Laser soldering of flexible leads

A method for soldering a lead to a pad having a solder plate formed thereon is described. In one embodiment of the method, a tape is disposed between the lead and a laser source, and the method includes the steps of aligning the lead with the solder plated pad, transmitting a beam from the laser source through the tape and to a location where a solder joint is to be formed, and the beam heating the solder plate thereby causing reflow of the solder plate to form a solder joint.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to soldering leads, and more 
particularly, relates to laser soldering microelectronic leads. 
2. Related Art 
Mounting microelectronic devices on printed circuit boards has become a 
popular method for packaging circuits for units such as computers and 
control systems. Known methods for mounting microelectronic devices on 
printed circuit boards include surface mount and through-hole mount 
methods. Particularly, in surface mount methods, a microelectronic device 
is disposed on a first surface of a printed circuit board, and leads of 
the device are soldered to and form connections with pads deposited on the 
first surface of the circuit board. The connections are made on the same 
surface, i.e., the first surface, on which the microelectronic device is 
disposed. With through-hole mount methods, a microelectronic device is 
disposed on a first surface of a printed circuit board and leads of the 
device are aligned with and passed through openings formed in the circuit 
board. Then, the leads of the device are soldered in the opening and form 
connections with conductors disposed on the printed circuit board. 
With surface mount devices, the number of leads extending from a 
microelectronic chip may exceed three hundred, for example. Lining the 
leads up with corresponding pads on the printed circuit board, and then 
maintaining the alignment during a soldering process is a difficult, and 
time consuming, task. 
To facilitate aligning and maintaining the leads in position relative to 
pads on the printed circuit board, a process generally known as 
Tape-Automated Bonding (TAB) has been implemented. A microelectronic chip, 
its corresponding leads, and polyimide tape typically compose a TAB 
device. The leads are bonded to the tape, and then the chip is bonded to 
an inner portion of the each lead. Then, an outer portion of the tape is 
etched away so that the leads extend from an outer surface of the tape. 
The leads from the TAB device are supported, at least partially, by the 
tape to facilitate aligning the leads with solder plated pads. The leads 
are then soldered to corresponding printed circuit board pads. 
Particularly, at the location where the tape has been etched away, the 
leads of the chip are exposed. These exposed leads are aligned with solder 
plated pads formed on the corresponding printed circuit board conductors, 
and the leads are then soldered using, for example, a hot blade. The hot 
blade is used for conductive heating to reflow solder plates on the solder 
pads. The hot blade is also utilized to hold the leads in contact with 
their mating pads while the solder is reflowed. 
Lead hold down is a common problem in most solder processes, and the 
problem is amplified when soldering flexible leads, which are typical in 
TAB devices. In known TAB processes, the exact point where the soldering 
is to occur is the exact point where the leads no longer are supported or 
held by the tape as a result of the tape etching process. With known TAB 
approaches, if the tape were not removed, utilizing a hot blade would 
result in burning the tape and possibly damaging the chip due to excessive 
heat. 
Other than using a hot blade, it is known to use a laser beam in a TAB 
process to reflow the solder. Particularly, using known laser-TAB methods, 
an outer portion of the tape is removed via an etching process, the leads 
are aligned with corresponding solder pads, and a laser beam is utilized 
to reflow the solder. Known laser tools which could be utilized in a 
laser-TAB process are described in commonly assigned U.S. Pat. No. 
4,564,736, entitled "Industrial Hand Held Laser Tool and Laser System", 
commonly assigned U.S. Pat. No. 4,681,396, entitled "High Power Laser 
Energy Delivery System", and commonly assigned U.S. Pat. No. 4,799,755, 
entitled "Laser Materials Processing with a Lensless Fiber Optic Output 
Coupler". 
In known laser-TAB processes, however, the problem of lead hold down is 
prevalent because the leads are unsupported by the tape at the location of 
soldering. At least with the hot blade method, the leads can be held down 
with the blade. Further, fumes result when utilizing a laser beam to 
reflow the solder, and the fumes can coat the laser beam delivery optics 
thereby deteriorating operation of the laser device. 
SUMMARY OF THE INVENTION 
The present invention provides a method for soldering microelectronic lead 
connections which not only protects beam delivery optics of a laser source 
from solder/flux vapors, but also permits implementation of easier hold 
down techniques, such as using mechanical clamps for holding the leads in 
a desired position. Particularly, rather than etching the tape to expose a 
portion of each lead, the tape is left in place, i.e., the tape is 
disposed over the leads. The TAB device is then placed on a corresponding 
printed circuit board, and the leads are aligned with corresponding 
conductors/solder pads. A mechanical coupler or clamp may be used to hold 
the TAB device in the desired position. A Nd(neodymium):YAG (yttrium 
aluminum garnet) laser source beam is then utilized to reflow the solder. 
Particularly, the tape is disposed between the laser source and leads. A 
beam emitted from the laser source is transmitted through the tape and 
couples with and heats the leads sufficiently to cause reflow of the 
solder plates. 
The tape supports the leads to facilitate and maintain alignment. The tape 
also blocks vapor emitted from the solder during the reflow operation 
thereby protecting the optical components of the laser system from 
solder/flux vapors. The present method therefore provides protection of 
the beam delivery optics and permits implementation of easier hold down 
methods for the leads.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring now more particularly to the drawings, FIG. 1 illustrates a 
portion 10 of a printed circuit board. As shown in FIG. 1, the circuit 
board includes conductors 12 which may be formed on/within board 10 using 
well-known techniques. Also using well-known techniques, each end portion 
14 of each conductor 12 has a solder pad 16 formed thereon. Each solder 
pad is plated with solder and is disposed so that it may mate with a lead 
of a TAB device, such as a TAB device 20 as shown in FIG. 2. 
More particularly, as shown in FIG. 2, TAB device 20 includes a 
microelectronic chip 22 having leads 24 extending therefrom. It should be 
understood that a typical TAB device has more leads than the number of 
leads illustrated in the TAB device shown in FIG. 2. Tape 26, such as the 
polyimide tape commercially known as Kapton and available from DuPont, is 
bonded to and supports leads 24 in a well-known manner. As illustrated in 
FIG. 2, tape 26 has been etched so that a portion of each lead 24 extends 
from the outer surface of the tape. 
The TAB device shown in FIG. 2 is constructed and used in accordance with 
well known methods. An illustration of device 20 is provided primarily for 
background purposes and to facilitate an understanding of the present 
invention. As should be apparent from FIGS. 1 and 2, TAB device 20 could 
be disposed on printed circuit board portion 10 illustrated in FIG. 1, and 
the leads of TAB device 20 would then be aligned with corresponding solder 
pads. The TAB device would be disposed so that the leads are between the 
tape and the printed circuit board surface, i.e., FIG. 2 is a bottom view 
of the TAB device. With the TAB device shown in FIG. 2, and once disposed 
on printed circuit board 10 as hereinbefore described, a hot blade could 
be utilized to reflow the solder plated pad thereby establishing 
electrical connections between the TAB device lead and the printed circuit 
board conductors. 
FIG. 3 illustrates a TAB device 30 which could be used in practicing the 
present method. Particularly, TAB device 30 includes a microelectronic 
chip 32, leads 34 extending from chip 32, and tape 36. Tape 36 extends to 
the end of each lead 34. In TAB device 20 shown in FIG. 2, tape 26 was 
etched so that an end portion of each lead is exposed, i.e., an end 
portion of each lead is not bonded to the tape. With the TAB device shown 
in FIG. 3, however, the end portion of each lead is bonded to the tape. 
It is contemplated that rather than terminating at an end portion of each 
lead, tape 36 could even extend beyond the end of each lead. Further, a 
second tape portion (not shown) could be utilized to "sandwich" the leads 
between two layers of tape. The second tape portion would have to be of a 
length so that a portion of one side of each lead is exposed so that the 
lead can be brought into contact with the solder plated pads, i.e., the 
end portion of the lead could not be totally enclosed by tape. 
TAB device 30, just as TAB device 20, may be disposed on portion 10 of the 
printed circuit board as shown in FIG. 1. The TAB device leads would then 
be aligned with the solder plated pads. Note that FIG. 3 is a bottom view 
of the TAB device and the chip leads would be disposed between the tape 
and the printed circuit board. Once aligned in the above described manner, 
a laser solder process as described below would be performed. 
Specifically, FIG. 4 is an enlarged view of a printed circuit board 40, a 
copper pad 42 and a solder plated plate 44, all shown in cross-section. A 
tinned copper lead 46 bonded to tape 48 is disposed on solder plate 44. 
Lead 46 and tape 48 are part of a TAB device. Tape 48 is illustrated in 
phantom in order to facilitate a view of the material layers. Once aligned 
in the manner illustrated, a mechanical clamp, or coupler, could be 
utilized to maintain the TAB device in the desired position. 
Subsequent to alignment, and as illustrated in FIG. 5, a laser beam 50 
emitted from a laser beam source (not shown) is directed to a location 
where a solder joint is to be formed. The laser source, for example, may 
be any of the devices described in U.S. Pat. Nos. 4,564,736, 4,681,396, or 
4,799,755, as hereinbefore mentioned. The laser beam is transmitted 
through the tape and couples with and heats the copper leads sufficiently 
enough to cause reflow of the solder plate as shown in FIG. 5. 
The tape supports the leads to facilitate and maintain alignment of each 
lead with a corresponding solder plated pad. The tape also blocks vapor 
emitted from the solder during the reflow operation thereby protecting 
optical components of the laser system from solder/flux vapors. The 
present method therefore provides protection of the beam delivery optics 
and permits implementation of easier hold down methods, e.g., using a 
mechanical clamp, for the leads. 
In an actual implementation of the present invention, the leads of a TAB 
device that were soldered were 1 by 4 mils in cross section and were 
soldered to mating pads when solder reflow was performed with laser energy 
transmitted through a 1 mil thick Kapton tape strip. The range of laser 
power used was 5-10 watts. The Kapton strip covered and was in contact 
with the tin copper leads. It should be understood, of course, that tape 
other than Kapton could be utilized in accordance with the present 
invention. Some polyimide tapes, however, may absorb the Nd:YAG laser beam 
and would prevent through-polyimide processing. Therefore, the tape 
selected must transmit, and preferably efficiently transmit, a 
corresponding beam which is being used to affect soldering. The Nd:YAG 
beam is of 1.06 micrometer wavelength and other tapes transmit such 
wavelength beams. 
It should also be understood that a laser source other than a Nd:YAG source 
could be utilized. If an emitted beam from a laser beam source can be 
substantially transmitted through the tape at a power level sufficient to 
bring about solder reflow, the laser beam source can be utilized. 
By providing that the tape remains disposed over the leads during the 
soldering process, the leads are better held in position relative to the 
pads, and registration and coplanarity of the leads and pads is 
facilitated. Particularly, a mechanical holder could be used to maintain 
the TAB in a selected position. Further, during materials processing, the 
polyimide tape prevents solder/flux vapors from coating the laser beam 
delivery optics, therefore eliminating the problem associated with the 
known systems. 
While a preferred embodiment of the present invention has been illustrated 
and described herein, numerous modifications, changes, variations, 
substitutions and equivalents, in whole or in part, will now occur to 
those skilled in the art without departing from the spirit and scope 
contemplated by the invention. Accordingly, it is intended that the 
invention herein be limited only by the scope of the appended claims.