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Production
Risky PCBs: Factory Floor Tests, Part 3
By Stanley Bentley P.E., Senior Technical Advisor, DIVSYS International
T
his series is about floor level checks that can be performed by a process engineer when they do not have access to lab instruments or the time to send out samples to a qualified lab. In this installment, we will address solder mask liftage, de-lamination and contamination.
Solder Mask Liftage. The LPI (liquid photo image) solder masks in use today are quite durable and very resistant to flaking or liftage if they are prop- erly applied, properly cured, and if the LPI is within the shelf-life recommen- dations of the manufacturer. The first approach is always to perform a “tape test” in accordance with
IPC-TM-650 section 2.4.1, Adhesion Tape Testing. The method is readily available, using 0.5 in. (12.7 mm) pressure-sensitive tape (3M 600), press the tape to the surface of a small feature (trace or pad) and rip it away at a 90° angle. What the standard does not clarify is a simple way to examine for any removed solder mask; there should be none. My recommendation is to press the tape onto a paper with a very white
finish. Any removed mask will be visible as small dots on the paper. Howev- er, this test does not give a good measure of the cure of the LPI mask. Use a selective soldering system with a 9 mm (0.35 in.) nozzle at normal soldering temperatures for the type of solder to be used. Create a program with a test pattern of stripes, 9 mm wide, across the
entire surface of the PCB with a separation of 25 mm (0.98 in.). This pattern should be fluxed first, using the flux that will be employed in production. Allow the flux to sit on the surface for at least five minutes before running the solder program. Do not preheat. The reason is that preheat will remove all of the volatiles. It is the volatile (usually alcohol) that allows the flux to pene- trate any openings or separations on the mask. Flashing off the volatile leaves the flux just on the surface of the mask where, unless you have a high pH, the LPI is impervious to the flux. An off contact of about 0.127 mm (0.005 in.) will ensure that the solder
temperature is close to the pot temperature, since selective solder systems lose heat through the chimney and nozzle. If you can see stripes where the sol- der fountain contacted the mask, then the mask was not properly cured and is at risk of liftage.
Delamination, or measling, has become more of an issue with the use of lead-free process laminates. This class of laminates was created to deal with the higher processing temperatures and longer dwell times above liquidus as- sociated with lead-free solder. The inert fillers added to this class of laminates are more prone to moisture absorption than their FR-4 cousins. For this reason, care must be taken in the storage and handling of these
laminates. If the vacuum-sealed packaging from the fabricator has been com- promised, it is advisable to perform a low temperature bake of the laminate before processing. Longer at a lower temperature is much better than faster baking at a higher temperature. I recommend 105°C (221°F) for 1-2 hours. I always caution when baking PCBs with HASL surface finishes that the
bake process will increase the intermetallic growth between the copper and the tin. Accelerated baking at higher temperatures is to be avoided as this will most always result in dewetting of the solder surface.
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PCB Contamination. Ionic contamination is all too prevalent on PCBs today. The increasing cost of the (very necessary) DI water is contributing to shortcuts in the process such as inadequate rinsing. This can manifest itself either on the surface of the PCB from the very active flux needed for the HASL process or residual ionics from plating and etching on the laminate surface which were then covered with the LPI solder mask. A standard ionic cleanli- ness test should be performed on incoming PCBs before they are accepted. I recommend the following for a standard product:
l Purchase an inexpensive conductivity tester. l Obtain from a drug store 91 percent IPA (alcohol).
l l
Obtain from a grocery store a quantity of sealable storage bags of sufficient size to hold the assembly.
Pull a sample quantity of the assembly and perform a standard cleanliness test to establish a typical operating range in µg/cm2.
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Take an equivalent number of the same assembly and put each in a sealable bag with enough of the IPA to cover about 1/3 of the assembly. l Note the volume of IPA for future use and always use the same volume. l Shake the sealable bag with the assembly and the IPA for 60 seconds. l Pour the IPA into a clean container and measure the conductivity.
l
Record the values you obtain and use these as your “expected” operating limits.
As a final caution, these hints are for the process engineer to aid trou-
bleshooting. The results are not laboratory values and are simply indicators of where to start the analysis. All too often, process engineers are given
process responsibility without the tools to solve the problem. Contact: DIVSYS International, LLC, 8110 Zionsville Road,
Indianapolis, IN 46268 % 317-405-9427 E-mail:
sales@divsys.com Web:
www.divsys.com r
July, 2016
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