Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


defect on PCB acrylic conformal coating

Views: 1924

#68867

defect on PCB acrylic conformal coating | 8 May, 2013

we are using acrylic conformal coating on the PCB. Under the same program, same parameters (not changing anything on the machine), and we always found one or two boards have some spots which are not coated (shown as picture). The location of the spot is random. The boards before or after the defect board are fine (no defect).what is the problems?

Attachments:

reply »

#68869

defect on PCB acrylic conformal coating | 8 May, 2013

What happens when you rework those non-coated areas? Please list your rework steps

Br, davef

reply »

#68875

defect on PCB acrylic conformal coating | 9 May, 2013

this is one of my project. I have tried to remove coating only on those areas with alcohol and thinner, then program the machine to coat the removed area again. But the result obviously shows that two different tones of coating on the board. I also tried to clean the whole board with alcohol and thinner, then recoat it. but the result tells that the spot get worse and more after the second time coating. I think it is because the thinner and alcohol are incompletely evaporating. the residue still leaves on the board to against the coating. finally, i am going to try a long waiting time to evaporating the residue in the board, and air spray the board before the second run.

But my question is what causes this spot defect?

reply »

#68878

defect on PCB acrylic conformal coating | 9 May, 2013

First, it appears that your solder mask and your coating material appear to be incompatible.

Second, IPA leaves a residue that's probably not compatible with your coating material, either.

IPC-HDBK-830 - Guidelines for Design, Selection and Application of Conformal Coatings talks to a process for evaluating the compatibility of coating materials with other process materials.

In his book [Cleaning and contamination of electronics components and assemblies, Brian N. Ellis, Electrochemical Publications Limited, 1986, 0901150207], BEllis recommended not using a solder mask if you intend to coat, provided you can get away with it.

reply »

Used SMT

ii-feed SMD Intelligent Feeder