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This invention relates generally to thermal testing of integrated circuits,
and more specifically to techniques for mounting a thermocouple on
multichip modules for measuring temperature during active operation of the
chips.
BACKGROUND OF THE INVENTION
The use of thermocouples permanently mounted onchip substrates to monitor
temperatures during the chip-manufacturing process is well known. Also,
permanently affixed thermocouples have been employed to measure the
temperature of chips during their actual operation in a completed computer
system.
Where permanently mounted thermocouples are undesirable or unnecessary, a
crude form of manual installation of a thermocouple has previously been
employed on the back of multichip modules used in earlier computers. Such
a thermocouple was temporarily installed for conducting tests to measure
module operating temperatures under various system conditions. The
thermocouple was attached directly to the module substrate using epoxy.
Each installation required careful tedious manual labor and the use of a
microscope to install the thermocouple. Extreme care was required to avoid
damaging the module, both in the installation procedure as well as in the
removal procedure after the test was completed. Many problems were
encountered due to factors such as human error in properly mounting and
locating the thermocouple. Also, there was often a problem of incomplete
epoxy adherence to hold the thermocouple securely in place. Each different
location in the substrate which was thermally tested often required a
completely new assembly and always required a customized manual
installation. In other words, once a test was completed and the
thermocouple removed, the thermocouple was not reusable in a subsequent
test without having to go through another tedious customized installation.
In some instances the removal procedure would even cause irreparable
damage to the thermocouple.
As more and more integrated circuits are packed together on single and
multilayered modules and boards, the collective generation of heat becomes
a serious problem. It is now common to provide either an air cooled or
liquid cooled heat sink on top of a multichip module, and to provide
multiple pins extending from the bottom for plugging into multilayered
circuit boards. A typical multichip module is the water cooled thermal
conduction module (TCM) described in the article entitled "Thermal
Conduction Module: A High-Performance Multilayer Ceramic Package", IBM J.
Res. Develop., Vol. 26, No. 1, January 1982, pp. 30-36. A typical
multilayered circuit board comprising a 20-layer composite is described in
an article entitled "A New Set of Printed-Circuit Technologies for the IBM
3081 Processor Unit", IBM J. Res. Develop., Vol. 26, No. 1, January 1982,
pp. 44. In a typical present computer system, a single TCM can contain as
many as 1800 pins, and nine of the TCMs can be mounted on a single
multilayered circuit board. It is not unusual for a single module to
dissipate several hundred watts of power, and future power consumption is
expected to be significantly increased for large individual modules.
In earlier times, when a component overheated and was damaged, it was
economical to merely replace it with another component. However, multichip
modules and multilayered circuit boards have become too expensive for
frequent replacement. This creates the need to provide more reliable
modules and circuit boards, and such reliability can only be assured by
keeping their operating temperatures below acceptable maximums. Therefore
it becomes very important to conduct thermal tests of the prototypes
during actual operation (i.e., when the module is plugged into the board)
to determine their expected operating temperatures, before embarking on
production. Additionally, it becomes very important to test the first
production units for excessive temperature during actual operation before
shipping the computer system to customers. In that regard, it is the
breakdown of pin lubricating oil at high temperatures that is one of the
dangers to be avoided by assuring that the modules do not overheat during
operation. Such oil breakdown can permanently damage the modules so as to
be inoperable. Finally, it may become desirable to be able to test units
which are in the field to locate excessive heating problems. Any time a
change is made to any part of a computer system, such change may have an
adverse effect on some other part of the system. The ability to make spot
checks during actual operation in order to monitor temperature is becoming
more of a necessity than merely a desirable option.
There is also a technique which has been developed in order to predict the
operational life of computer components, called dynamic burn-in. In such a
technique, the computer system is deliberately stressed beyond its normal
operating limits for the purpose of accelerating early-life defects. Such
a dynamic burn-in will necessarily cause the circuits to heat beyond
normal specifications, and it is very desirable to have a thermocouple
unit monitor the temperature of critical locations on a module or board to
be sure the temperature does not exceed acceptable tolerances during the
testing procedure.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
A primary object of the invention is to eliminate much of the repeated
manual labor in thermal testing of multichip modules by providing a
thermocouple template which is reusable.
Another primary object is to provide a thermocouple template which carries
multiple thermocouples for simultaneously testing various locations on the
same module and/or board for the purpose of determining chip module
temperatures or board temperatures, and also the temperature gradients
between such different locations.
A further important object is to form the template of a thin non-conductive
material to avoid interfering with the mounting and operation of the
module and/or board during the testing operation.
An additional object is to design the template for mounting on the pin side
of the module by putting holes in the template in a pattern which matches
the pattern of the module pin topology on the substrate.
Yet another object is to use very thin tape for holding the thermocouple in
place on the template and for protecting the template from the pin
lubricants and for preventing the non-conducting material from
disintegrating due to repeated use and handling.
The invention includes a unique method of making the thermocouple template
including the steps of drilling holes in an insulating sheet using the
pre-determined pin pattern of the module; cutting out a piece of the
insulating sheet to a certain size; marking the position on the piece
where the thermocouple bead(s) will be located; fabricating the
thermocouple from two wires including welding the bead at one end of the
wires and installing at the other end of the wires a connector plug of the
type insertable into a data logging unit; placing each thermocouple in
their proper position on the drilled sheet; applying a thin layer of tape
to hold the thermocouple against one side of the drilled sheet; and
piercing holes in the tape to correspond with the holes in the drilled
sheet.
The completed thermocouple template is easily installed or removed from a
module by sliding it over the module pins which allows for repeated use of
the template on several modules without additional use of specialized
equipment or labor. The shape and thickness of the template allow the
module/template composite to be installed on a board without damaging the
module and without interfering with the normal operation of the chip
circuits or the board circuits because of electronic noise, distortion and
the like. Moreover, by using the thermocouple template, it is possible to
obtain a direct temperature measurement in some critical areas, and a
temperature of sites which are closely proximate to other critical areas,
both of which are more accurate alternatives than attempting to predict a
maximum temperature based on the thermal behavior of very remote but more
easily accessible sections of the component.
In addition, up to four thermocouples have been installed on a single
template in order to simultaneously monitor the thermal changes on the
surface of a single module. Of course it is possible to install more if
needed, all within the spirit of the invention. Accordingly, in the
preferred embodiment, at least one edge of the template includes an
enlarged border displaced from the module pins so that one of the
thermocouples can be installed on such border to provide a base
temperature for comparison with the temperatures detected by the other
thermocouples. It was also found preferable to wrap the thin layer of tape
on both surfaces of the sheet for additional strength and better sealing
and better protection of the insulating sheet, and to extend the useful
life of the template.
cl BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram showing a single terminal thermocouple embodiment
and a four terminal thermocouple embodiment mounted on the pin side of two
separate multichip modules;
FIG. 2 is a simplified schematic drawing showing a sectional view of a
single terminal thermocouple template mounted under a module which is
plugged into a multilayer board;
FIG. 3 is a fragmentary closeup view showing how a single terminal
thermocouple is positioned between the pin holes on the insulated sheet of
material;
FIG. 4 is an exploded view of a four terminal thermocouple template;
FIG. 5 illustrates the presently preferred manner of wrapping a thin layer
of tape on both sides of the insulated sheet without any overlapping on a
four terminal thermocouple template;
FIG. 6 is a bottom view of a four terminal thermocouple template mounted on
the pin side of a TCM unit;
FIG. 7 is a bottom view of the pin pattern of a multichip module having
1800 pins, and showing a set of typical locations for the beads of three
thermocouples on an insulated sheet designed for mounting on one quadrant
of the multichip module; and
FIG. 8 is an enlarged schematic drawing showing an appropriate mounting
space for a thermocouple template between two exemplary electronic
components.
DETAILED DESCRIPTION OF PREFERRED AND ALTERNATE EMBODIMENTS
Generally speaking the invention contemplates the use of a thermocouple
template which is custom designed for repeated testing of the thermal
characteristics of a multichip module having a predetermined pin pattern.
Typically the pins are designated for different functions such as signals,
specified positive and/or negative voltages, electrical grounding, and the
like. The beads for each thermocouple are precisely located on a template
between the pins so as to be adjacent to the critical pins when the
template is mounted on the pin side of the module. For example, it is
often desirable to have at least one of the thermocouple beads located
directly under the center of one of the chips.
When it is considered necessary to conduct a temperature monitoring test,
the thermocouple template is pushed onto the pin side of the module before
plugging the module into its matching holes on a printed circuit board.
Various operations are then performed on the computer system or portions
thereof while dynamically recording the temperature changes on a data
logger which is connected to the two wires of the thermocouple. Such
operations can be diagnostic routines and normal instruction processing,
which can also include specialized testing operations such as dynamic
burn-in.
After completing the test, the module is unplugged and the thermocouple
template removed from the pins and reused again on a different module or
reused again at a later time on the same module. In order to facilitate
such reusability, a thin layer of tape on both sides of the template
serves to hold the thermocouple wires and bead in proper position, while
at the same time protecting the insulating sheet from disintegration
because of repeated use and handling as well as preventing absorption of
the lubricating oil from the surface of the pins.
In the preferred form, the template covers only one of the quadrants of the
module, since monitoring the temperature of one-quarter of the substrate
is usually sufficient. For example, by considering the chip pattern in the
module and/or the placement of the module on the board, it is often
possible to tell which of the quadrants is most likely to have the highest
operating temperature, and therefore that is the quadrant on which the
template is mounted. It is important, among other things, to be sure that
the temperature does not reach the breakdown point for the pin lubricant.
If that occurs, some lubricants turn into a varnish-like substance which
opens up the pin contact with its matching component thereby causing loss
of conductivity. Larger or smaller thermocouple assemblies can be used if
needed to cover less than a quadrant or else the entire substrate, all
within the spirit of the invention.
While a certain precision is required in the assembly of a thermocouple
template of the present invention, it is a relatively simple matter to
mount the thermocouple template before testing and an equally simple
matter to remove the thermocouple template after testing. The risks of
physically damaging the module or its pins during such mounting and
removal is virtually eliminated. Moreover, consistency of test results is
increased since the thermocouples can be remounted in the very same
location for each and every test. Thus, a set of norms can be established
for any particular computer system, and subsequent testing in the research
lab, or on the production line, or in the field can be considered very
reliable without the need for highly skilled persons to administer the
test. The size of the thermocouple template unit is small and compact
making storage between tests very easy. Finally, the cost of materials is
negligible and the time spent to assemble them into a finished
thermocouple template is short compared to the time and expense required
to manufacture and install replacement modules and/or replacement boards
damaged due to overheating.
Referring now to FIG. 1, a number of multichip modules 10 are typically
pluggable into a board such as a laminated printed circuit board 12. A
single terminal thermocouple template 14 is mountable on the pin side of a
module 10 with a single monitoring site 16 to which is connected a
conventional two-wire thermocouple 18 having at its loose end a
conventional connector 20. A conventional data logger such as a
multi-input unit 22 transforms the changes in voltage across the
thermocouple into temperature changes which are then recorded for
immediate checking as well as future analysis. Also shown is a four
terminal thermocouple template 24 mountable on the pin side of a module 10
with four separate monitoring sites 26, 28, 30, 32 respectively connected
through two-wire thermocouples 34, 36, 38, 40 to connectors 20. Thus, test
data can be simultaneously obtained from identically located sites on
different modules, and/or from differently located sites on the same
module.
Although various type of thermocouples can be employed with the present
invention, suitable results were obtained by using a data logger for
recording ANSI type T (Copper-Constantan) thermocouples with miniature
connectors (Omega type NMP or equivalent). Also, it was found helpful to
use a straight microprobe tool for enlarging holes in the thermocouple
assembly and for removing the template from the substrate. Flat tipped
tweezers were also found to be helpful in the removal step.
The relative dimensions in FIG. 2 are exaggerated in order to show the
various components of a thermocouple template actually mounted on the pin
side of a multichip module during a testing operation. A substrate 42 is
used for carrying multiple chips 44, with some form of cooling unit such
as 46 often located on top of the module immediately adjacent the chips.
On the bottom of the module are numerous pins represented by 48 which pass
through an insulated sheet 50 and two layers of tape 60 down into the
board. On the top surface 52 of the insulated sheet 50 is a double wire
thermocouple 54 which has its end wires 56 stripped of insulation in order
to be joined together through a welded bead 58. A thin layer of tape 60
covers the top surface 52 and holds the double wire thermocouple as well
as its welded bead securely in position. In the preferred form, the tape
also covers the bottom surface of the insulated sheet 50. Through
experimentation it was found desirable to cover the bead 58 and stripped
wire ends 56 with a layer of epoxy 62 to provide electrical insulation
from the pins as well as additional attachment to the sheet. After the
layer of epoxy is applied, a resistance meter is used to check the
sufficiency of the insulating qualities of the epoxy layer. It will be
appreciated by those skilled in the art that there are various types of
multichip modules having various densities and patterns of pins of
differing lengths which are pluggable into spring loaded or friction fit
receptacles housed in all kinds of circuit boards, and the illustration of
FIG. 2 is merely a schematic representation for purposes of illustration
only.
A closeup view in FIG. 3 shows how a two wire thermocouple is positioned
between holes 66 on the insulated sheet. These holes are drilled in a
pattern which is identical to the pattern of the pins of the multichip
module. By providing a bend 68 immediately before the welded bead
connecting the two wires, it is possible to precisely locate the
monitoring site of the bead as well as provide stable mounting on the
sheet 50. The exploded view of FIG. 4 clearly shows an alternate structure
for a thermocouple template having four thermocouple 70. In this version,
a perforated sheet 72 is sandwiched between a top tape layer 74 and a
separate bottom tape layer 76. An assembly ID 78 is permanently marked on
one corner to indicate which module and which testing site locations are
associated with this template. A handle 79 is provided to facilitate the
mounting and removal of the template relative to the pins of the module,
and a tape extension 80 is provided to cover the handle. Widened edges 82,
84 on the outer perimeter of the sheet 50 have no perforations and
therefore provide a neutral site for measuring temperature as well as add
strength and stability to the template.
In contrast to the separate tape coverings of FIG. 4, the preferred
structure has the thermocouples mounted on the top surface 86 of the sheet
(See FIG. 5) and held there by a continuous central portion 88 of tape
which has tapered flaps 90 extending outwardly in all four directions. The
flaps are intended to be folded downwardly in the direction of the arrows
in order to cover the bottom surface (not shown) of the sheet, and an
additional flap 92 is provided for folding down to cover the bottom of the
handle. It is highly desirable to keep the overall thickness of the
thermocouple template small, typically less than one mm throughout its
entire area. Accordingly, the shape of the flaps is designed to prevent
tape overlap and to seal the edges of the sheet while still leaving an
access passage at the corner for the thermocouple wires. Conformity to
these thickness specifications is also assured by using insulating paper
of 0.3 mm thickness and transparent tape #681 made by the 3M Company of
approximately 0.1 mm thickness. Epoxy used to insulate the bead and bare
wire ends was ScotchWeld brand epoxy. Additionally the thermocouple wires
are not allowed to overlap each other on the insulator sheet. Finally,
care must be taken in welding the bead to avoid creating too thick a bead.
FIG. 6 shows the underside of a TCM unit 100 having the usual components
such as a base plate 102, corner brackets 103, interior flange 104, and
substrate 106. The number of pins 108 is not actually shown in the
drawing. The drawing does show how a four terminal thermocouple template
110 appears mounted thereon with the pins 112 passing completely through
the template. All of the pins in the quadrant covered by the template are
in the potential monitoring area for any of the four thermocouples.
FIG. 7 does show the actual pattern of pins on a multichip module 114
having 1800 pins in a pattern. Also shown is the manner of precisely
locating each of the three thermocouples A, B. and C. In other words, the
thermocouple bead is to be located immediately adjacent the pin or pins
which are identified by their coordinates shown at the bottom of the
drawing. Of course, the invention is not limited to the specific pin
pattern shown or to the number or location of testing sites shown, but the
illustrated embodiment of the invention shown in FIG. 7 serves to indicate
the incredible number of pin positions found on multichip modules while at
the same time showing the capability of the invention to monitor sites
immediately adjacent to each and every one of those pins in the particular
area covered by the template.
The schematic drawing of FIG. 8 shows how a thermocouple template can be
positioned for thermally monitoring electronic components while they are
interconnected. More specifically, a multilayered board 120 has a
conductive sleeve 122 for receiving a lower end 124 of a contact spring
126. The contact spring has a bifurcated upper end 128 which is aligned to
engage one of many pins 130 extending from the pin side of a TCM type of
multichip module. The features of a TCM are well known in the art,
including substrate 132, chip 134, hat 136, cold plate 138, and piston 140
which is held against the exposed face of the chip by a spring 142. It is
apparent that the overall thickness of the thermocouple template is
sufficiently thin so as to fit between the two interconnected components
without interfering with their normal operation, thereby allowing easy
testing of various predetermined sites simultaneously or sequentially to
measure their respective temperatures. It will be seen that the exposed
face of the chip is inaccessible to direct temperature monitoring, but the
unique shape and construction of the thermocouple template nevertheless
still allows the proximity of the testing site to be close enough to the
chip and other pertinent areas to provide satisfactory testing.
It is to be understood that the length of the thermocouple wires between
the template and the data logger can be extended in order to reach modules
located almost anywhere. In this regard, wire lengths of 16 feet have been
successfully used. The type of wire preferably used is 36 gauge having a
diameter of 0.13 mm (0.005 inches). Finer wire can also be used.
As a result of experimentation, a presently preferred sequence of assembly
steps has been developed as follows. First a drawing is prepared much like
FIG. 7 to show the size and layout of the thermocouple template for a
specified multichip module. Using that drawing, holes are drilled in the
insulator sheet to match the pin pattern of the module, a piece is cut out
of the sheet, and the piece is marked to show the position(s) where the
thermocouple bead will be located. The required lengths of the
thermocouple wires are cut to size, and the bead connecting the two wires
together is formed by soldering and/or welding. In forming the bead, the
two exposed ends of the wires can be wound together before welding or can
be positioned a short distance apart and then joined through a bead formed
by conventional arc welding. Although in some instances the bead will have
a diameter approximately the same as the diameter of the wire, it is still
acceptable to have an enlarged diameter bead so long as the bead diameter
is less than three times the diameter of the wire.
The connector plug is now attached at the other end of the two wires. At
this stage the thermocouples are checked for proper operation. If the test
is satisfactory, the bead and exposed wire near the bead are coated with a
thin coat of epoxy which is allowed to dry. The thin coat of epoxy is then
tested to assure that it provides complete electrical insulation of the
bead and exposed wires. In making the thermocouple bead, it is desirable
to minimize the length of exposed wire welded to form the bead. As a
further precaution, the wire adjacent to the connector is doubled over so
that it lays on the surface of the connector plug, and such doubled over
portion is taped to the connector for strain relief. This prevents the
fragile wires from breaking loose inside the connector during handling.
Next the thermocouple is placed on the drilled cutout piece of insulator
sheet so that its location and orientation matches with the drawing, and
it is temporarily held in place with small strips of tape. In earlier
experimental versions, small beads of epoxy were used instead of the tape
strips. By this point it is important to have marked the thermocouple
connectors with identification numbers such that they can be correctly
identified later. Where multiple terminal thermocouples are involved, they
are successively mounted in their appropriate places on the insulator
piece. A portion of tape is now cut to be a size large enough to cover the
thermocouple(s) as well as the entire surface on which they are mounted,
with enough left over to fold over the edges to the other side (See FIG.
5). Lay this piece of tape flat on a clean work surface with the adhesive
side facing up, and place the thermocouple/insulator piece composite on
the adhesive tape with the thermocouples facing the tape. Avoid wrinkling
the tape while pressing the composite in position by applying uniform
pressure against the composite so that it sticks to the tape. The wires of
the thermocouple should not be allowed to cross over each other on the
insulator piece, since the completed thermocouple template must have a
minimum thickness throughout. Cut out the excess adhesive tape around the
corners such that when it is folded over to the other surface of the
insulator cardboard piece, there are no tape overlaps. Fold the tape over
to the other surface and thoroughly press the tape so that it is securely
adhered on both surfaces of the insulator cardboard piece to seal the
cardboard while at the same time holding the thermocouples in position.
Where necessary, additional pieces of adhesive tape may be applied to
cover over any exposed areas on the back side of the insulator piece,
avoiding overlap and wrinkles.
Place the assembled template on a soft flat surface such as balsa wood,
with the thermocouples facing up. Take a dental pick having a straight
pointed end and use it to pierce holes in both layers of adhesive tape
corresponding to the holes previously drilled in the insulator sheet.
After piercing all of the holes, try the assembly out on a module,
enlarging the holes as needed to assure easy mounting and dismounting of
the thermocouple template on the pin side of the multichip module. All the
thermocouple wires for each assembly can now be braided together starting
from near the assembly and proceeding toward the connector end. This
prevents the individual wires from becoming tangled together or tangled
with wires from other assemblies. All of the wires of the completed
assembly can now be suitably wrapped and held such as by a small piece of
tie-wrap, so that the completed testing template unit can be stored in an
appropriate clean box when it is not in use. It will be appreciated that
the foregoing assembly steps require no specialized tools and no expensive
materials, and the completed thermocouple templates can be reused over and
over again in order to provide accurate and reliable test data for
multichip modules and adjacent components which data is dynamically
acquired during actual operating conditions.
While specific embodiments of the invention have been shown and described,
it will be appreciated by those skilled in the art that various changes
can be made and various revisions can be adopted, all within the spirit
and scope of the invention as set forth in the following claims.
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