Laser soldering method and apparatus

An electronic device (10) is surface mounted to bonding pads (11) of a substrate (12) by first mounting the device within an aperture (24) of a glass plate (17). The leads (14) of the electronic device are coated with solder and pressed onto the bonding pads by the reusable glass plate (17). While pressing the plate against the leads, the plate is scanned with a laser beam (16) directly over each row of bonding pads. The laser light is of an appropriate wavelength such that the glass plate converts the laser light to heat, which melts the solder to bond the leads to the bonding pads.

TECHNICAL FIELD 
This invention relates to processes for bonding conductors to bonding pads 
of a substrate. 
BACKGROUND OF THE INVENTION 
Electronic systems are typically made by defining complex integrated 
circuits on semiconductor chips, bonding the chips to circuit package 
substrates, and in turn bonding the packages to printed circuit boards. 
The most common bonding technique is wire bonding, in which an instrument 
(a thermode) thermo-compression bonds wires to a bonding pad of one 
element, such as a chip, and then pulls the wire and makes a bond on a 
bonding pad of a second element so as to form an arcuate self-supporting 
wire bridge between the two bonding pads. An alternative technique that is 
rapidly coming into increased use is known as reflow soldering or surface 
mounting, in which the leads of a device are "tinned" with solder and 
placed on an array of bonding pads. Heat is then applied to melt or reflow 
the solder between the leads and the bonding pads to make a solder bond. 
Direct heating of circuit assemblies to melt or reflow the solder has been 
applied, for example, by hot air convection and by infrared heating. A 
disadvantage of such techniques is that the entirety of the unit, 
including active elements of the chip, must be heated to a temperature 
above the melting point of the solder. To avoid this and to localize the 
heating, laser beam soldering has been proposed, in which the heat is 
applied by a laser beam directed against leads overlying the bonding pads. 
Differences in absorption and reflectivity resulting from differing 
surface properties may create problems; also, it may be difficult to 
position the beam with the required accuracy; finally, the laser beam may 
burn or char the substrate. 
The need for easier methods for surface mounting devices on substrates is 
expected to become greater as the density of circuits on integrated 
circuit chips increases. As the number of leads extending from chips and 
chip carriers increases, it becomes more difficult to use surface mount 
techniques to solder them with consistency and uniformity. One advantage 
that thermal compression bonding has had over reflow solder techniques is 
that it applies a uniform compressive force to each lead that is being 
soldered, whereas, with reflow soldering, different leads may rest with 
different degrees of force on the different bonding pads. With either 
method, as density increases it becomes more difficult to align precisely 
the leads with the bonding pads. 
Some, but not all, of these considerations are addressed in U.S. Pat. No. 
4,785,156 to Benko et al., which shows the use of a fused silica member 
overlying pre-tinned leads to be soldered. A laser beam impinging on the 
silica member is converted to heat, which is sufficient to reflow the 
solder and effect a solder bond. 
SUMMARY OF THE INVENTION 
The invention is an improvement of a method for surface mounting an 
electronic device having a plurality of leads extending from it in a 
cantilever fashion. The free ends of the leads are tinned and aligned with 
a row of bonding pads such that each lead overlies a bonding pad. The 
leads are then pressed onto the bonding pads with a glass plate that 
bridges the leads of an array and forces each of them into contact with a 
corresponding bonding pad. While the plate presses against the leads, the 
plate is scanned with a laser beam along a line directly over the row of 
bonding pads. The laser light is of an appropriate wavelength such that 
the glass plate converts the laser light to heat, which melts the solder 
to effect a bond. Since the glass plate is transparent to visible light, 
the operator can look through it to align the leads with the bonding pads. 
According to another feature, an aperture is made in the glass plate, and 
the electronic device is mounted in the aperture such that opposite sides 
of the aperture compress opposite lead arrays of the device; in this 
manner, the electronic device can be held within the aperture by friction. 
The operator can then manipulate the plate to align the leads with the 
bonding pads. Additionally, two or more electronic devices can be held 
within two or more apertures in the glass plate to permit simultaneous 
registration of the various lead arrays with corresponding bonding pad 
arrays of the substrate. After the soldering operation, a stopper element 
is abutted against the electronic device to permit the withdrawal of the 
glass plate without affecting the solder bonds. 
It will be appreciated that the invention lends itself well to either 
manual or robotic placement of electronic devices onto substrates with the 
leads precisely positioned on bonding pads. During the reflow operation, 
the glass plate bears down on the lead array and compresses it against a 
bonding pad array with a substantially uniform force. This is a 
significant improvement over other surface mount techniques in which a 
lead that is accidentally bent upwardly with respect to other leads of the 
array may not touch the bonding pad sufficiently to insure a reliable 
bond. These and other features, benefits and advantages of the invention 
will be better understood from a consideration of the following detailed 
description taken in conjunction with the accompanying drawing.

DETAILED DESCRIPTION 
Referring now to FIGS. 1 and 2, there is shown an electronic device 10, 
which may be an encapsulated semiconductor chip, which is to be soldered 
to bonding pads 11 of a substrate 12. Substrate 12 may be a circuit 
package substrate or, alternatively, electronic device 10 can be a circuit 
package and substrate 12 can be a printed circuit board. Each bonding pad 
11 constitutes part of a circuit on the substrate which, for brevity, has 
not been shown. The electronic device 10 contains electrical leads 14 
which have been "tinned"; that is, their surfaces opposite the bonding 
pads have been coated with solder. 
In accordance with the invention, the solder coatings are melted and the 
leads 14 are soldered to bonding pads 11 by directing a laser beam 16 to 
impinge on a glass plate 17. The glass plate absorbs the laser beam 16 and 
converts it to heat without itself becoming significantly damaged. The 
heat in turn is conducted to leads 14 and is sufficient to melt the solder 
coatings on the leads. After the solder operation, a stopper member 19 is 
abutted against the electronic device 10 so that glass plate 17 can be 
removed without impairing the soldered joints. Thereafter, the glass plate 
can be reused for other solder operations. 
As is customary in the fabrication of integrated circuit devices, the leads 
14 contain first and second substantially right angle bends. A first lead 
portion 20 extends from the electronic device to the first right angle 
bend, a second lead portion 21 interconnects the first and second right 
angle bends, and a third lead portion 22 extends from the second right 
angle bend and makes contact with a bonding pad. As shown in FIG. 2, an 
aperture 24 is defined in the glass plate 17. The aperture is fitted 
around the electronic device 10 such that inner surfaces of the plate 
defining the aperture abut against second lead portions 21 of the leads 
14. The aperture is sized such as to compress slightly opposite arrays of 
leads when the aperture is fitted around the lead arrays. The leads are 
sufficiently spring-like, and have a sufficient Young's modulus within the 
elastic limit, that the electronic device can be held by friction within 
the aperture. As a consequence, the glass plate 17 can be used as a holder 
of the electronic device 10 and can be manipulated by an operator to align 
the leads 14 with respect to corresponding bonding pads 11. The glass 
plate is transparent to visible light so that the operator can see through 
it to align the leads with the bonding pads. 
After alignment has been made, a downward force is preferably exerted on 
the glass plate 17 which presses each lead 14 firmly against a 
corresponding bonding pad 11. Thus, if any of the leads of the array is 
bent slightly upwardly, it will, nevertheless, be forced into firm contact 
with a bonding pad. Encapsulated integrated circuit chips presently being 
used typically have 24 leads extending from each of its four sides, and so 
the likelihood is fairly great that one of the cantilevered free ends 
might be slightly bent so as to prevent it from fully contacting a bonding 
pad in the absence of any compressive force. Another advantage of the 
applied pressure is that the amount of solder for making the bond can be 
minimized. With conventional surface mount, if one uses a sufficiently 
thick solder coating to assure a bond to all bonding pads, one may run the 
risk of excessive solder causing a spurious short-circuit between adjacent 
bonding pads. 
The light beam 16 is typically generated by a carbon dioxide laser emitting 
light with a CW power of about fifteen watts at a predominant wavelength 
of 10.6 microns. Fused silica glass is sufficiently absorbent of such 
wavelength so as to convert it to heat in accordance with an illustrative 
embodiment of the invention. A glass plate which 17 has been used 
experimentally has a thickness of forty mils, which provides structural 
strength along with an appropriately short path for heat conduction to 
each lead. As the sizes of devices are further miniaturized, it is to be 
expected that the preferred values for laser power and glass thickness 
will also drop. It is expected that practical glass thicknesses will be 
between about five and forty mils. The thickness of the bonding pads and 
leads provides an inherent gap 25 between the glass plate 17 and the 
substrate 12 so as to prevent a significant heating of the substrate by 
the glass plate. Experiment shows that with apparatus of the type shown, 
the heat applied to substrate 12 is insufficient to damage a conventional 
printed circuit board made of glass filled with epoxy. 
As shown in FIG. 2, the beam 16 scans the glass plate 17 along scan lines 
27 to provide heat for the solder bond. Apparatus for causing a laser beam 
to scan in a controlled manner along a straight line is, of course, well 
known and for the sake of brevity will not be described. The generation of 
heat along lines 27 localizes the heat and prevents any damage to the 
substrate 12 or the electronic device 10. On the other hand, if the glass 
plate 17 were not used, a scan along lines 27 would impinge on the 
substrate and could cause damage to it, particularly a printed circuit 
substrate. 
The electronic device 10 may be an encapsulated chip having major 
dimensions of 750 by 750 mils, which may typically have twenty-four leads 
extending from each side, each lead having a thickness of seven mils and a 
width of eith mils. It is apparaent that the glass plate can be 
conveniently used by an operator as an aid for aligning the free ends of 
the leads with corresponding bonding pads 11. In the interest of clarity, 
the various bonding pads 11 have not been shown in FIG. 2, but it is to be 
understood that the substrate 12 contains four rows of bonding pads, upon 
each of which one of the free ends of leads 14 is to located. It can be 
appreciated that the apertured glass plate can be of considerable 
assistance to an operator in appropriately positioning the electronic 
device on the substrate. On the other hand, the plate can also be a 
significant aid in assisting automatic or robotic placement, and, for 
automatic placement, there may not be a need for making the plate 17 
transparent to visible light. A different combination of laser wavelength 
and apertured plate material may then be selected such that the plate 
converts all or much of the laser light to heat. 
FIG. 3 illustrates schematically one example of how the invention can be 
used for automated placement of lead arrays on bonding pad arrays. A 
substrate 30 contains generally rectangular bonding pad arrays 31 and 32 
upon which it is desired to align the lead arrays of electronic devices 34 
and 35. A glass plate 36 contains apertures 37 and 38 for respectively 
holding electronic devices 34 and 35 in the same manner as described 
previously. Indexing rods 40 and 41 permit vertical movement of the glass 
plate 36 and assure its predetermined registration with substrate 30. 
Apertures 37 and 38 are located in glass plate 36 such as to align the 
lead arrays of devices 34 and 35 respectively, with row arrays 32 and 31 
of the substrate 30. Thus, one initially inserts devices 34 and 35 
respectively, in apertures 37 and 38 and then lowers the glass plate 36 
until the free ends of the lead arrays (not shown) are contacted to the 
bonding pad arrays 32 and 31. As before, the glass plate presses each lead 
against a corresponding bonding pad during a laser scan around the 
periphery of each aperture in the manner depicted in FIG. 2. A stopper 
member, not shown, may be included above each of the devices, in the 
manner depicted in FIG. 1 by stopper member 19, for the purpose of holding 
the devices to the substrate after the soldering operation as the glass 
plate 36 is removed in an upward direction. 
While the electronic devices we have shown have cantilevered leads 
extending from all four sides, it is apparent that the invention can be 
used wherever one has an array of leads to be bonded to a corresponding 
array of bonding pads. The holding action of the apertures on the 
electronic device does not require the leads extending from all four 
sides, and it will work quite well if leads were included on only two 
opposite sides or, perhaps, from one side. One skilled in the art could 
devise a scanning mechanism to make the laser beam 16 scan along a 
continuous rectangular line around the periphery of aperture 24. In some 
instances, the weight of the glass plate could provide sufficient downward 
force on the leads during the heating operation that other force on it 
would not be necessary. Various other embodiments and modifications may be 
made by those skilled in the art without departing from the spirit and 
scope of the invention.