Integrated circuit package with molded cell

An integrated circuit package encapsulates a volatile memory chip and a backup battery for preserving data in the event of loss of main power supply. The package includes a lead frame assembly encapsulated within a body of non-conductive material, with the memory chip being mounted onto a base plate on one side of the lead frame. The battery is supported in offset relation by axial power leads on the opposite side of the lead frame. The integrated circuit chip, the battery, the lead frame assembly and the gold interconnect wires are completely encapsulated within the molded package body.

FIELD OF THE INVENTION 
This invention relates generally to packaging for semiconductor devices, 
and in particular to an integrated circuit package for encapsulating a 
semiconductor integrated circuit such as a memory chip and a backup 
battery for preserving memory data in the event of loss of main power 
supply. 
BACKGROUND OF THE INVENTION 
Conventional electronic circuit packages for semiconductor integrated 
circuit chips are adapted to enclose and seal the chip devices, while also 
providing heat dissipation, structural support, electrical attachment of 
device leads to external pin connectors, and electrical interconnection 
with other devices in the package. Such packages may be formed of one or 
more layers of a non-conductive material, with the semiconductor chip 
embedded within one of the layers. Flexible metal leads are extended from 
an interconnect region surrounding the chip to edge mounted connector pins 
for connecting the device input/output terminals to a printed circuit 
board socket in a host electronic circuit. 
An important integrated circuit product which is implemented on an IC chip 
encapsulated within an integrated circuit package includes a volatile 
semiconductor memory such as the static random access memory (SRAM) which 
is characterized by low power consumption and high memory cell density. 
The generation of valid logic signals and the retention of data in such 
integrated memory circuits having volatile memory cells depend in part on 
maintenance of power supply voltages within specified limits. In 
conventional integrated circuit memory devices, internal circuits sense 
the external source voltage being applied to determine if it is sufficient 
for reliable operation. In response to a low voltage condition, control 
signals are generated which cause active chips to be de-selected and 
maintained in standby condition. This is usually carried out by means of 
true and complement chip select signals, CS and CS, respectively, which 
inhibit read/write operations until the low voltage condition has been 
corrected. 
During the period that a memory chip is in the unselected condition it is 
necessary to maintain the charge levels of the storage capacitors in the 
volatile memory cells so that stored data will be retained. Otherwise, the 
information stored in the memory cells, including programs and data, will 
be lost when main power is removed. Although the loss of power does not 
result in memory circuit damage, the loss of stored information requires 
that the memory be reloaded with programs and data before processing can 
be reestablished. 
DESCRIPTION OF THE PRIOR ART 
It has been proposed to solve the data loss problem by using an additional 
pin terminal on memory semiconductor circuits and that the additional 
terminal will be supplied with backup power from a remote source to 
maintain the data in the memory cells. However, there are now established 
standardized pin patterns for most integrated circuit memories; 
consequently, the addition of another pin dedicated to a remote backup 
power supply would not be compatible with standard pin patterns, and would 
require a substantial redesign of existing circuits. 
Accordingly, there exists a need for a semiconductor memory package for 
encapsulating a memory chip and a backup battery wherein the socket area 
and standard pin configuration are not affected, and stored data are 
retained despite a loss of the main power supply. 
A substantial portion of the cost and size of a packaged chip is 
attributable to package fabrication, and two important design criteria in 
addition to providing a reliable electrical connection are cost 
effectiveness and space efficiency. A need thus exists for an improved 
device package for safely supporting an integrated circuit ship and a 
backup battery wherein the package is provided with pin connectors formed 
therein for plug-in compatibility with standard printed circuit board 
sockets, and the packaging space required for supporting the backup 
battery is minimized. 
Some packages for integrated circuit memory devices include a battery 
molded within one half section of a two-part package. In that 
construction, a chip is loaded onto the base plate of a lead frame and 
wires are bonded between I/O pads and respective internal leads. The mold 
is heated, and molding resin is then injected into the heated mold cavity. 
Consequently, the lead frame and IC chip are encapsulated by the resin 
within one molded half section. A small battery and other discrete 
components, for example a crystal, are mounted within a second half 
section. The second half section includes connector pins accurately 
positioned for engaging finger leads in the lead frame of the first molded 
half section. The dual section arrangement has served well for many 
product applications. However, the additional height imposed by the second 
half section produces a package which exceeds the maximum height limit 
established for critical space product applications. 
Accordingly, a need thus exists for an improved device package in which a 
semiconductor circuit device, a lead frame assembly and a backup battery 
are encapsulated within a single molded body of non-conductive material, 
wherein the packaging height dimension is less than the height of 
conventional two-part device packages which include a backup battery. 
SUMMARY OF THE INVENTION 
The present invention provides an improved package for encapsulating an 
integrated circuit device, including a backup battery, and it overcomes 
the foregoing backup battery limitations by mounting the integrated 
circuit device onto a base plate on one side of a finger lead assembly, 
and by mounting an axial lead battery on the opposite side of the finger 
lead of a lead frame assembly. In this arrangement, the battery has 
positive and negative axial leads and which support the body of the 
battery in offset relation above the finger lead frame assembly. The 
integrated circuit device substrate is bonded to the underside of the base 
plate by a layer of adhesive. The backup battery, integrated circuit 
device and finger leads are totally enclosed within the molded body of the 
package, without altering the socket area or the pin configuration. 
Although the package height is increased to totally encapsulate the backup 
battery, the package height does not exceed the maximum limit established 
for critical space product applications. 
Operational features and advantages of the present invention will be 
appreciated by those skilled in the art upon reading the detailed 
description which follows with reference to the attached drawings, wherein 
:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the description which follows, like parts are indicated throughout the 
specification and drawings with the same reference numerals, respectively. 
By way of example, the invention is described in combination with a static 
random access memory (SRAM) which is implemented by monolithic CMOS/LSI 
techniques on an N-type silicon semiconductor chip. It will be 
appreciated, however, that the packaging assembly of the invention may be 
used to encapsulate and provide backup battery power for discrete as well 
as other integrated devices, and has particular utility for volatile 
memory integrated circuits having multiple input/output nodes. 
Accordingly, it should be understood that the invention in its broadest 
aspects may be incorporated in any moldable package which houses one or 
more circuit devices requiring backup power, including but not limited to 
discrete, micro-discrete and integrated circuit components, and hybrid 
combinations of discrete and integrated devices. 
Referring now to FIG. 1 and FIG. 2, there is shown an exemplary 
semiconductor chip package 10 incorporating the present invention. The 
package 10 supports and encapsulates an integrated circuit chip 12 having 
input/output nodes 14. The IC chip 12 may be, for example a 2K.times. 8 
static random access memory (SRAM) circuit which is characterized by low 
power consumption, high memory cell density and which is implemented on an 
N-type silicon substrate by complementary metal oxide semiconductor (CMOS) 
technology. 
The exemplary package 10 has a conventional dual-in-line pin configuration 
including twenty-four external connector pins arranged in two parallel 
rows with 600 mil spacing along the longitudinal edges of the package. The 
input/output nodes 14 of the integrated circuit chip 12 are electrically 
connected to selected connector pins 16 by conductive finger leads 18 of a 
lead frame assembly 20 as shown in FIG. 3. 
The inner lead fingers 18 are radially spaced with respect to a central 
base plate 22 and are integrally formed with the connector pins 16. 
Linking segments 20L of the lead frame assembly 20 are ultimately trimmed 
away during manufacture, whereby each inner lead 18 is electrically 
connected to a single connector pin 16. Transport side rail strips 24, 26 
on the outer perimeter of the lead frame 20 are also cut away during trim 
and form operations in the last stages of manufacture, after molding has 
taken place. 
The inner tips of the conductive fingers 18 are radially spaced about the 
base plate 22 in an interconnect region R. The inner tips of the 
conductive fingers 18 are relatively narrow, and the fingers expand 
substantially as they radiate outwardly from the base plate 22. The lead 
frame assembly 20 includes base plate finger leads 28, 30 which extend 
from opposite edges of the base plate 22 in alignment with the 
longitudinal axis Z of the integrated circuit package 10. 
The external connector pins 16 and inner finger leads 18 are initially 
coplanar during molding, as shown in FIG. 3. After molding, the connector 
pin portions 16 are bent through a 90 degree angle along the longitudinal 
side surfaces of the package during the trim and form operation. However, 
the inner finger leads 18 and base plate 22 remain coplanar as shown in 
FIG. 1 and FIG. 2. 
The semiconductor chip package 10 provides a standard external pin pattern 
for electrically connecting the input/output nodes 14 of the semiconductor 
chip 12 to a socket on a printed circuit board of a host electronic system 
or on some other semiconductor package. The chip package 10 includes a 
molded body 32 of non-conductive material, for example a polymer such as 
polyetherimide or epoxy resin. In this arrangement, the finger lead 
assembly 2, the semiconductor chip 1 and a backup battery 34 are embedded 
and encapsulated within the molded body 30. 
Preferably, the backup battery 34 is a axial lead cell which is 
hermetically sealed so that its electrolyte will not evaporate when it is 
exposed to the elevated temperature conditions of the transfer molding 
procedure. Additionally, it is preferred that the battery 34 have a 
non-linear internal resistance which increases rapidly to a high current 
limiting value in response to short circuit current flow. This is 
desirable because the backup battery 34 will be shorted by the lead frame 
during assembly, and during transfer molding, until the lead frame has 
been trimmed. For some applications, it is desirable that the backup 
battery 34, in addition to being hermetically sealed and having short 
circuit protection, is also rechargeable so that its charge level can be 
restored to its rated value after assembly has been completed. 
In one exemplary embodiment, the battery 32 is a 2.8 volt DC cell having a 
substantially cylindrical body of 0.1575 inch diameter and a body length 
of about 0.42 inch. It is essential that the battery 34 be rated for high 
temperature duty, since it will be exposed to high temperatures during 
wire bonding and transfer molding. Otherwise, the electrolyte within the 
battery will evaporate and the battery charge will be destroyed. An 
example of a suitable axial lead battery 34 is a lithium-iodine cell which 
can be obtained from Catalyst Research Company of Baltimore, Md., under 
Model No. B35. That cell is rated at 2.8 volt DC and 35 Mah has a 70 
degree C. shelf life of 10 years, and can survive elevated temperature 
conditions of 225 degrees C. for three to five minutes without failure, 
which is more than adequate for the transfer molding procedure 
contemplated herein. The battery 34 has a positive terminal 34P and a 
negative terminal 34N formed on opposite ends of the battery body. 
The semiconductor chip 12 is bonded to the underside surface 22A of the 
base plate 22 by a conductive deposit of silver-filled epoxy adhesive such 
as Ablebond.TM. 84-1. The input/output nodes 14 are electrically connected 
to selected conductive fingers 18 by fine gold wires 36 having a diameter 
of 1.3 mil. Bonding of the gold wires 36 to the conductive fingers 18 and 
I/O nodes 14 is preferably by the conventional thermosonic ball bonding 
technique. 
The negative and positive terminals 34N, 34P of the backup battery 34 are 
electrically connected to finger leads 28, 38, respectively, prior to 
encapsulation, preferably by resistance welding or by soldering. The 
finger leads 28, 38 serve as negative and positive interconnect leads for 
conducting current from the backup battery 34 to the interconnect region 
R. As can be seen in FIG. 1, the battery 34 has negative and positive 
axial leads 40, 42 which support the body of the battery 34 in vertical 
offset relation above the lead frame assembly 20. Each axial lead 40, 42 
is terminated by a freestanding, compliant gull-wing lead 40A, 42A, 
respectively, having bonding feet 40F, 42F, respectively. The interconnect 
leads 28, 38 have distal end portions 280, 380, respectively, which serve 
as bonding pads for electrical attachment to the battery leads 40, 42, 
respectively. The dimensions of the gull-wing leads are selected to 
provide a battery stand-off clearance A of 0.030 inch above the base plate 
22, and a total axial length including axial leads) of about 1.25 inch. 
After the device 12 has been attached to the base plate mounting surface 
22A, the ends of the fine gold wires 36 are then connected between the 
chip I/O nodes 14 and the respective finger leads 18. A gold wire 36P is 
bonded between the positive interconnect lead 38 and the positive backup 
voltage node 14P of chip 12. The negative backup voltage node 14N is 
electrically connected to the negative interconnect lead 28 by a gold wire 
36N. 
The lead frame assembly 20 is then placed in a multi-cavity split mold. The 
mold cavity is closed in a transfer molding machine and a non-conductive 
encapsulant material such as polyphenolene sulfide is injected in fine 
pellet form from a nozzle. The pressure at which this injection takes 
place is closely controlled to prevent damage to the gold wire bonds. 
Under the appropriate pressure and temperature, the pellets melt and flow 
into channels within the mold and fill the cavities around the lead frame 
assembly 20, thereby completely encapsulating the lead frame 20, the 
backup battery 34, IC chip 12 and gold wires 36. The resin is cured while 
still in the mold by the applied heat and pressure. Further curing takes 
place in an oven. 
As a result of the foregoing transfer mold procedure, the package 10 is 
produced in the form of an elongate, generally rectangular molded body 32 
of non-conductive material. After removal from the mold, the linking 
segments 20L between adjacent pins 16 in the lead frame assembly 20 are 
cut to separate and electrically isolate the pins and conductive finger 
leads from one another. Additionally, the transport side rails 24, 26 are 
also cut and separated from the molded assembly. 
The lead frame 20 material preferably comprises a conventional metal alloy, 
such as a tin-plated nickel or iron alloy or, alternatively, a tin-plated 
copper alloy such as CDA 194. It will be appreciated that during assembly, 
the connector pins and inner conductive leads are structurally 
interconnected by the linking segments 20L and by the side transport side 
rails 24, 26, preferably stamped from a continuous metal strip. The 
connecting sections remain attached to the connector pins for handling 
purposes only and are severed during trim and form operations in the last 
stages of manufacture, after molding has taken place. 
It will be understood that a selected one of the external pins 16 is 
adapted for connection to a primary power supply node which provides a 
voltage V.sub.CC which is typically at +5.0 volts DC. Similarly, another 
external connector pin is adapted for connection to a ground node of a 
host electronic system for providing a ground-reference GND. Other pins 
are dedicated for true and complement chip select signals, CS and CS, a 
signal CLK for synchronously clocking data to and from the monolithic 
integrated circuit 12, as well as various other I/O signals which are 
produced by the host electronic circuit. A comparator and switching 
circuit (not illustrated) compares the voltage V.sub.CC from the primary 
power supply of the host electronic circuit with the voltage of the backup 
battery 34 and automatically connects the highest detected voltage to 
power the integrated circuit 12. 
Because of the vertical offset A, the backup battery 34 is vertically 
spaced from the top surface of the base plate 22 and is centered 
longitudinally within the package 10. By virtue of this arrangement, the 
finger lead assembly, the backup battery, chip and gold wires ar 
completely encapsulated within the compact molded body 32. 
Although the invention has been described with reference to a preferred 
embodiment, and with reference to a package which encapsulates and 
provides backup battery power for an integrated circuit device, the 
foregoing description is not intended to be construed in a limiting sense. 
Various modifications of the disclosed premolded battery package as well 
as alternative applications thereof will be suggested to persons skilled 
in the art by the foregoing specification and illustrations. It is 
therefore contemplated that the appended claims will cover any such 
modifications or embodiments that fall within the true scope of the 
invention.