Stackable double-density integrated circuit assemblies

Integrated circuit (IC) packages having leads projecting in the vertical direction are provided with male and female locking elements of snap fasteners that allow a pair of IC packages to be stacked into a module so as to align the leads of one package with the leads of another package. The leads of the packages are soldered to a PCB that carries the external conductors to be connected with the inner circuits of the packages. The male locking element on one of the IC packages is tightly engaged with the female locking element on another IC package to prevent the soldered leads of one package from touching the leads of another package. Multiple modules are positioned on the PCB to double the packaging density of the PCB.

TECHNICAL FIELD 
The present invention relates generally to integrated circuit packaging, 
and more particularly, to integrated circuit (IC) packages arranged in a 
stack to produce a double-density IC assembly. 
BACKGROUND ART 
In an electronic system, components must be mounted on some interconnection 
network for communication to other parts of the system. The components of 
various types and configurations can be attached to a printed circuit 
board (PCB) to make the board a functional electronic device. To reduce 
costs of the assembly, semiautomatic or automatic insertion of components 
into PCBs are employed. However, multi-leaded components, e.g. integrated 
circuits, present problems for semiautomatic or automatic assembly because 
of lack of uniformity of body size and shape, difficulty of maintaining 
desired lead conditions, and feed problems at the input to insertion 
mechanisms. Successful mechanized assembly for integrated circuits is 
being achieved by using relatively standard integrated circuit (IC) 
packages. 
Suitable packages for an integrated circuit must satisfy a series of 
requirements. They must be strong enough mechanically to withstand the 
stresses occurring during use and provide easily established and reliable 
electrical connection from the circuit inside to the external environment. 
Leaded integrated circuit packages can be formed in various configurations. 
For example, the Small Outline Integrated Circuit (SOIC) package 
illustrated in FIG. 1 includes an epoxy body 2 having two rows of leads 4 
projecting from opposite edges of the body 2. An IC die 6 enclosed within 
the body is coupled to the leads 4 via gold bonding wire 8. The leads are 
preferably perforated between the lead bonding pads and just inside the 
body edge. Perforations 10 provide mechanical strength to aid in the 
adhesion of the epoxy to the leads. The SOIC packages have leads of 
gull-wing configuration. As shown in FIG. 1, each lead extending outwardly 
from the package sidewalls is bent outwardly to form a foot parallel to 
the top and bottom surfaces of the package, and parallel to the printed 
circuit board (PCB) to which it is mounted. 
Another common configuration for IC packages is the Small Outline J-leaded 
(SOJ) package illustrated in FIG. 2. The SOJ package has two rows of leads 
extending outwardly from the opposite sidewalls and bent in the shape of a 
"J" under the package body. 
Square or quad IC packages have a body of a square shape with leads 
extending from all four sides of the body. FIG. 3 illustrates the Quad 
Flat Pack (QFP) package having leads of gull-wing configuration, and FIG. 
4 shows the Plastic Leaded Chip Carrier (PLCC) with J leads on all four 
sides of the package. 
Component manufacturers offer wide tolerances of lead profiles for IC 
packages. For example, as shown in FIG. 5A, Motorola provides foot lengths 
from 26 mils to 30 mils for the same part number of SOIC packages. 
Signetics offers SOIC foot lengths from 16 mils to 50 mils for the same 
part number (FIG. 5B), and National provides SOIC foot lengths from 20 
mils to 50 mils for the same part number (FIG. 5C). Also, as shown in 
FIGS. 5A-5C, various lead widths are available for the same part number. 
To provide an IC package assembly, for example, a memory module of very 
high capacity, a group of IC packages are usually assembled on a PCB 
carrier connected with a system PCB or a mother board that carries the 
circuits to be connected with the inner circuits of the IC packages. As 
illustrated in FIG. 6, such a PCB carrier may have multiple packages 
mounted on a printed board and interconnected with each other by printed 
conductors on the printed board. 
Due to space limitations in some portable applications, for example, in 
laptop computers, an area occupied by a PCB carrier on a mother board for 
the same memory capacity should be minimized. To obtain higher packaging 
density, two IC packages may be combined in a stack, wherein one package 
is arranged on top of another. However, in portable systems, the packages 
in the stack may be moved with respect to each other. As a result, 
corresponding leads of the packages may touch each other, and cause a 
short-circuit. 
To fix IC packages with respect to each other, a package stack may be 
inserted into a casing attached to a PCB. An example of such a casing 
having metallic lamination layers can be found in U.S. Pat. No. 5,279,029. 
Alternatively, a socket for receiving a package stack may be provided on a 
PCB, for example, see U.S. Pat. No. 4,696,525. 
However, the prior art devices for stacking IC packages occupy substantial 
areas on a PCB and, therefore, reduce packaging density. Also, they 
include complicated mechanical arrangements that increase the 
manufacturing cost of a PCB. 
Accordingly, it would be desirable to fix IC packages arranged in a stack 
to each other without reducing PCB packaging density. 
Also, it would be desirable to provide a reliable high-density IC package 
assembly that is low in cost and easily implemented. 
DISCLOSURE OF THE INVENTION 
Accordingly, one advantage of the present invention is in providing a 
high-density IC package assembly wherein packages arranged in stacks are 
fixed to each other without reducing the packaging density of the 
assembly. 
Another advantage of the present invention is in providing a high-density 
IC package assembly that is low in cost and easily implemented. 
The above and other advantages of the invention are achieved, at least in 
part, by providing an electronic package assembly comprising a plurality 
of IC packages having top and bottom surfaces and leads projecting in a 
direction perpendicular to the top and bottom surfaces. The packages are 
combined in at least one stack including one of the IC packages having its 
bottom surface arranged on the top surface of another package. At least 
one pair of male and female locking elements are fixed on adjacent top and 
bottom surfaces of the IC packages arranged in the stack for attaching the 
IC packages to each other so as to prevent the leads of one of the IC 
packages from touching the leads of another IC package. 
In accordance with one aspect of the invention, the male and female locking 
elements attach the IC packages to each other so as to establish an air 
gap between the IC packages to allow forced cooling to be provided. 
In accordance with another aspect of the invention, a pair of the male 
locking elements and a pair of the female locking elements are provided at 
each of the adjacent top and bottom surfaces for rigidly orienting the IC 
packages with respect to each other. 
Preferably, the male and female locking elements are made of elastic 
material, to provide tight engagement with each other. 
In accordance with a further aspect of the invention, the leads of all of 
the IC packages are directly soldered to a surface of a printed-circuit 
board having a row of contact lands designated for each of the IC 
packages. These rows may be displaced with respect to each other in a 
longitudinal direction. 
Still other objects and advantages of the present invention will become 
readily apparent to those skilled in this art from the following detailed 
description, wherein only the preferred embodiment of the invention is 
shown and described, simply by way of illustration of the best mode 
contemplated of carrying out the invention. As will be realized, the 
invention is capable of other and different embodiments, and its several 
details are capable of modifications in various obvious respects, all 
without departing from the invention. Accordingly, the drawings and 
description are to be regarded as illustrative in nature, and not as 
restrictive.

BEST MODE FOR CARRYING OUT THE INVENTION 
Although the invention has general applicability in the field of electronic 
packaging, the best mode for practicing the invention is based in part on 
the realization of a structure composed of IC packages with gull-wing and 
J lead configurations. However, it is to be understood that the present 
invention is also applicable to assemblies that include electronic 
packages of any type. 
Reference is now made to FIG. 7 of the drawings showing a pair of IC 
packages 50a and 50b having gull-wing leads 52a and 52b, respectively. 
Male locking element 54 is fixed on the bottom surface of the top package 
50a, whereas female locking element 56 is fixed on the top surface of the 
bottom package 50b. 
As shown in FIG. 8, the male locking element 54 on the top package 50a can 
be pressed manually or mechanically into an opening 60 of the female 
locking element 56 on the bottom package 50b to engage the packages 50a 
and 50b with each other. The locking elements 54 and 56 are made of 
elastic material, e.g. plastic, to provide tight engagement. Hence, each 
pair of the opposite locking elements on the packages 50a and 50b 
constitutes a snap fastener 70 for attaching the packages 50a and 50b to 
each other. 
FIG. 9 illustrates an IC package assembly 80 composed of the packages 50a 
and 50b arranged in a stack. As discussed above, the wide range of lead 
profiles is available for the same type of an IC package. Thus, the lead 
profiles of the packages 50a and 50b may be selected so as to avoid 
mechanical contacts between the corresponding leads 52a and 52b. An air 
gap is maintained between the packages 50a and 50b to allow forced air 
cooling to be provided. The packages are rigidly oriented by the male and 
female locking elements 54 and 56 so as to align the leads of one package 
with respect to the leads of another package during a placement procedure 
before soldering to a PCB. As a result, the soldered packages will not 
shift with respect to each other. Thus, the locking elements 54 and 56 
prevent the leads of the top and bottom packages from touching each other 
to avoid a short-circuit. 
As shown in FIG. 10, the leads 52a and 52b of the IC package assembly 80 
may be soldered to a PCB 90 having two rows of contact lands 92a and 92b 
arranged along both edges of the assembly 80. Printed conductors 94 are 
provided to connect the contact lands 92a and 92b to other contact lands 
on the PCB 90. The feet of the leads 52a are soldered to the contact lands 
92a, whereas the feet of the leads 52b are soldered to the contact lands 
92b. As illustrated in FIG. 1OA showing a top view of the assembly 80 in 
FIG. 10, corresponding lands 92a and 92b are aligned with each other so as 
to position the leads 52b under the leads 52a. 
Alternatively, as shown in FIG. 11, the corresponding contact lands 92a and 
92b may be displaced so as to align the lands 92a with a gap between the 
lands 92b. As illustrated in FIG. 11A showing a top view of the assembly 
80 in FIG. 11, the packages 50a and 50b are displaced so that leads 52a 
are positioned above gaps between the leads 52b. The land displacement 
allows solder paste printing of the lands 92a and 92b to be facilitated. 
Reference is now made to FIG. 12 showing another embodiment of the present 
invention. QFP packages 100a and 100b having gull-wing leads 102a and 
102b, respectively, that project from all four sides of the packages may 
be arranged in a stack using a pair of male locking elements 104 and a 
pair of female locking elements 106 on each of the packages. The top 
package 100a has the locking elements 104 and 106 arranged near the 
corners of its bottom surface, whereas the locking elements 104 and 106 of 
the bottom package 100b are provided near the corners of its top surface. 
The locking elements of the same type are arranged in the opposite corners 
of the packages. The male and female locking elements 104 and 106 rigidly 
orient the packages 100a and 100b during placement so as to align the 
leads 102 and 104 with respect to each other. The tight engagement of the 
corresponding locking elements on the packages 100a and 100b prevents the 
leads 102a and 102b from touching each other. 
As shown in FIG. 13, the present invention allows IC packages having 
different lead configurations to be stacked. For example, a top package 
100a has a gull-wing configuration of leads 112a, whereas a bottom package 
100b has J leads 112b. Pairs of male and female locking elements 114 and 
116 are provided near corners of the opposite surfaces of the packages 
110a and 110b. The lead profiles of the packages may be selected during 
placement to avoid mechanical contacts between the leads 112a and 112b in 
a module 120 composed of the packages 110a and 110b. Alignment recesses 
118a and 118b on the packages 110a and 110b allow the packages to be 
oriented during placement. 
As illustrated in FIGS. 14 and 15, multiple modules 80 and 120 shown in 
FIGS. 9 and 13, respectively, may be positioned on a PCB so as to double 
the packaging density of the PCB compared to a conventional PCB. Although 
the drawings show that each of the modules 80 and 120 is composed of two 
IC packages, it is to be understood that any number of the packages can be 
stacked. 
In summary, the results and advantages of the DIP package assembly of the 
present invention can now be more fully appreciated. IC packages having 
leads projecting in the vertical direction are provided with locking 
elements of snap fasteners that allow a pair of IC packages to be stacked 
into a module so as to align the leads of one package with respect to the 
leads of another package. The leads of the packages are soldered to a PCB 
that carries the external conductors to be connected with the inner 
circuits of the packages. The tight package engagement provided by the 
locking elements prevents the soldered leads of one package from touching 
the leads of another package. Multiple modules are positioned on the PCB 
to double the packaging density of the PCB. 
Accordingly, a high-density IC package assembly is provided wherein 
packages arranged in stacks are fixed to each other without reducing the 
packaging density of the assembly. 
It will also be recognized that the high-density package assembly of the 
present invention is low in cost and easily implemented. 
In this disclosure, there is shown and described only the preferred 
embodiment of the invention, but, as aforementioned, it is to be 
understood that the invention is capable of use in various other 
combinations and environments and is capable of changes or modifications 
within the scope of the inventive concept as expressed herein.