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Recent Ask the Experts Questions
July 9, 2020
Adding Chlorine to a DI (Deionization) Water System
We have had a recent issue of bacteria accumulation in our DI (Deionization) reservoir. In order to combat this we have put in a floater (similar to one found in swimming pools) containing chlorine tablets. Is this dangerous to use in our system? What limit should the chlorine levels be in the DI reservoir?
D.G.
July 8, 2020
Confused About IPC-A-610 Class 2 vs. Class 3
My company is confused about IPC-A-610 Class 2 and Class 3. Some believe that Class 3 is for aerospace and Class 2 is not. The Class or standard used in Aircraft is what the OEM will use in the design submitted to the FAA and has nothing to do with IPC-A-610. Where did this thinking come from?
T.G.
July 7, 2020
0201 Pick & Place Nozzle Plugging
We are assembling PCBA's containing 0201 passive components. We have a problem with solder paste getting in the 0201 P&P nozzles and plugging them up. We have been using a .005" thick stencil and with a 1 to 1 ratio on everything except the 0201 passives that have a 10% reduction. I believe what is happening is when the 0201 passives are placed, the paste is squished out to the sides and gets on the P&P nozzles.

Our stencil vendor is recommending using the same aperture size but going to a .004" think stencil. I am concerned about reducing the volume by 20%. Has anyone tried a 4 mil stencil that had the apertures enlarged to maintain volume while overprinting the ends to keep solder away from the 0201 nozzles?
C.T.
July 6, 2020
Blow Holes and Disturbed Joints
We are finding many blow holes and disturbed joints after reflow. Our PCB is 1.7 mm thick and baked prior to reflow at 120 C for 6 hours. The process is non RoHS. Peak temp for reflow is 220 C. What could be going wrong?
P.P.
July 2, 2020
Lifted Lead on SOT Component
We are having a high rejection rate with a SOT883 component where one of the small pads lifts off after reflow resulting in an open contact.

What should we do in our process to eliminate this problem?
R.D.
July 1, 2020
Allowable Bow and Twist on Round PC Fab
On my PCB fab drawing I specified - Overall board bow and twist should not exceed 0.005 in/in as per the latest revision of IPC-A-600 standard. I would like to know how you interpret it? If my fab is round, 11 inches in diameter, what is the allowable bow and twist?
B.D.
June 30, 2020
Mixed MSL Baking
If the highest MSL-rated component on a PCB had a 27-hour bake time at 125C (1.4-2.0mm, MSL 3) and the PCB had to be baked to replace a different component that required an 18-hour bake time (1.4-2.0mm, MSL 2), what is the preferred avenue of approach?

1. Bake the PCB for 27 hours at 125C because that's the highest rating of a component on the PCB, regardless of the component needing rework.

OR

2. Bake the PCB for 18 hours at 125 because that's the rating of the component needing rework, regardless of the other components.
B.B.
June 29, 2020
Step Stencil Squeegee Angle
If you have step stencil, what angle should we use for the squeegee when printing solder paste, and what other parameters are important?
P.P.
June 26, 2020
Solder Balling Splash After Reflow
A lot of balls are observed after reflow process along and under SMD components. (see images)We have not found the root cause, any ideas are welcome.

Details:
  • PCB finish surface: HAL
  • Solder Paste: SN62MP100AGS90
  • Reflow oven: Vapor Phase with Galden
  • Problem seen with one specific PCB supplier but not with another one
  • Rebaking the pcb before process do not solve the problem

B.G.
June 25, 2020
Application Using No-Clean and Water Soluble Fluxes
If we use a no-clean solder paste for SMT, is it acceptable to use a water-soluble solder wire for subsequent processes (soldering through hole components by hand, not touching up the original SMT)?

We have some difficult assemblies where water-soluble wire just works better for hole fill.
D.B.