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Den-On Instruments RD-500V/SV All-In-One Rework Stations
Am I at Risk to Use This PCB?
By Stanley L Bentley, P.E., Senior Technical Advisor, DIVSYS International, LLC
T
here is only one way to answer this question, and that is with an exam- ination by a qualified laboratory. However, production pressures, lab- oratory lead times and cost usually require some “less than definitive”
answer. Even with access to a laboratory and control of the lead time, I sometimes
find it necessary to get some type of an answer “on the fly”. This usually oc- curs at a customer location with the product running. To address this need for less than perfect guidance, I have developed a series of simple tests that can be performed at the floor level with equipment generally available to the process engineer. The following is one of my top ten!
Poor Wetting and De-wetting of the solder. Poor wetting of the attach- ment pad is usually manifest in an irregular attachment to the solder pad and obvious areas where no solder adhered. I also find this with pads where the solder does not “wet” the pad to its edges. In the solder process, the liquid sol- der must melt and dissolve the surface finish (no matter the type) in order for the tin-to-base metal intermetallic to form. In the case of ENIG (Electroless Nickel Immersion Gold), the base metal
will be nickel after the very thin gold layer is dissolved into the solder. There is no visual examination of the ENIG surface that will give an indication of the solderability of the nickel. The following test works with all finishes, but it is particularly suited to ENIG. Remove an array or single board depending upon how the PCB is nor-
mally processed. Do no preparation (such as baking) to the array. Perform the normal paste print process. Do not place any parts. Hold (delay) the array for the normal hold time and placement time of
this board. Run through the fusion oven with the correct profile. Perform any post cleaning that would normally occur. If SMT both sides, run the array through the second side process. Once complete, examine the attachment locations under a microscope at
RD-500V
The ALL-IN-ONE advanced technology SMT rework station is compatible with multiple types of SMT components, including 01005s!
Features:
» 2 Mega Pixel full HD camera » 3-Point Auto-profile
» Highly accurate temperature control (sampling intervals of 0.1 sec)
» Powerful built-in 1000W heaters on both top and bottom side
» Area heater that includes six quick-heating 600W IR heaters
» Precise profiling w/detailed temp curves for up to 30 zones
The system supports industrial large and small-scale circuit boards, including 01005 components. Additional features include two pressure sensors, contact-less cleaning, precision control of the Z-axis On-the-Fly profile programming and more.
email:
info@seikausa.com web:
seikausa.com
os neles fce 3528 Torrance Blvd. Suite 100 Torrance, CA 90503 Main: 310-540-7310 Fax: 310-540-7930
Atlanta fce 1580 Boggs Rd. Suite 900 Duluth, GA 30096 Main: 770-446-3116 Fax: 770-446-3118
10-20X . The fused paste should have a uniform and rounded appearance on each pad. The paste will attempt to form a parabolic shape in each dimension of the pad. If the pad is square (not common), the shape will be a uniform parabola. With rectangular pads, the shape will be a curve in both axis. If this is the result, the PCB is perfectly solderable. As you see degradation from this perfect shape, you will see corresponding soldering issues that require touchup in your process. Look very closely at the second side of the SMT process. Every surface
finish suffers some degradation when exposed to the Paste-Fuse-Clean process. Therefore, the second side is always the least solderable. This side is also where poor surface finishes will have the most impact on your solder con- nection. Unfortunately, it is also the side that usually has the most difficult- to-solder parts. One other benefit of this solderability test (and the reason for no prepa-
ration of the PCB) is that if you have moisture in the PCB or poor curing of the innerlayers, the PCB will show signs of delamination or “Measeling”. A variant of this test is also useful. The joint strength of ENIG is sig-
nificantly lower than HASL (Hot Air Solder Level). The strength of the ENIG join is entirely dependent upon the condition of the underlying nickel barrier metal. This condition can be a series of things from passivation, cracks, or even captured contaminants all of which are easily covered by the immersion gold.
My “Push/Pull Pin Test”.
Bend a small diameter solid wire (28-30 AWG) into an “L” shape. Push this small “L” into the (unfused) solder paste of several SMT pads
on the PCB. Do not make any attempt to “help” the solder connection by moving the
wire around on the pad. Complete the fusing and cleaning process. Do the same thing on the second SMT operation (if there is one). When the process is complete, grasp the exposed end of the small wire
and pull at a 45 degree angle to the PCB until you pull the wire and pad from the PCB. Do this for all wires you installed. Examine the failure mode of the solder connection. If the wire pulled the pad from the PCB, then the ability of the solder to “wet” the pad is sufficient. This is correct even if the solder joint is not as “pristine” as a perfect joint. The reason is that the failure mech- anism is the attachment of the copper pad to the “butter coat” epoxy of the PCB. The joint strength does not need to be any better than the attachment of the pad to the PCB. All of my “Top Ten” are as follows in no particular order because on any
given day, one of them will be #1: poor wetting and de-wetting of the solder; weak solder joint strength; paste print coverage; insufficient hole fill of through-hole components; poor wetting of “topside” pads; solder mask liftage; de-lamination or “Meazeling”; contamination; warpage; low Tg (Glass Transi- tion Temperature). My goal has always been to have no rework. One step in this direction
is to not process PCBs that do not pass these tests. Contact: DIVSYS International, LLC, 8110 Zionsville Road, Indianapo-
lis, IN 46268 % 317-405-9427 fax: 317-663-0729 E-mail:
sales@divsys.com Web:
www.divsys.com r
August, 2015
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