System for chip joining by short wavelength radiation

Defective chips are removed from a substrate package. The package is cleaned. Replacement chips with solder bearing elements are replaced in the position(s) of the defective chip(s). Silicon chips are less damaged by heating with light wavelengths substantially shorter than infrared radiation, when the radiation is directed upon the upper chip surface and the lower chip surface carries circuitry and solder balls. Radiation is absorbed by the upper chip surface and converted there directly to heat, protecting the circuitry below. An argon-ion laser beam confined to a given chip is directed upon the upper surface of the chip to be soldered in place. A thin laser beam can be scanned under computer control across a chip to heat the areas of a chip above solder balls. Automatic temperature control of the chip can be provided by a heat detector or chip condition detector and a program controller in a feedback loop controlling laser power.

DESCRIPTION 
1. Technical Field 
This invention relates to metal working systems and more particularly to 
systems including assembly into electrical circuits of integrated circuits 
which are replaced to repair the assembly. 
2. Background Art 
In reworking of LSI multichip packages in which solder ball packaging is 
employed, in the past, removal of defective chips has involved pulling or 
ripping away the old chip, cleaning the remaining solder away 
mechanically, reinserting the replacement chip and then subjecting the 
entire package to the standard solder reflow heating which involves 
heating the entire package including every chip. This has been in many 
cases undesirable since the reheating has proved to be damaging to the 
chips which were not being replaced. 
U.S. Pat. No. 3,735,911 of Ward, commonly assigned, describes a flame 
heating system producing infrared heat, which includes an infrared 
temperature sensor for controlling the temperature of the heated chip by 
turning off the flame after the chip temperature exceeds a permissable 
value. 
U.S. Pat. No. 3,402,460 of Smith describes direct laser heating of leads 
and an exposed P- or N-type area to which the lead is to be bonded. The 
laser beam reaches directly to the lead and a doped silicon region of a 
semiconductor device heating the conductor to fuse it to the silicon. The 
laser wavelength was selected to be 10,600 A (neodymium laser) in order to 
produce light near 11,000 A which is the critical wavelength of silicon at 
which the absorption coefficient of silicon is relatively low so that the 
energy of the laser beam will pass through the silicon and will not be 
concentrated at the surface of the silicon chip to avoid thermal damage 
because the heat penetrates deeply into the wafer rather than 
concentrating at the surface of the wafer. 
U.S. Pat. No. 3,614,832 commonly assigned also teaches laser welding of 
leads to a chip by directing the beam at the top of a chip to join a 
conductive strip 21, a lead 13 and metallic contacts 17 by a solder reflow 
technique using 90% lead, 10% tin solder, where the lead and the contact 
are precoated with tin solder. The variety of laser is not identified. 
U.S. Pat. No. 3,970,819 describes heating the back side of a semiconductor 
chip with a laser to reduce the breaking strength of the chip so 
individual devices on the chip can be separated into individual chips. 
U.S. Pat. No. 3,934,073 shows a technique for using a laser to bond a lead 
to a land on a chip. The difference is that the lead is supported on a 
glass substrate and the laser beam must be shone through the glass to the 
lead. A Korad ruby red laser was used. No suggestion of an opaque 
substrate, application to the back of the chip, etc. is disclosed. 
U.S. Pat. No. 3,435,186 is of interest as to laser machining of the back of 
an object but the substrate is transparent and the coating is opaque. 
U.S. Pat. No. 3,485,996 of Chiou et al shows passing laser light through a 
transparent substrate to heat metallization to weld it to lands on a 
substrate and a chip, with feedback control of the laser based upon what 
metal is struck by the beam, gold or alumina. 
In D. Giacomo et al "Prevention of Land Opens During Infrared Rework of 
Chips," IBM Technical Disclosure Bulletin 20, No. 8, 3216-3217 (January 
1978), a chip is heated by a source of radiation, the portion of the 
spectrum above 1 micron is filtered so that only the spectrum of 0.25 to 1 
micron is allowed to irradiate the chips.

DISCLOSURE OF INVENTION 
This invention is performed in a chamber from which air has been evacuated. 
The chamber is then backfilled with an inert gas in order to prevent 
oxidation of the solder and the pads and to prevent carbonization and 
breakdown of the flux used in the soldering process. Because the process 
must be performed in an enclosed, sealed chamber which prevents easy 
access to the work, an automatic process maximizes efficiency. As only 
some chips need to be replaced and as heating of a package carrying 
several chips to high temperatures involves the risk of damaging the 
chips, an important advantage of the present invention is that it provides 
a way in which an individual chip can be heated to a sufficient 
temperature to solder it in place without unduly heating the neighboring 
chips upon the substrate package. In addition, this process takes into 
account the discovery that chips will be less damaged by heating with a 
source of light having wavelengths substantially shorter than infrared 
radiation, when the radiation is directed upon the upper surface of a 
silicon chip (or the like) when the lower surface of the chip carries 
circuitry and solder balls. Since silicon is opaque to shorter 
wavelengths, the radiation is absorbed by the upper surface of the chip 
and converted directly to heat there, protecting the metallization, 
transistors and/or other elements of the circuitry below. Specifically, a 
short wavelength argon-ion laser beam confined to the surface area of a 
given chip is directed upon the upper (back) surface of the chip to be 
soldered in place. The laser light is absorbed by the upper surface of the 
chip and is not transmitted through the silicon of the chip, which has a 
critical wavelength at 11,000 A, at which the absorption coefficient of 
the silicon is very low. The light has been selected to have a wavelength 
at which the silicon appears to be opaque, i.e., the absorption 
coefficient is very high. As a large source of noncoherent light with 
desirable wavelength characteristics has not been identified to date, a 
coherent source of light such as a laser whose intensity can be controlled 
by a temperature monitor is the preferred source. 
FIG. 1 shows a cylindrical vacuum chamber 10 containing a work support 11 
carrying a ceramic substrate package 12. A plurality of multicircuit chips 
14 is supported upon the ceramic substrate package 12. Replacement chips 
14 are to be electrically and mechanically secured to conductive contact 
pads upon the package 12 by means of conventional solder balls 15. 
In a process known as "reworking," a chip 14 which is defective is removed 
from package 12 by ripping or pulling the chip away and cleaning away the 
solder remaining upon the pads, as is well known in the art. 
This invention pertains to the process of replacing the chip removed with a 
new chip of similar characteristics but without the defect. The chip 14 to 
be joined to the package 12 has solder balls 15 and flux beneath it which 
must be protected from oxidation, as explained above. Thus, prior to 
joining the chip 14 and the package 12, the cylindrical vacuum chamber 10 
is evacuated by roughing pump 19 and diffusion pump 20 connected by 
conduits 21 through opening 22 in the sidewall of chamber 10. 
Subsequently, the chamber 10 is refilled via valve 62 and line 61 with an 
inert gas such as argon, nitrogen, hydrogen, neon, xenon, krypton, helium, 
or mixtures of the above, etc. for the purpose of protecting the solder 
balls 15, the pads and the flux while they are being heated. 
In FIG. 1 three chips 14 are shown to represent a large number of chips in 
a single row with several rows of chips extending back into the page. The 
chip 14 in the center is the one which is being joined to package 12 by 
means of heat from a square cross-section laser beam 31 for the square 
chip 14 (chips of different shapes would be heated by appropriately shaped 
beams). The source of the beam 31 is a laser 26, preferably an argon-ion 
laser which emits energy at wavelengths from 4579 A to 5145 A. Those 
wavelengths are so short that the chip 14 having a silicon substrate has a 
high absorption coefficient of about 1.5.times.10.sup.4 cm.sup.-1 at those 
wavelengths. This means that the entire incident energy of the laser beam 
31 is absorbed within the first 10 micrometers of thickness of the chip 
14. Since the chip 14 has a substrate thickness of several hundred 
micrometers, the circuitry on the lower surface of the chip is protected 
from overheating by the large energy absorbing capacity of the silicon in 
chip 14. The chip 14 is heated relatively uniformly because of the shape 
of the laser beam 31 and the absorption of the heat on the top surface of 
chip 14 from which it passes very quickly to the lower surface because the 
chip is so thin. 
As stated above, the laser beam 31 is shaped to match the shape of the top 
surface of a chip 14. For a square chip, a square beam 31 is projected. 
The beam 29 from laser 26 is not square and it does not have the usual 
Gaussian variation in intensity of usual laser reflecting mirrors. 
Instead, the beam 29 leaving laser 26 is cylindrical and is of relatively 
uniform intensity everywhere within the cross-section of the beam. This is 
achieved by replacing the usual mirrors by more concave mirrors 
(reflectors) at opposite ends of the laser light source within laser 26, 
with the back mirror 100% reflective and the front mirror about 85% 
reflective. Preferably, a confocal configuration in which the distance 
between the mirrors is equal to twice the focal length is employed. 
A mirror 30 is placed to receive cylindrical beam 29 and to reflect it down 
upon chip 14 in a square configuration. Mirror 30 is a beam integrator 
which transforms the circular beam 29 into the square beam 31. For a 
rectangular chip 14 the mirror 30 will produce a rectangular beam of a 
substantially uniform cross-sectional intensity. 
The beam 31 from mirror 30 passes to chip 14 through a transparent window 
16 (calcium fluoride or glass) which is clamped by an annular metallic 
frame 17 which is secured to a flange 19 by nuts and bolts 18. A calcium 
fluoride window 16 is used where infrared light must pass through to a 
sensor outside the chamber window 16. Window 16 is sealed to flange 19 by 
means of a conventional elastomeric O-ring seal 9 of the variety usually 
employed in vacuum systems. A substantial quantity of the light from beam 
31 is reflected from chip 14 back up to mirror 32 which is a preferably 
flat mirror which redirects the light back down upon chip 14 in order to 
conserve energy. All mirrors used must be highly reflective at the laser 
frequency used. The laser 26, laser shutter 27, mirror 30 and mirror 32 
are all rigidly supported by means now shown in a way which will be 
obvious to one skilled in the art. 
The chamber 10 and the vacuum pumps 19 and 20 are supported upon a pedestal 
now shown which is slidably supported for horizontal motion in x and y 
directions at right angles independently by cranking a screw drive to 
place a specific chip 14 desired beneath the beam 31. 
Thus, with laser shutter 27 blocking beam 29, the chamber 10 can be moved 
into the position required to place the desired chip in line with beam 31. 
Then the shutter 27 is opened (as shown) to permit the chip 14 to be 
heated to solder it to the package 12. 
The maximum temperature of a chip 14 occurs at its top surface while the 
laser beam 31 is directed upon it. In accordance with another aspect of 
this invention, the temperature of the upper surface of chip 14 is 
monitored by an infrared radiometer or detector 36 (of the type available 
commercially from Barnes Engineering). Detector 36 has its maximum 
sensitivity at the appropriate wavelength in the infrared range of 
wavelengths suitable to detect the desired process temperature. 
Detector 36 provides an output signal varying as a function of temperature, 
which signal is supplied by cable 37 to a feedback control system 38 
connected via cable 39 to laser 26. System 38 is programmable to produce a 
predetermined temperature profile such as a Eurotherm Controller adapted 
to use for control in industrial environments which adjusts the output 
intensity of laser 26. There is a temperature profile such as shown in 
FIG. 2 which rises as shown above the melting point of solder balls 15 to 
prevent thermal shock. The upper level is 20.degree.-50.degree. C. above 
the melting point of the solder balls 15. Thus, chip 14 is heated 
automatically to the desired temperature which varies as a function of the 
intensity and duration of the laser beam 31. System 38 is discussed below 
in detail in connection with FIG. 3. 
While the beam 31 shown here is broad enough to cover the entire chip, it 
is better where the locations of the solder balls 15 are irregular over 
the surface of a chip 14 to provide a pencil thin laser beam (see FIG. 4 
below) which can be directed across only certain regions of a chip below 
which the solder balls 15 are to be found. There is no need to heat the 
entire chip 14 to heat the solder balls 15 in such a case. Furthermore, 
our analysis of thermographic types of infrared photographs of the 
temperature of chips has shown in areas where no solder balls rest below 
the heated area that there is a very substantial heat buildup in the chip 
which is undesirable. Obviously, the solder balls 15 provide an excellent 
heat sink for escape of heat from the lower surface of the chip. In 
addition, heat transfer laterally along the chip is relatively far slower 
as compared with heat transfer through a thin chip 14. In such a case, the 
laser beam must be scanned rapidly across the chip by a rotating and 
scanning mirror 30 operated under computer control in order to reach only 
the areas desired in the same manner as electron beam scanning systems 
employed in exposure of thin film resists and electron beam machining. 
Thus, only the areas which need heat will receive it. Several scans of a 
chip 14 are required in such an embodiment before the chip has been heated 
sufficiently to permit the solder to melt, moving the chip 14 into final 
position bonded to the package 12. 
The work support 11 includes an upper copper block 42 with holes 50 for 
receiving pins 51 at the bottom of package 12. Block 42 and block 41 
sandwich heating coils 40 between them. Coils 40 are employed to heat the 
package 12 to about 200.degree.-250.degree. C. prior to application of the 
laser beam 31 so that the heat from beam 31 in the amount required to melt 
the solder balls 15 is reduced. Thermal shock of chip 14 and package 12 is 
reduced in this way. Blocks 41 and 42 (composed of copper for good thermal 
conductivity) are brazed together by means of a copper braze and 
appropriate flux. The coils 40 within are protected electrically from the 
copper and silver by a layer of insulation. Block 42 is hollowed out to 
provide cooling cores 43 filled with coolant. The cores 43 are 
interconnected (by means not shown) and are connected to a pump and 
radiator, as is well known to those skilled in the art. The coolant in the 
cooling cores provides a means of rapidly cooling the package 12, after 
the chip 14 and the package have been joined together. 
The block 42 includes a number of apertures 50 which correspond in location 
to the pins 51 extending down from the bottom of the package 12. That 
arrangement is employed in order to maximize the thermal contact between 
block 42 and package 12. 
Coil 40 is heated electrically by A.C. voltage source 46, wires 45 and 
switch 44, when the switch is closed. When the chips have been joined by 
the laser, switch 44 is opened and coolant is circulated in the cores 43. 
Referring again to FIG. 2, the temperature profile of a chip from time 0 
rises for an initial interval of about 5-15 sec. S.sub.1 from a low 
temperature of T.sub.0 to a high temperature of T.sub.1. Subsequently to 
that for an interval of 20-40 sec. t.sub.1, the temperature remains at 
value T.sub.1. That should be enough time to enable an adequate soldering 
connection to be made, in most cases. Subsequent to that interval, the 
temperature drops to level T.sub.2 during interval S.sub.2. At that point, 
the chip has been cooled below the melting point of the solder balls 15 
and the process has ended essentially. 
FIG. 3 shows a block diagram of the control system 38, etc. The chip 
condition detector 36 which is referred to above as an infrared radiometer 
or detector (shown in FIG. 3 also) can be any form of sensor which will 
detect the condition of the chip 14 as a function of heating by the laser 
beam. It is even conceivable that the detector could be an "Individual 
Chip Joining Monitor," by S. I. Tan, IBM Technical Disclosure Bulletin, 
21, No. 6, 2551-2 (November 1978) which uses an x-y array of detectors 
shown in block diagram form to detect motion of the chip as the solder 
melts. 
Control system 38 includes program controller 50 with start switch 51 and 
monitor lights 80, 82 controlled by lines 60, 62 which show where the 
heating program of FIG. 2 is in its cycle. Controller 50 starts a manually 
adjustable repeating cycle clock 53 by sending out a signal upon line 52. 
Clock 53 energizes a set point programmer 57 which is set to heat along 
line S.sub.1 in FIG. 2 to temperature T.sub.1 at the end of a manually 
preset time interval of from 5 to 15 sec. as stated above. The temperature 
T.sub.1 is also preset manually by a manual control on the programmer 57. 
Line 65 from clock 53 connects line 63 to comparator 72 via switch 64 
during heating along line S.sub.1 and during the time t.sub.1 while 
temperature T.sub.1 is maintained. Next clock 53 turns off line 54, 
resetting programmer 57 to its start. After time t.sub.1 programmer 57 is 
disconnected from comparator 72 as clock 53 disconnects line 65 by opening 
relay 64, while energizing set point programmer 59 and line 71 to close 
relay 70 to connect line 69 to comparator 72 during cooling along line 
S.sub.2 to temperature T.sub.2 for the last interval of the program 
described here. Temperature T.sub.2 is held by programmer 59 to the end of 
the cycle when programmer 59 is reset as line 56 is turned off by clock 
53. Actually, a number of other program cycles can be employed as well, 
but the above description is believed to illustrate how a series of 
increasing, fixed and declining values of temperature control signals can 
be generated to control a comparator 72. The comparator receives the 
output of detector 36, which is compared with the current input to 
comparator from line 63 or 69. If the temperature of the chip is too high, 
an output on line 74 is generated to decrease the signal to output 
controller 75 which adjusts the input to the laser power supply in laser 
unit 26. If, on the other hand, the comparator 72 determines that the 
signal on cable 37 is less than the signal on the other comparator input 
63 or 69, then a signal on line 73 generates an increase by the output 
controller 75 in its input to the power supply to laser 26. 
FIG. 4 shows a fragmentary plan view of the structure of FIG. 1 modified to 
include a modified mirror scanning system with two mirrors driven 
alternatively by manual or computer (microprocessor) control. A fragment 
of a package 12 is shown looking down through a window 16 (not shown since 
it is transparent). Upon package 12 are several chips 14 which may number 
far, far greater than shown in both the vertical and horizontal 
directions. A pencil thin laser beam 29 is shown directed at mirror 30' 
which is tilted slightly about a vertical axis to direct the beam 31' to 
the right onto another closely spaced mirror 30" which is tilted up 
slightly about a horizontal axis in the plane of the page parallel to the 
laser beam 29 so that a doubly reflected laser beam 31" hits the upper 
right hand chip 14 in the back row rear in FIG. 1, (where the back row 
cannot be seen since it is a sectional drawing and all the chips are in 
line). By operating mirror 30' all of the chips in the back or upper row B 
can be scanned in a straight line if mirror 30" remains fixed, from R 
(right) to C (center) to L (left), across back row B. If mirror 30' on the 
other hand is held fixed in place, then the laser can be scanned down 
column R from row B to row F to any other row (not shown) which is 
desired. By operating both mirror 30' and mirror 30" in the appropriate 
sequence one at a time or both together, a scan in the x and y directions 
can be combined as in the well known computer plotters to scan straight 
lines as well as complex curves. To drive mirror 30' (x direction), a 
motor 80 is provided connected by cable 82 through switch 86 or 88 to 
computer 85 or manual control unit 90. Similarly, mirror 30" is driven by 
motor 81 which is connected to cable 83 which in turn is connected through 
switch 89 or 87 to manual control 90 or computer 85 respectively. Motors 
80 and 81 are synchros when switches 88 and 89 are closed, permitting 
turning of manual controls 92 and 93 by a human operator who is 
supervising the process manually. Alternatively, under automatic process 
control, motors 80 and 81 are stepping motors. Both sets of motors can be 
connected to the same shaft for alternative use during development of the 
proper process controls for optimum cycling of the chips 14 of a specific 
configuration, or where a single chip is to be removed under manual 
control rather than entering control information into the microprocessor 
85. Of course, the simplest control technique is to turn both of the 
mirrors 30' and 30" with knurled wheels (not shown) on the shafts 
connected thereto. Scanning should be performed in fractions of a second 
and repeated over all the lines required to heat the various rows of 
solder balls 15 under the chip 14. Thus, the most efficient process is to 
scan with the assistance of the computer 85, which can drive the stepping 
motors 80 and 81 extremely rapidly. 
Industrial Applicability 
This invention is useful because the laser frequency can be chosen to 
provide a minimal penetration depth into the chip. In addition, the energy 
can be easily directed to the chip from a convenient distance which allows 
various monitoring gear to be close to the chip without interference. 
As to alternative heat sources to lasers, various noncoherent and other 
coherent energy sources have been tried with limited success. A 
photographic projection bulb has been used heretofore but had to be placed 
so close that monitoring was not possible so that necessary control of 
energy level was impossible. A mini oxy-hydrogen flame has been used. 
Xenon arc lamps are inadequate because they employ 10 Kw which must be 
focussed into a tiny aperture and filters cut the intensity further. 
For successful application of a laser to this problem, many factors of a 
subtle nature need to be combined to achieve the usefulness of the system 
described herein. 
Ultraviolet radiation is also undesirable because of its deleterious 
effects upon the flux. Thus, a narrow bandwidth source is preferable and a 
laser of the appropriate frequency is ideal.