Yes! Thanks for your post Raye. I actually used it to measure microminiature MEMS gyroscopes. We had damage occurring somewhere in the process. We measured the strain from these gauges connected to a datalogger, from the manufacture of the MEMS, through the delivery truck and plane flight, through our receiving department and through our entire build process at one of the client companies I work for. Guess what was damaging the MEMS? The inline cleaning process! Nothing else even showed up on the data, except for the cleaning process. It turns out the cavitation from the wash spray, coupled with (normal) vibration through the conveyor really shocked the heck out of these parts. So we switched to a manual wash process using (believe it or not!) an ultrasonic cleaning tank followed by a gently manual rinse in DI water and complete blowoff. The sweep frequencies in the US cleaner allowed for excellent cleaning without the cavitation known to destroy internal wirebonding, or in this case, the microminiature spinning gyros. Very counterintuitive, but it worked extremely well. Those MEMS gyros are working extremely well and have been for about 15 years or so.
Odin
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From: TechNet <[log in to unmask]> On Behalf Of Rivera, Raye
Sent: Tuesday, October 6, 2020 11:05 AM
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Subject: Re: [TN] Bending a Board Assembly
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I recommend the strain gauge method Richard referenced. We had a similar problem, and ran the board through the suspect process with strain gauges in place. You then plot strain vs. strain rate and anything that falls on the wrong side of the acceptable line represents too much bending. In our case we had to revise the fixtures and run again to get to an acceptable process. We've also used this to measure bending during shipping.
Best regards,
RAYE RIVERA
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-----Original Message-----
From: TechNet <[log in to unmask]> On Behalf Of Stadem, Richard D
Sent: Tuesday, October 6, 2020 8:56 AM
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Subject: Re: [TN] Bending a Board Assembly
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Hi, Jack
It depends on the design of the PWB first of all. If it is a multilayer PWB with internal traces connected to vias, the flexing puts huge stresses on the internal trace connections to the via barrel and, worse yet, any trace separations that occur are often very intermittent and can lead to latent failures during operation in the field.
As far as the components, most axial and compliant-leaded SMT components can handle a small amount of flexing without any issues. The caveat there is how much flexing and whether the solder used is Sn36, because the lead in that solder provides at least a one-time relaxation factor (pretty much overnight, depending on the amount of flex strain), after which it will hold its shape without any great decrease in reliability. The question again is how much flex are you considering with respect to the PWB size, measured in the maximum RATE of warpage per centimeter in any direction. If the CCA must be a lead-free assembly, then you would want to try using Kester KL100D (I think that is the name) which is a SN100 alloy doped with small amounts of other elements that gives it a modulus of elasticity and other properties very similar to SN63. An alloy like SAC305 is simply too hard and brittle, and the resultant solder joints under stress have very little relaxation and this leads to crack propagation or pad fracturing from the PWB. As far as the leadless chip caps, resistors, and glass body diodes, it is important to orient these perpendicular to the intended flex curve, as they are quite often small enough to straddle the curve without a huge amount of strain. Larger leadless ceramic chips are a major issue, however.
And if you want to actually measure the strain, that is easily done. Strain gauges are available for purchase that can be mounted on the PWB (in any direction or length) to measure the strain using small sensors. The process is well-documented somewhere in IPC-TM-650, I believe it is section TM 2.4.41.2A if my ancient memory serves me, but it is there. And I remember that the source of the strain gauges used is also in that particular test method.
Now don't tell old Walt that I gave you this info, because he thinks I get paid to sit around all day and quote IPC specs by memory. That is simply not true. Or is it? LOL!
Odin
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From: TechNet <[log in to unmask]> On Behalf Of Jack Olson
Sent: Tuesday, October 6, 2020 10:13 AM
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Subject: [TN] Bending a Board Assembly
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We are considering a change to a manufacturing process which will save time and money, but will cause extra stress on the completed board assembly.
Q1) I'm familiar with bow and twist for bare boards, but is there data on flexing a completed assembly? I'm sure it depends on board thickness and material, etc. but is there danger to the board itself? or does it always break a component first?
Q2) I'm NOT familiar with stress on components. How can I determine whether we are in danger of damaging components, especially SMDs?
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