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Recent Ask the Experts Questions
January 20, 2020
Dross Particles Sticking to PCBs
We are wave soldering using 63/37 tin-lead solder. After wave, we are finding solder dross particles sticking the the PCB surface. We clean the dross from the solder bath and yet have the same problem.

Do we likely have a contamination problem with the solder bath? What else could cause dross particles to stick to our PCBs?
K.U
January 17, 2020
Conformal Coating in Low Humidity Environments
Whereas conformal coating is excellent for high humidity protection - is there any performance impact when subjected to very low humidity environments?
B.L.
January 16, 2020
Mixing Different SAC305 Solders
We have been using SAC305 bar solder from one supplier. We now want to source it from a different supplier.

Should we be concerned if we mix the new bar solder in with the current solder in our wave soldering systems?

Are there any reliability concerns?
M.K.
January 15, 2020
Package-on-Package Rework
We need to perform some Package-on-Package (PoP) rework. The only package we need to rework is the top package. Can we rework the top package only, or should we remove both packages and then replace them both?
K.S.
January 14, 2020
How to Remove Oxidization from SMT Component Leads?
How do you suggest we remove oxidization from SMT component leads? The component leads are seriously oxidized due to long time storage. Are there companies that can rework oxidized SMT components?
J.J.
January 13, 2020
Causes of Annular Ring Dewetting
I have a question about selective annular ring dewetting. This failure occurs on Electroless Nickel/Immersion Gold (ENIG) Plating for Printed Boards after wave soldering on through hole mounted components. It has a huge financial impact since the defect is rejetcable per IPC IPC A-610 and PCBA cannot be reworked.

What are the reasons for this phenomena? What process parameters, both PCBA manufacturing and bare board manufacturing are critical? If this is related to bare boards manufacturing, how can we discover the failure before the PCBA assembly process?
R.L.
January 10, 2020
Deionized Water Sample Testing
We have an in-line sensor connected to a resistivity meter at the exit pipe of our deionized water system. The resistivity readings of the deionized water are above 15 megohms. The meter is calibrated per the manufacturer recommendations and the sensor has been verified with acceptable results.

We have taken samples of the deionized water to an outside lab for testing. The resistivity results of the samples tested by the outside lab have been approx. 1 megohm.

Is there any explanation for this big discrepancy? Can the deionized water samples change by exposure to the atmosphere? How should we transport samples of deionized water to lab for testing?
P.C.
January 9, 2020
Toe Fillet Requirements on Gull Wing Components
Is a toe fillet required on gull wing components? If it is not a requirement why is that so? We have some gull wing components that have no toe fillet, but all the other requirements for the solder connection are met.
A.C.
January 8, 2020
Conformal Coating Over Heat Sinks
We have PCB assemblies that will be conformally coated using a had spray acrylic coating. Should we apply masking to the heat sinks to prevent them from being coated? If the conformal coating is applied over the heat sinks, will it impact their performance?
F.S.
January 7, 2020
Dendritic Growth and Contamination at BGA Sites
We have 180 assemblies where our subcontractor replaced five BGA memory devices. Unfortunately they utilized active flux and then did not perform an automated cleaning, instead they hand washed.

We are now experiencing latent failures due to dendritic growth. (see photo) Is there any way to recover these boards?
R.D.