Subject: Heat sinks for ICs in CBM computers


As I promised last week, here is the article regarding adding heat sinking
to the IC chips in a C-64. (Sorry it took so long to post this. I am new
on this system and no one around here can answer my questions. I had to
figure this out for myself... sure hope it works this time!   Ray C.)


     Solid state devices such as Integrated Circuits (ICs) fail for two
basic reasons, assuming they were not defective from the factory...  1.
Overvoltage, either from a defective power supply or a "spike" on one of
the lines in or out of the device, and 2. Excessive heat, which exceeds
the device limits. Long-term temperature cycling also leads to breakdown
after an unpredictable amount of time. The causes of overvoltage by way of
the power supply are almost predictable... the Commodore "black brick" has
a history of catastrophic failure, sometimes taking the computer with it.
After-market repairable supplies are much more reliable because they are 
built with higher current capability than the original. Spikes (static 
electricity or voltage surges) can destroy ICs in the computer through one
of the "accessory" ports. Both of these hazzards are preventable to a
degree... the use of a higher quality supply and simple precautions such
as never plugging anything in (cartridges, mouse, etc.) when the computer
is turned on just make good sense. Static damage can be kept to a minimum
by grounding everything that will be used with the computer before
connecting it. 
     A less predictable but no less important concern is the fact that
repeated heating and cooling of semiconductor devices (especially ICs and
power transistors/diodes) under normal use, even within their "safe"
temperature range, will eventually cause them to fail. It is usually the
devices that generate the most heat in normal operation that fail first.
It's a bit like bending a piece of metal back and forth until it fatagues
and breaks. Repeated heating and cooling does the same thing to the
structure of the microscopic components inside an IC or a transistor. 
     You could leave the computer on 24 hours a day... some systems run
that way. A BBS system is a good example. I know of one system that has
run without a breakdown for years (except for the hard drive power
supplies). For most of us, however, the computer doesn't get enough use to
justify the expense of a system running all the time. An alternate
solution is to reduce the extremes of temperature inside the ICs by use of
cooling fans and/or heat "sinks", and thereby reduce the thermal stress on 
the devices. 
     Microprocessors in new IBM type computers are running so fast
nowadays (60 to 100MHz) that heat sinking and direct fan cooling is
absolutely necessary... those chips would overheat and fail in a very
short time without some method to dissipate that heat! Some of the
Commodore 64 ICs run too hot to touch in normal operation. There are two
simple modifications you can do to improve the reliability of your C-64,
namely, increasing the airflow inside the case, and heat-sinking the ICs. 
     The physical design of the C-64 limits the airflow over the chips.
Tiny air vents in several places in the upper and lower half-shells offer
minimal convection cooling. That cardboard and tinfoil shield, although of
questionable usefullness as an interference filter, is great at blocking
the airflow and holding all the heat inside the case. I have run tests to
see how effective that shield is in reducing radio and TV interference. I
found little difference after removing the shield. In fact, the 1541 (with
all shields in place) generates more "noise" than the C-64. Without the
shield, the C-64 might be more prone to pick up noise from external
sources such as a monitor or TV, but I have never found that to be a
problem (none of my computers has the shield anymore). The increased
airflow without the shield inside the C-64 case helps to keep the chips
cooler. 
     Three of the large ICs tend to run hot: the PLA, the SID and the VIC.
These are the ones that fail most often and would benefit most from "heat
sinking"... drawing away the normally produced waste heat from the device
by means of a metal "fin" attached to the case of the IC. The C-64C (flat
case) and the C-128 have a shield plate with "fingers" that touch each of
the larger ICs and dissipate some of their heat into the surrounding 
air. The SX-64 has individual heat sinks glued to the larger chips. Most
C-64s had no such sinking. 
     Remove the screws from the bottom of the computer, but leave the
keyboard in place, then run the computer for half an hour, lift the
keyboard and feel the tops of all the chips... some will be quite hot!
Those are the ones that most need the help. You can purchase commercial
heat sinks that fasten to the top of the chips with clamps, adhesive
strips or glue. Radio Shack and other parts houses can supply them in a
variety of sizes. If you want to make your own, any thin metal stock will
do. The only consideration is that it be flexable enough to bend without
breaking, and stiff enough to hold its' shape when bent... it must be flat
on the bottom for intimate contact when glued to the chip. I use strips of
"tin" cut from large juice cans. With tin snips, cut a piece of metal
about 4" long and about 1.5" wide from the "un-ribbed" part of the can,
and bend it to fit the top of the IC. Careful! Those cut ends are razor
sharp! The top of a 28 pin chip such as the PLA is approximately 1.5" long
and 5/8" wide. The heat sink is fashioned into a "U" shape with "ears"
that extend out to either side. The sink ears are made high enough to
clear all components on the PC board, but not touching the keyboard when
it is reattached: bend the ears over about 1/2" from the top of the IC. 
The large "surface area" is necessary since the airflow inside the case is
minimal to begin with. 

                 SIDE VIEW OF HEAT SINK ATTACHED TO IC
                  
  |<-------------- APPROX 3" --------------------->|                                                                                                                                                         
  ~~~~~~~~~~~~~~~~~|             |~~~~~~~~~~~~~~~~~~
            .5" /\ |<---5/8"---->|<----- HEAT SINK
              \/   |_____________|
             ----  =============== <------ ADHESIVE
                   |~~~~~~~~~~~~~|
                   |______|______|<----IC
  ___________________!________!______________<----CIRCUIT
                     !        !                    BOARD

     Use fast-set epoxy (15 min) to attach the sink to the IC. If you have
several sinks prepared ahead of time, you can mix one batch and glue them
all before the epoxy starts to set. Print the name or number on the top of
the sink with a felt tip pen so you can still identify the chips when
you're done. Be careful not to use too much glue (one small pea-sized drop
is plenty), or it will ooze out the sides and down into the socket. (Note:
not all chips are socketed.) Spread the epoxy over the sink (or on top of
the chip) and gently press it into place. I don't recommend super glue as
the surface of the metal will not be flat enough for a good bond. Heat
sink compound (white silicon "goo") is sticky enough if the computer will
never be moved, but I don't recommend it either. If the metal sink falls
into the computer, it will most certainly short something out. Epoxy forms
a good bond even with irregular surfaces as it fills the "voids" and
provides sufficient heat transfer to the sink. Just make sure the metal is
free of dirt, oil, fingerprints, etc. 
     As I stated, the chips that benefit most from sinking are the large
heat producing ones, like the PLA (CBM #906114-XX, 82S100PLA, or 93459),
the SID (CBM #906112-01 or 6581), and the VIC (CBM #906109 or 6567). The
VIC chip is already sinked in later C-64s. It's inside a metal box... the
cover has a tab that touches the VIC chip. If yours doesn't have one, sink
it. You can use the "finger" test to see if any other chips in your
computer run hot enough to need sinking (too hot to touch after 1/2 an
hour with the case closed). The only other chip I would sink would be the
MPU (6510), although I have only seen a few bad ones. Make sure the heat
sink fins don't touch any circuit components and cause a short! 
     The two VIA (6526) chips run cool to the touch and don't really need
sinking. Since they connect to the external ports, they are more sensitive
to static "zaps". The ROMs run warm but seem to last forever anyway...
same with the eight RAM chips (4164), at least until the power supply
regulator fails... when RAM chips get hot to the touch, they're gone! 
Normally, they run cool. 
     Simalar modifications can be made to the 1541 disk drive. It's power
supply is internal, making the IC heat problem even worse than in the
computer. The problem with adding heat sinking to the drive is the space
limitations inside the case... it's pretty crowded in there, but it can be
done. Find the chips that get the hottest, and make the necessary
modifications. The heat sinks you make will have to be cut to fit the
space allowed. With these simple modifications, you can keep your little
'64 system humming along for many more years. :-)

                                 Ray Carlsen   CET



