PHIL
IN A PAST LIFE I DID SOME VERY HIGH VOLTAGE DESIGN AND FOUND THAT FR4 TO BE
A BAD CHOICE. THE WEAVE TENDS TO LEND TO ARCHING. WE USED A MATERIAL THAT
WAS A BLOWN RENFORCEMENT (RANDOM DIRECTION OF FIBER AND SHORT FIBERS). I
ALSO BELIVE THAT THE BOARD DID NOT USE A RESIN BASE MATERIAL, BUT A POSSIBLE
PLASTIC. SORRY DON'T REMBER THE NAME OF THE MATERIAL. WILL SEE IF I CAN DIG
IT UP. ABOUT 10 YEARS AGO



    THANKYOU
    JOE KEARY
    SR. PCB DESIGNER
    SYSTRON AND DONNER 

-----Original Message-----
From: Phil Nutting [mailto:[log in to unmask]]
Sent: Wednesday, June 27, 2001 5:36 AM
To: [log in to unmask]
Subject: [TN] Choice of PWB substrate for "oil bath" operation


Hi All,

Here is yet another opportunity for you to lend your wealth of knowledge as
you have many times in the past.

We build high voltage power supplies and some of our supplies require the
multiplier and feedback boards run at voltages of 160keV or higher.  For
these applications we "submerge" the boards in Shell Diala AX oil used as a
dielectric.

Our manufacturing process currently has the boards (FR4) soldered using an
OA flux and then washed in a Batch DI wash which dries for 20 minutes at up
to 150°F.  The boards are then  routed to the next level of assembly, at
which point they may be "sunk" in oil right away or could sit on the
assembly shelf for some period of time before going into the oil.

It is my understanding that FR4 will absorb some amount of moisture.  Is FR4
the best choice for this application?  

Should the boards be baked to drive out any residual moisture right before
"sinking" in oil?  If so, for how long and at what temperature?

While we are on this subject I have some questions about the cleanliness of
the oil.  Up until recently we weren't pushing our output voltages as high
and the level of air, moisture and particulates in the dielectric oil didn't
matter.  Now with the higher voltages (and they are going to go higher) we
see the need to monitor, process and control the "quality" of the oil.  At
the moment we are looking at heating the oil and storing it under a vacuum
to remove the water and oil.   Do any of you have experience with this type
of application?  I'd like to get some benchmarks so I have an idea just how
nuts we have to go in processing the oil.  One of my first places will be to
call Shell oil and get their input, but I wanted to tap into the vast
knowledge base of this esteemed group as well.

Thanks in advance,

Phil Nutting
Manufacturing Engineer
Kaiser Systems, Inc.

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