Conformal Coating of Circuit Boards

[scott.john.mockry@bangate1.TEK.COM]

Anton,

The MAC-02 (ur-Quattro),  MAC-11 (5000TQ), and 
the MAC14 (200TQ, 10V) ECU's I have looked at
all have used the conformal coating. They have the coating
on both sides of the circuit boards as well as on the
ribbon cables connecting the two boards (MAC11/14).

In order to remove the EPROM from these circuit boards
you usually have to  clean off the coating on the top of the 
EPROM as well as on the back side pin area prior to 
de-soldering this chip.  The conformal coating has a tendency
 of pooling underneath the EPROM chip and acting like a glue
 to hold the EPROM  on the circuit board This makes removal 
a real pain in the butt. Even when you use a vacuum de-soldering 
tool it is sometimes better to just clip all the pins off the EPROM 
to assist in the removal and to avoid pulling any runs off the board. 

I have not recoated the few ECU boards that I modified by
adding an EPROM socket but it would be a good practice to get into. 
This is especially true in areas where high temperature/high humidy 
could cause  a problem,  (Dave Head are you listening out there in 
Florida?)
 The Audi ECU's are mounted inside the vehicle which helps  protect 
them a little but  the typical automotive environment is pretty harsh for 
electronic
components with the wide swings in temperature and humidy.

Most of the oscilloscope instruments we manufacture do not use any
conformal coating on the circuit boards even though the instruments  are
rated for use in 95% Relative Humidy and at 40-50 degrees C.
Our board build facility does run the boards through an extensive
cleaning procedure to remove any flux residue or contamination 
from  the boards  after they go through the flow solder and SMT 
reflow machine. 

Scott M.