Thermal mismanagement wreaks havoc with an avionics display dumping lots of heat
Thermal mismanagement wreaks havoc with an avionics display dumping lots of heat.
In the early 1980s, I worked for a company that built avionics displays
and display drivers. This was "back in the day" before video driver
circuitry was so compact. The equivalent circuitry to today’s ~20-square-inch video driver card would have taken a whole box, and a large
one at that!
In order to alleviate the size issue, they would sometimes
use a "Stroker" (as opposed to "Raster") display to simplify the
circuitry and save power and cooling. A Stroker display is one that
relies on directly steering the beam with the CRT deflection coils to
form map outlines, letters, and numbers on the display.
analogous to how you would draw the letter "A" with a pencil (go ahead,
try it). Where you actually draw a line consider the beam "on."Where
you move a space without drawing, consider the beam "off." This was an
ingenious solution to display design problems, although the display DID
suffer from a pronounced flicker when the screen got too crowded.
For instance, when the CPU wanted to draw a line on a 640 X 480
display, it would execute two writes to the X and Y position registers
(e.g. 100/100 and 200/200) which would in turn draw a line from
position (100,100) to position (200,200) on the screen. It would have
to re-execute these writes to the registers periodically to keep the
line displayed on the screen. The refresh rate was dependent on the
persistence of the phosphor on the screen.
A friend of mine was tasked with designing the display driver cards for
the new display. The old display drivers gave off quite a bit of heat,
due to cooling the TO-3 packaged driver transistors.
TO-3 is a diamond-shaped package (when viewed from the top), about
1.5 X .75 inches in size.
In the 1980s it was probably the highest
heat-dissipation package available. Due to the difficulty of
dissipating heat in avionics, the TO-3s were conduction cooled. This
meant that the circuit board had a beryllium-copper heat sink laminated
onto it and the ICs as well as the driver transistors were mounted
through it. The heat sink was thermally connected to the ICs with
heat sink grease and to the chassis with "Wedge-Lok" ™ fasteners.
This laminated construction was expensive to design and build (plus
there were occasional problems with circuit traces shorting to the
heat sink under vibration), so the engineering team opted for a
different sort of design which, in retrospect, seems more like a "Rube
Goldberg" special than a legitimate design.
The TO-3s were bolted and soldered
to the board through a small heat sink block that just surrounded the
TO-3 package and was slightly taller. A heat sink plate was then screwed
to the heat sink block which was itself fastened with "Wedge-Loks" ™ to
the chassis. A "Sil-pad" ™ was used in each of the junctions to promote
This design was marginal due to the thermal conductivity of the extra
interface between the heat sink block and the heat sink plate. To add
insult to injury, the heat sinks were joined with stainless steel screws
mated to Helicoil ™ inserts in the heat sink block. Although the thermal
design simulation looked OK, I rather doubt that anyone seriously
looked at the screw/ Helicoil ™ contribution to thermal resistance.
All of this analysis is in retrospect, however. The actual failure was
kind of scary. While the aircraft was tested on the ground with
piped-in chilled air, everything worked OK. The problem occurred when
the aircraft was at altitude. Commonly, aircraft cabins are pressurized
to an 8,000 foot altitude (this rarified air makes cooling more
difficult) and this is what ultimately caused the reported failure. The
equipment operator reported smoke in the cabin, the equipment was shut
down, an in-flight emergency was declared, and the crew went on oxygen
while they tried to vent the smoke from the cabin.
The "slagged" mass of circuitry was evaluated and the following
conclusion was drawn: the TO-3 package had heated up sufficiently to
melt the solder used to connect its leads to the circuit board. The
solder then ran down and shorted the high-power driver transistors,
causing burning of the circuit board laminate. The problem was
alleviated (as opposed to "fixed") by soldering the TO-3 leads with
high-temperature solder (90/10 Sn/Pb). Additionally, the Sil-pads ™
were upgraded to ones with lower thermal resistance, and the heat sink
screws were tightened routinely. A routine fix for a potentially deadly
Dwight Bues is a Georgia Tech Computer Engineer with 27 years
experience in Computer Hardware, Software, and Systems and Interface