Note: Descriptions are shown in the official language in which they were submitted.
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DEVICE HEADER AND MFTHOD OF MAKING SAME
This invention relates to an improved header
suitable for use with semiconductor devices and, in
particular, to a header with a multilayer coating
overlying its entire surface and a method ~or fabrication
of such a header.
BACKGROUND OF THE INVENTION
Semiconductor devices are typically fabricated
by attaching the device, e.g~ a light emitting element or
laser, to a mount or header. A header comprises a base
plate with two majox surfaces and a heatsink affixed to
one of the major surfaces. The heatsink is usually of a
good thermal conductor-like copper and has a device
mountiny area to which the device can be soldered or
bonded. The header may also include a stud, pins, or the
like on the other major surface suitable for mounting the
header onto an external support.
It is known to metallize the header, e.g. with
nickel and gold, prior to bonding the light emitting
element thereto. Electroless nickel is widely used
because of the excellent coverage it is known to provide
on the intricately shaped headers. Further, deposi-tion of
electroless nickel is convenient on a variety of base
metals with minimal surface preparation. The nickel layer
acts as a barrier to prevent any diffusion of copper into
the light emitting device. The electroless nickel layer
is usually followed with a layer of gold. The gold is a
protective layer with good conductivity which also
enhances solderability of the device.
There are problems, however, in using the
electroless nickel for the above application. First, the
most convenient-to-deposit forms of electroless nickel
contain a certain amount of phosphorus. When the mounting
area of the copper heatsink is heated during the process
of soldering the light emitting device thereto, the
phosphorus migrates to the surface forming detrimen-tal
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intermetallis compounds with the solder. These
intermetallic compounds can adversely affect the thermal
and electrical conductivity, as well as the physical
strength, of the solder bond.
Also, it is desirable -to have a sharp edge on
the copper heatsink terminating the mounting area.
Because of its excellent coverage characteristics, an
electroless nickel layer tends to be somewhat non-
conformal thereby rounding off and compromising the
sharpness of the edge of the heatsink.
It is known that the n:ickel deposited by an
electrolytic process is more conformal and of the highest
purity. Unfortunately, the electrolytic nickel solution
does not "throw" in-to all of the contours and crevices of
the header as well as the electroless nickel solution
does. Therefore, it is harder to get an overall coverage
on the header using electrolytic nickel. Further, the
copper heatsink should be treated in a nitric-sulfuric
etch solution preparatory to deposition with electrolytic
~ickel. This solution is known to attack the steel base
plate and stud of the header.
Attempts to selectively etch the copper by
masking the steel parts with waxes, photoresists and the
like have proven to be impractical for production
purposes. Applying, delineating, and removing mask
materials from large numbers of headers is a highly
tedious task. Further, many masking materials leave an
organic residue on the metal parts detrimental to
subsequent plating.
It would be desirable therefore, to have a
device header which could combine the advantages provided
by the electroless and electrolytic nickel processes and a
method for fabricating such a header.
SUMMARY OF THE I NVENT I ON
A device header with a multilayer coating
overlying its entire surface, and a method of making said
header, are disclosed. The multilayer coating comprises
an electrolytic nickel layer and a gold layer in the
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device mounting area of the header, whereas the rest of
the header is coated with electroless nickel, a irst gold
layer, electroly-tic nickel, and a second gold layer. In
the fabrication, the electroless nickel layer is deposited
over the entire header followed by the first gold layer.
Upon removing these layers from the de~ice mounting area,
the first gold layer remaining on the rest of the header
ac~s as a mas~ for the etching of the mounting area
preparatory to deposition of elec-trolytic nickel and the
second gold layer. The header has the advantage of the
excellent coverage of electroless nickel over most of its
surface, but with the advantage of high purity
electrolytic nickel in the device mounting area.
BRIEF DESCRIPTION OF THE DRAWING
FIGURE 1 is a cross-sectional view of a light
emitting device mounted on a header of the present
invention.
FIGURE 2 is an enlarged cross-sectional view of
the device mounting area of FIGURE 1 illustrating an
emhodimen~ of the multilayer coating of the present
nventlon.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention will be described with
reference to a stud-type header, although the invention is
equally applicable to any type of device header.
Referring to FIGURE 1, the device header 10 comprises a
base plate 12 having first and second major surfaces 14
and 16 with a mounting stud 1~ mounted on the ~irst major
surface 14. The mounting stud 18 has an electrical lead
20 extending axially through an opening in the base plate
12 and a threaded portion 22 suitable for mounting on an
external support. The base pla-te 12 -and stud 18 are
typically of steel. The electrical lead 20 is isolated
from the mounting stud 18 by an electrically insulating
material 24 such as a glass or plastic encapsulant. A
copper or copper alloy heatsink 26, with a device mounting
area 26a, is mounted on the second major surface 16 and a
light emitting device 28 is mounted on the heatsink 26.
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One typical copper alloy comprises about 0.5 percent
tellurium and the balance substantially copper. An
electrical wire 30 connects the electrical lead 20 to one
side of the light emitting device 28 with the second
electrical contact to the light emitting device 28 being
made through the base plate 12 and the stud 18. A
multilayer coating 32 overlies the surfaces of the base
plate 12 and the outer surface of the mounting stud 18
including the threaded portion 22. A cover 33 with a
light transmissive window 35 mounted therein is attached
to the second major surface 16 and encloses the light
emitting device 28. The window 35 is so positioned in -the
cover 33 that a light beam emitted by the device 28 will
pass through the window 35.
FIGURE 2 is an enlarged view of the heatsink 26
and the device mounting area 26a which shows the various
layers which make up the multilayer coating 32. A layer
of electroless nickel 34 overlies the entire header except
the device mounting area 26a. The electroless nickel
layer 34 can be of any desired thickness but at a minimum
should be of a thickness to provide complete coverage of
all surfaces of the base plate 12, stud 18 and heatsink
26. A first gold layer 36 overlies the electroless nickel
layer 34. The first gold layer 36 can be deposited by any
convenient method, e.g. electrolytic plating, electroless
deposition, evaporation, sputtering and the like. The
~irst gold layer 36 should be of a sufficient thickness to
act as a protective mask during chemical etching of the
device mounting area 26a. Between 1.0 and 3.0 micrometers
(~m) of electrolytically deposited gold are suitable for
this purpose.
A layer of electrolytic nickel 38 overlies the
device mounting area 26a and the first gold layer 36 over
the rest of the heatsink 26, base plate 12, and stud 18.
The electrolytic nickel layer 38 should be of a high
purity nickel and should be of a thickness sufficient to
prevent diffusion of copper from the heatsink 26 to the
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light emitting device 28. Thicknesses of 0.5 to 5.0 ~m
are suitable for this purpose with an electrolytic nickel
layer 38 about 1.5 to 2.5 ~m in thickness being preferred.
Overlying the electrolytic nickel layer 38 is a
second gold layer 40 which, as the first gold layer 36,
can be deposited by any convenient means. The second gold
layer 40 is a protective layer for the unit and also
allows for convenient bonding of the light emitting device
28. Therefore, the thickness of the second gold layer 40
should be in accordance with a desired bonding or
soldering technique. For thermocompression bonding of the
light emitting device 28 to the heatsink 26 thicknesses
for the second gold layer 40 from about 2.0 ~m to 10 ~m
have been found suitable with 4.0 to 6.0 ~m being
preferred.
Thus, the coating 32 over the heatsink 26, base
plate 12 and s-tud 18 comprises the electroless nickel
layer 34, the first gold layer 36, -the electrolytic nickel
layer 38 and the second gold layer 40, whereas the device
mounting area 26a is covered with a coating 32a, unigue to
that area, which comprises only the electrolytic nickel
layer 38 and the second gold layer 40.
The method of the present invention will now be
described. The stud 18 and heatsink 26 can be a~fixed to
the base plate 12 by known welding or brazing techniques.
To prepare the header for the electroless nickel layer 34,
the header can be treated in solvents and alkaline
cleaners. Since bright dips for copper are known to
attack steel, they should not be used here so as to avoid
damage to the base plate 12 and stud 18. After depositing
the electroless nickel layer 34, the first gold layer 36
can be deposited thereover. Initially, these two layers
34, 36 cover the entire base plate 12, stud 18 and
heatsink 26, including the device mounting area 26a.
At this point the electroless nickel layer 34
and the first gold layer 36 are removed from the device
mounting area 26a. These layers 34, 36 can be removed by
physical or mechanical means e.g. machining, cutting,
3~!
milling and the like. This can be accomplished
while removing the electroless nickel layer 34 and the first
gold layer 36.
After removing these two layers 34
and 36, the exposed copper in the device mounting area
26a can be suitably bright dipped. Any copper etchant
can be used, e.g. a solution comprising about 60
to 70 percent by volume of nitric acid and about
30 to 40 percent by volume of sulfuxic acid. This
type of a bright dip step is known in the art as a
necessary procedure for proper adhesion of
electrolytic nickel to copper and especially to
copper alloys. Although this etchant is known to
attack steel, the first gold layer 36 acts as a
protective mask for the base plate 12 and stud
18 so that the device moun-ting area 26a can be
selectively etched.
The next step is to deposit the
electrolytic nickel layer 38 which will cover the
device mounting area 26a and, further, will overlie
the first gold layer 36 on the rest of the header.
The electrolytic nickel layer 38 is a highly pure,
conformal layer which will maintain the sharp
corner imparted to the device mounting area 26a and
will preven-t phosphorus contamination during
subse~uent bonding.
The electrolytic nickel layer 38 is
followed by a second gold layer 40 which covers the
entire header lO. The light emitting device 28 can
3Q be soldered or bonded to the second gold layer 40 in
the device mounting area 26a by means known in
the art.
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The advantage of such a header 10 is that the
excellent coverage qualities of the electroless nickel
layer 34 are provided where necessary, i.e. over the
intricate shape of the header in toto, whereas the high
purity, highly conformal qualities of the electrolytic
nickel layer 38 provide excellent conditions for bonding
~ the light emitting device 28 to -the device mounting area
26a.
