability into
Telephone Circuit Packs", IPC-TR-206, April, 1978)
Based on this work, I investigated the release rate of SA flux
residues,
finding it even faster than rosin fluxes. (See W. G. Kenyon,
"Synthetic Activated (SA) Flux Technology: Development,
Commercialization, Benefits and Future Applications", Internepcon
Japan, 24 Jan. 1986)
6. In the late 1980's, the materials and acceptance of the no clean or
low residue or acceptable dirt concept became widely accepted and
implemented on the designs of the time. Outsourcing to contract
assemblers (both in the US and
overseas) became widely practiced, so much of the former 'in-house'
cleaning
expertise disappeared.
7. This was seen at IPC as the number of company sponsored volunteers
dwindled. Could we take on projects today to develop an updated ionic
test
for pr
ocess monitoring? Find enough participants to conduct statistically
sound
round
robin testing?
Bill Kenyon
Global Centre Consulting
3336 Birmingham Drive
Fort Collins, CO 80526
Tel: 970.207.9586 Cell: 970.980.6373
**************Need a new ride? Check out the largest site for U.S. used
car
listings at AOL Autos.
(http://autos.aol.com/used?NCID=aolcmp00300000002851)
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