[tinwhiskers] Re: Conformal Coating ? When Reliability Goes Astray
- From: "Fritz, Dennis D." <DENNIS.D.FRITZ@xxxxxxxx>
- To: <tinwhiskers@xxxxxxxxxxxxx>
- Date: Fri, 14 Mar 2008 09:47:52 -0400
A reference that I worked on concerning tin whisker mitigation is the JEDEC/IPC
document JP002. It can be downloaded free from this site:
http://www.jedec.org/DOWNLOAD/search/JP002.pdf
While the document is exactly 2 years old, it contains a reasonable
bibliography of tin whiskers articles that a consensus group felt were
reliable. You might check there to see what Tom Woodrow of Boeing reports on
the ability of conformal coats to mitigate whiskers - reference A-49. I heard
him speak at SMTAI in September of 2006 on his findings, but I think you have
to pay SMTA for that article.
Denny Fritz
SAIC.
________________________________
From: tinwhiskers-bounce@xxxxxxxxxxxxx on behalf of Bob Landman
Sent: Thu 3/13/2008 4:38 PM
To: tinwhiskers@xxxxxxxxxxxxx
Subject: [tinwhiskers] Conformal Coating ? When Reliability Goes Astray
We are told that conformal coatings are a successful tin whiskers mitigation
strategy. Correct me if I'm wrong, but I don't believe it's been demonstrated
to any significant degree that any conformal coating on the market today will
"prevent" tin whiskers from punching through the coating.
I note that it's popular to use the word "mitigate" and that's a word that is
not as strong a word as "avoid" or "prevent". I await someone who can do the
math on how statistically significantly conformal coatings "mitigate" tin
whiskers.
A dictionary states that the word means "To moderate (a quality or condition)
in force or intensity; alleviate."
By how much? The word itself gives us no clue.
If a whisker can grow from one pin on an IC package, then certainly, it can
also grow from adjacent pins as well and then don't we have the perfect
opportunity for shorts?
I just read the article below on reliability of conformal coatings that I
thought worth sharing if we are going to have to count on such a coating to
save our lives.
-Bob Landman/H&L Instruments,LLC
SMT
Conformal Coating - When Reliability Goes Astray
By Umut Tosun, Ms.ChE., ZESTRON America
Proper functioning of electronic assemblies under the most stringent conditions
can be guaranteed only when conformal coatings perfectly adhere to board
surfaces. The usual requirement for such proper adhesion is attained through
the highest cleanliness level on the assembly surface. Cleanliness level
assessments can be performed by implementing quick, innovative, and economical
analytical cleanliness procedures.
In automotive-, military-, and aerospace-related industries, electronic
assembly requirements have increased steadily over the years. Increasing
package density brings higher standards for assembly cleanliness during the
manufacturing and assembly process. The use of assemblies under harsh climatic
conditions, such as temperature fluctuations and moisture exposure, increases
the risk for malfunctions. Thus, failure mechanisms such as leakage current and
electrochemical migration are initiated through environmental influences
(Figures 1A and B).
With the introduction of lead-free solder pastes, increased amounts of rosin
and activator content also must be taken into consideration. The latter has
proved responsible for an increase in corrosion-related malfunctions, and
reductions in the reliability and life of electronic assemblies.
Coating as a Reliable Protective Measure
Protecting electronic assemblies with conformal coatings is an important,
necessary measure to ensure reliability of electronic products. As conformal
coating is usually the last step in the manufacturing process, application
failures may have a drastic effect on production costs and lead to unavoidable
field failures. To guarantee optimum adhesion of the protective coating and
prevent subsequent crack formation or delamination, it is of utmost importance
to ensure the highest cleanliness level for assemblies prior to coating.
Minimum Surface Cleanliness
The minimum cleanliness requirement prior to conformal coating application is
specified in the J-STD 001 D standard. Accordingly, the following methods are
required for proper qualification:
. Visual inspection with 20 or 40× magnification (according to IPC
A610D);
. Measurement of flux residues (257.95 µg/inch2 for Class-3 assemblies);
. Measurement of ionic contamination (10.06 µg/inch2 eq. NaCl);
. Evidence of other organic impurities;
. SIR measurement during or after climatic storage.
Visual inspection can be performed with a microscope. No visual impurities on
the assembly should be observed during this scrutiny.
The amount of resin on assemblies plays a significant role as it directly
influences conformal coating adhesion. Resin residues can lead to insufficient
adhesion and result in coating delamination. Acknowledging the threshold for
Class-3 assemblies with a set limit of 257.95 µg/inch2, one has to be aware
that this value is equivalent to the amount of resin that can be found around a
single soldered joint. However, the amount of resin left by lead-free solder
pastes has increased enormously due to their changed composition. Over the
years, resin residues have been detected by means of extensive and lengthy
procedures such as high-pressure liquid chromatography (HPLC). They now can be
detected easily through a quick chemical test, such as the resin test. This
method ensures that residual resin is identified and removed effectively.
In assessing the surface's cleanliness according to the J-STD 001D standard,
the ion equivalent represents an important test method as well. A high ionic
equivalent value indicates the existence of a large amount of hygroscopic
impurities. Over time, the found impurities might lead to a coating
delamination, ultimately resulting in failure (Figure 2).
Other organic impurities, such as flux residues, influence coating quality and
trigger failure mechanisms underneath the conformal layer. In accordance with
J-STD 001D standard, the presence of organic impurities either can be tested
with infrared spectroscopy or detected with a discoloration method such as the
flux test. By means of a color reaction, organic acids used as activators in
fluxes specifically are revealed by the flux test on the assembly. This
innovative and non-destructive test method naturally provides a visual
distribution of the critical residues on the board surface.
Surface insulation resistance (SIR) measurement is done to demonstrate the
surface insulation resistance, as a high degree of insulation ensures that
electrical signals on the assembly are not distorted. Flux residues and
conductive impurities may cause leakage current bridges, thereby leading to
malfunctions. While performing SIR measurements, operators store a comb
structure in a climatic exposure test cabinet and the surface resistances
between the individual comb structures are measured.
All previously mentioned methods ensure detection of the various impurities
that could disrupt the functionality and viability of a conformal coating.
Nevertheless, the integration of a cleaning process usually is required to be
able to maintain all of the production thresholds set by the J-STD 001-D
standard. The cleaning process should not only remove impurities, but also
guarantee the proper adhesion of the conformal coating to minimize long-term
risk of crack formation and delamination.
Conclusion
Associating an appropriate cleaning process with innovative surface cleanliness
tests will yield an economical and cost-effective solution. A proper coating
process will result, and process and operational reliability of coated
assemblies will increase. The Society for Corrosion Protection endorses this
statement in its latest "Use and Processing of Conformal Coatings for
Electronics Assemblies" guideline. Authored by a consortium of coating
manufacturers, this guideline provides a selection of reliable and economical
process solutions. When using optimized cleaning and qualification test methods
in production steps preceding the coating process, the adhesion of coatings is
ensured, thus preventing field failures of the coated assemblies. With the
arising board complexity, partnerships between manufacturing process engineers,
cleaning process suppliers, and equipment manufacturers become increasingly
important.
Umut Tosun, M.S. Chem. Eng., accredited cleaning expert and application
technology manager, Zestron, may be contacted at u.tosun@xxxxxxxxxxxxxx
<http://smt.pennnet.com/Articles/mailto:u.tosun@xxxxxxxxxxxxxx> .
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