We recently received our first contract to assemble Class 3 circuit board assemblies. Are there any changes we need to make in our cleaning operations to switch from Class 2 to Class 3?
L.L.
Experts Comments
It is unclear as to what
specification you are working under. There is no general specification
for "Class 3" cleanliness. Apparently, there is a
specification accompanying your contract that will dictate exactly what
conforms to "Class 3" cleanliness in your particular
application. Refer to that specification for the exact
requirements. Undoubtedly, it will require a verification step, so
expect either a particle count per MIL STD 1246 or similar spec, or some type
of solvent rinse of the surface with a GC / FTIR scan of the rinsing agent
associated with the part. Unless it requires 100% verification (as
on many NASA and black DoD programs), once you have proven the process you
should be able to maintain QC by sampling.
Rick Perkins President Chem Logic Rick Perkins is a chemical engineer with more than 33 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.
IPC
TM650 describes cleanliness testing methods and required results.
Unfortunately, the cleanliness results outlined in both military and IPC
standards were written so long ago, they are, in practical terms, obsolete.
IPC allows up to 10 ug NaCl equivalent /in2. This
number is far too high. Due to the nature of ROSE testers (the industry's
approved cleanliness testing method), contamination is detected then averaged
over the entire assembly's surface area. Because actual contamination is not
equally distributed, actual contamination levels are concentrated in some areas
of the assembly and less concentrated in others. Because of this, one must also
consider other factors in determining an allowable contamination level. One
must consider the density and diversity of component population, standoff
height, flux application method, the device's intended environment, and flux
type. Most importantly, one must consider the cost of failure. There may be
differing costs for failure between a class 2 device and a class 3 device. The
more critical the reliability, the lower the allowable contamination level.
While industry standards allow 10 ug NaCl
equivalent /in2, I would initiate an internal requirement of 2 ug NaCl
equivalent /in2, particularly for class 3 devices.
Mike Konrad President Aqueous Technologies Mr. Konrad has been in the electronic assembly equipment industry since 1985. He is founder and CEO of Aqueous Technologies Corporation, a manufacturer of automatic de-fluxing equipment, chemicals, and cleanliness testing systems.
The answer is "it depends."
Whether or not you need to make any changes to cleaning operations depends
entirely on the requirements that your customer sets forth. If your customer is
asking you to work to J-STD-001, for example, they would specify any cleaning
requirements according to Section 8 of that document. You'd want to review the
requirements and the test methods and compare to your existing process. This
would tell you whether your existing process is capable of meeting the desired
level of cleanliness. In the event that your customer has failed to
communicate cleanliness requirements, a discussion with the customer is
suggested to avoid any potential for misunderstanding.
Also recognize that
there may be a need for documentation of cleaning results for Class 3 products.
You may need to be able to demonstrate that cleanliness was tested on the
product, and have records of the results. Again, open communication with the
customer is essential to ensure you are meeting their needs with regard to test
methods, contamination limits, testing frequency and reporting.
Fritz Byle Process Engineer Astronautics Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
Yes,
there are certain spec changes, for example amount of rosin based residues left
behind on the board surface differs from Class 2 to Class 3 assemblies. We
would highly recommend reviewing the:
IPC
J-STD-001-E, Requirements for Soldered Electrical and Electronic Assemblies;
Section 8 - Cleaning Process Requirements and additional industry
specific made addendums
IPC A 610-E Acceptability of Electronic
Assemblies, Section 10.6 - Cleanliness.
Umut Tosun Application Technology Manager Zestron America Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".
It is really comes down to your company having data
showing you can meet and control your process to the limits called out in IPC
class 3 requirements. If you have not recently certified your cleaning
process meets the JSTD-001 class 3 requirements of surface insulation
resistance(SIR) and Ion Chromatography (IC) you should consider this
course. These are complex and time consuming test and only need to be performed initially and when significant process changes are made. You should select an appropriate test assemble with similar component types and
use your flux, soldering process, and cleaning material sets when performing
these tests.
Resistance of solvent extract (ROSE) testing is
recommended to control the cleaning process on a ongoing basis once the IC and
SIR testing passes. The important thing with ROSE testing is not the
absolute value (assuming you pass) as much as the cleanliness data trend day
to day.
There are not many inspection differences related to
cleaning between class 2 and 3 in the IPC JSTD-001 assembly requirements and
the 610 inspection requirements. You may want to send a lead inspector
for re-certification to IPC 610 if they are not certified.
Steve Stach President Austin American Technology Founder and President of AAT. Steve holds numerous patents and has authored numerous research papers and articles in cleaning and soldering. Steve is a founding member of the Central Texas Electronics Association and is a past Director of IMAPS. Steve is active on several IPC cleaning committees.
