Date: Fri, 30 Apr 2004 16:16:06 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Hot times under the rear seat
In-Reply-To: <001501c42ee6$c526d9f0$e4e75e44@none9c7r46o4i1>
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At 02:10 PM 4/30/2004 -0500, Stan Wilder wrote:
>So far mine has lasted 29 years without any problems.
How many years? No vanagons in '75, right? ;->
'Tany rate, with the Digifant system VW brought the ignition drive circuit
into the ECU so it now has two power circuits and consequently more
internal heat dissipation. The older ones only have the FI drive, ignition
is an independent system and the Fairchild ignition module has a biggish
heat sink and is bolted to the vehicle body.
"Thermal stress" typically refers to mechanical stresses on solder joints
(and other things too) resulting from cyclic heating and cooling of power
devices (driver transistors and the like) -- this has a distinct tendency
to fracture the joint* which causes intermittent operation and often a
blown device. That failure may begin a cascade of component failures in
some circuits. For example, if your monitor display suddenly collapses to
a vertical line, pretty good chance that the horizontal drive output
transistor has tried to pull itself out of the board and suicided when it
succeeded. I've heard rumors that the Digifant ECU ignition drivers have
this problem.
Cure is to equalize heating of device and board (heat sinks, fans); secure
or redesign the device mounting so that thermal cycling produces less
mechanical stress on the joints (e.g. mechanically secure the device and
use flexible wires from device to board); and reduce the total number of
thermal cycles by leaving the system powered on**. If the Digifant ECU has
a thermal-cycling problem a fan might be very helpful, particularly if the
air were directed to the parts that fail.
*Vibration has a similar effect on components that are supported by their
leads, and adds to the thermal problem. Self-mounted power transistors with
three leads in line [very common] are I think particularly subject to
thermal-stress failures because the leads can use each other for leverage.
Anyone wanting to see a fractured solder joint -- AFM pullup resistor in a
Digijet board -- pmail me as it will be a while before I get it
posted. This was a vibration fracture I imagine, but they look the same.
**Power-cycling also produces significant electrical stresses. I once --
idea suggested by an article in an industry magazine -- designed and built
a burn-in cabinet that would take 5-1/4" floppy-drive controller boards 200
at a time and power-cycle them at x-minute intervals for 24 or 36 hours or
some such. It worked very well except for the big airco unit freezing up
on humid days; the drives had lots of problems but early board failure
wasn't one of them.
:)
david
