Dowel-less mold chase for use in transfer molding

Transfer molding equipment, used to encapsulate semiconductor die, is provided with clamps mounted to the mold base to position and align mold chases. Previous designs position and align the top and bottom mold chases using dowels on the mold base received in holes in the chases. The invention reduces the downtime of the mold equipment, and decreases the time required to replace the chases when a different package is to be produced.

FIELD OF THE INVENTION 
This invention relates to the field of semiconductor manufacturing. More 
specifically, it describes an improvement in the manner in which the top 
and bottom mold chases used to encapsulate the die in plastic are attached 
to the mold base, thereby making the equipment less likely to require 
servicing and making the chases easier to replace when a different package 
is to be produced. 
BACKGROUND OF THE INVENTION 
Many types of semiconductor devices are made in much the same way. A 
silicon wafer is masked, doped, and etched to produce a plurality of die 
on the wafer. Each die is separated then attached to a lead frame, with 
several die attached to each lead frame. The lead frame forms the 
input/output leads which will allow data to be transferred between the die 
and the electronic device into which the semiconductor components will be 
installed. 
After the die are attached to the lead frame, the die are encapsulated in a 
protective layer of plastic material. The equipment which molds the 
plastic around the die comprises several parts. A bottom mold chase 
receives a bottom cavity bar, the cavity bar comprising a void which will 
be filled with plastic. A lead frame having die is placed on the bottom 
cavity bar, and a press holds a top cavity bar, which is received in a top 
chase, against the bottom cavity bar. The top and bottom cavity bars form 
a top and bottom void around the die. All this equipment together forms a 
mold base. Molten plastic is pumped into the top and bottom cavities to 
form the top and bottom halves of the package around the die. Each of the 
plastic packages formed on the lead frame are separated and the excess 
plastic is trimmed away from the lead frame. The plurality of bodies 
formed on each lead frame are separated, and the metal is trimmed and 
formed to produce the final semiconductor component. 
The mold chases, which are rectangular in shape, have a hole jig ground in 
each corner. The jig grinding presently costs approximately $50 for each 
hole, adding $200 to the cost of each chase. Dowels on the mold base are 
received by the holes in the corners of the chases, the chases being 
aligned in this manner. After the dowels receive the chase, the chase is 
bolted to the mold base to hold it firmly in place. The mold base can 
receive several top and bottom chases at once, thereby increasing the 
productivity of the transfer molding equipment. 
Semiconductor manufacturing facilities produce several package types. When 
a new type of package is to be produced the chases are removed and a 
different type of chase is installed on the dowels. Several different 
package types may be produced weekly. 
Each time the chases are removed there is friction between the chases and 
the dowels. Since the dowels are made of a much softer material than the 
chases, metal is worn away from the dowels. Due to forces involved with 
removing and replacing chases, the dowel holes on the chases can wear and 
shift location or become damaged, thereby contributing to subsequent 
alignment problems. Over time the dowels become smaller in diameter and 
the chases are somewhat loose on the dowels, thereby producing incorrect 
alignment between the top and bottom chases. In addition, allowable 
tolerances in the placement of the holes in the corners of the chases can 
force the dowels further apart or closer together, thereby wearing the 
dowels even faster and causing the base of a dowel to become loose. The 
loose dowels cause misalignment of the top and bottom chases. This 
misalignment eventually makes the semiconductors produced with the worn 
dowels unusable, and the dowels must be replaced at no small cost. 
Another drawback with the dowel arrangement is that it makes replacing the 
top and bottom mold chases time consuming. Each chase must be removed from 
the dowels on the mold base, and different chases installed which takes a 
skilled operator approximately 20 minutes of downtime. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide transfer molding equipment which 
will eliminate the use of dowels to position the mold chases, thereby 
replacing one unreliable component with a more reliable component. A 
second object of the invention is to provide transfer molding equipment 
which allows a reduction in the time required to replace the mold chases. 
Another object of the invention is to provide transfer molding equipment 
which provides a means for easily aligning the upper and lower mold chases 
thereby eliminating package-to-leadframe misalignment and package mismatch 
due to chase misalignment. A fourth object of the invention is to provide 
transfer molding equipment which has the advantages listed above while 
allowing the conversion of equipment of previous designs to the inventive 
means. 
These objects of the present invention are attained by using a system of 
clamps and fixed alignment blocks to align the chases rather than by using 
dowels. The alignment blocks and clamps therefore replace the dowels on 
the mold base and the holes in the mold chases, thereby lessening the 
downtime of the molding equipment. The alignment blocks and fixed portions 
of the clamps are permanently bolted to the mold base. 
By using two alignment blocks and two clamps for each chase, the chases can 
be aligned in the x and y directions to within 0.0002". Dimensional errors 
which contribute to misalignment top to bottom can be easily adjusted by 
the use of shims placed between the alignment block and the chase. 
The inventive design additionally provides the advantage of allowing chase 
replacement in approximately five minutes, rather than the 20 minutes 
normally required. Two or more molds can use a common alignment block, 
thereby simplifying the design of the invention as described in the 
detailed description of the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a top view, and FIG. 2 is a side view, of one embodiment of the 
invention showing four bottom mold chases 10 mounted to a mold base 12. A 
center block 14, permanently attached to the mold base 12 by attachment 
means 15 such as bolts or dowels, contacts all four mold chases 10. Two 
other fixed alignment blocks 16, which run laterally down the center of 
the mold base 12, contact two chases each. The fixed alignment blocks 16 
are permanently attached on one end to the mold base 12 with attachment 
means 17 such as dowels or bolts, and are held in position on the opposite 
end by the center block 14. Eight clamps 18 are shown with two clamps 18 
for each chase -0. Figure 1 shows the dowel holes 20 on the chases 10, but 
the dowel holes 20 are not used with the invention. This illustrates the 
use of chases designed for a mold base of a previous design on the 
inventive mold base 10. Bolts 21 hold the chases 10 to the mold base 12. 
Each clamp assembly 18 has a fixed portion 22 bolted to the mold base 12. 
Through the fixed portion 22 of each clamp assembly 18 is a threaded hole 
24 which receives a threaded shaft 26. On one of the two shafts 26 which 
positions a chase 10 is a planar end cap 28 which contacts a large portion 
of the end of the chase 10. This end cap 28 serves to dam the end of the 
channel (not shown) on the chases 10 to prevent plastic from flowing out 
the end of the chase 10. With this embodiment, the clamp assemblies 18 
which contact the side of the mold chases 10 do not have end caps. On the 
end 30 of the shaft 26 opposite the end cap 28 is means for rotating the 
shaft 26, many such means being known by one skilled in the art. A 
preferred method is a recess in the end of the shaft which receives an 
Allen wrench. Rotating the shaft 26 moves the shaft 26 toward o away from 
the alignment block 14, 16 opposite the shaft 26 which should provide just 
enough force to hold the chase 10 into place. The primary function of the 
clamp assembly 18 is to align the chase 10 by contacting the chase 10 with 
the alignment blocks 14, 16. After the clamps 18 align the chases 10, 
bolts 21 in the four corners of the chases 10 hold the chases 10 in place 
on the mold base 12. 
There are two clamp assemblies 18 for each chase 10 as shown, mounted to 
the mold base 12 at 90.degree. angles to each other. The two clamps 18 
position the chase 10 and hold it in place until the chase 10 is bolted 21 
to the mold base 12. If care is taken to position the clamps 18 and 
alignment blocks 14, 16 of the bottom half of the mold base 12 with the 
clamps and alignment blocks of the top half of the mold base, the top and 
bottom chases will properly align. If the chases wear from contact with 
the end caps 28, shafts 26, or alignment blocks 14, 16, shims can easily 
be used to ensure alignment of the top and bottom chases. 
A hole 32 and channels 34 are for channeling molten plastic, and are 
detailed in the description of FIG. 3. 
FIG. 3 details of one chase 10 of the Figure embodiment. A lead frame with 
semiconductor die (not shown) is placed onto each of the bottom cavity 
bars 36. Each die on the lead frame (not shown) fits into a bottom cavity 
38 formed in the bottom cavity bar. A top chase (not shown) is pressed 
onto the bottom chase and held firmly against it. The top assembly also 
has cavity bars with cavities that match the bottom cavities 38. Channels 
34 on the center block 14 lead to channels 40 on the chase 10. In order to 
protect the die from the injected plastic, a layer of polyimide or other 
plastic is applied to the die prior to the molding procedures. 
Molten plastic is pumped under pressure through the channels (no shown) 
running laterally through the center of the center block 14. The plastic 
comes to the surface of the center block 14 through a hole 32 in the 
center of the center block 14. The plastic runs through the channel 34 in 
the center block 14 to channels 40 in the chase 10. The plastic flows to 
the cavity bars 36 and fills cavities 38 in the cavity bar 36 and flows 
around the die (not shown) and portions of the lead frame (not shown) to 
encapsulate the semiconductor die on the lead frames. The end cap 28 on 
the clamp 18 dams the plastic and prevents it from running out the end of 
the chase. Therefore, there are end caps 28 only on the clamps 18 which 
are at the end of the chase 10, while there is no end cap on the clamp 
which contacts the side of the chase. After the plastic has cooled and 
solidified, the upper and lower chases are separated, and the lead frames 
are trimmed and formed, thereby providing a packaged semiconductor die. 
What has been described is a specific configuration of the invention, as 
applied to a particular embodiment. Clearly, variations can be made to the 
original design described in this document for adapting the invention to 
other embodiments. For example, the invention can easily be modified by 
one of ordinary skill in the art to operate in top gating molds. Also, 
other clamp elements and other types of clamps can be adapted for use with 
the invention. Therefore, the invention should be read as limited only by 
the appended claims.