1. Field of the Invention
The present invention relates generally to a printed circuit board (PCB) for use in chip-on-board (COB) packages and the COB packages using the same. More particularly, the present invention relates to a PCB which has protrusions or trenches within an encapsulation region on the PCB in order to prevent warping of the COB package after the encapsulation process.
2. Description of the Related Arts
As semiconductor integrated circuit devices become more complex and achieve greater speed and functionality, larger numbers of input/output (I/O) pins are required to realize the enhanced performance parameters of the new designs. At the same time, however, the overall size of the devices must continually be reduced to meet the need for employment in smaller and smaller electronic devices. To meet these conflicting needs of higher pin densities in smaller areas, so-called chip-on-board (COB) packages have been developed. In COB packages, a semiconductor integrated circuit device (IC) is directly mounted on a printed circuit board (PCB), thereby eliminating the need to construct a separate lead frame for connection to the device.
COB packages are heavily used for memory cards. As is well-known, memory cards have been developed for many uses such as storing captions, sounds, or static images generated by electronic entertainment equipment and digital still cameras, as substitutes for hard disks of computers, and as components of portable information storage and retrieval devices. By packaging several memory chips onto a single board, memory cards can attain a large storage capacity.
Memory cards include miniature cards, compact flashes, and SmartMedia. Among these, miniature cards and compact flashes suffer from high overhead cost and bulky size because both require built-in controllers.
Integrated circuit (IC) cards such as SmartMedia or solid state floppy disc cards (SSFDC), which are used for storing digital signals, contain COB packages with built-in flash memory chips. These IC cards enjoy advantages over earlier memory cards, including smaller size, good portability, and the ability to easily expand or upgrade the memory capacity without having to change the I/O pin name conventions. SmartMedia is used for storing information from, for example, digital still cameras or personal digital assistants (PDAs), and may be used in either an internal or removable mode. Moreover, SmartMedia is expected to be used more extensively in the future for a wide range of applications since it has a greater storage capacity than floppy diskettes or ID cards using a magnetic strip or tape, and is easy to store and handle.
The conventional PCB utilized in COB packages is described with reference to a plan view in FIG. 1. The conventional PCB 10 consists of thin board body 11 made from a glass-epoxy resin or BT (Bismaleimide Triazine) resin. A circuit pattern of conductors 13, which serve as electrical signal transmission pathways, is formed in the upper surface of the PCB, for example by using a copper foil and etching process. The bottom surface of the PCB is also provided with a pattern of conducting material which serves as a set of external contacts 15 as shown in FIG. 3. The conductors 13 of the circuit pattern in the upper surface of the PCB 10 are electrically connected to corresponding external contacts 15 by way of via holes 16 in the board body 11.
The circuit pattern of conductors 13, the external contacts 15, and the inner wall of the via holes 16 are coated with a conductive plating material, for example gold (Au) which does not suffer oxidation. A chip bonding area 12 is also formed on the upper surface of the board body 11. Usually, a cavity is produced in the bonding area 12 to reduce the thickness of the board body 11 where the chip will be attached and thus reduce the overall height of the finished package.
The PCB 10 is usually manufactured in the form of a strip like a conventional lead frame strip. On the strip, each individual PCB 10 is linked to guide rails 19 by way of tie bars 18.
FIG. 2 and FIG. 3 depict the conventional COB package 100 employing the PCB 10 described above. The COB package 100 includes a semiconductor chip 20, a conventional PCB 10, bonding wires 22 and an encapsulant 23. The chip 20 is attached to the chip bonding area 12 on the upper surface of the PCB 10, and the chip 20 is electrically connected to the circuit pattern of conductors 13 by using bonding wires 22 made from gold (Au) or aluminum (Al). Thus, the chip 20 which is electrically connected to conductors 13 of the circuit pattern via the bonding wires 22, is also electrically connected to the external contacts 15 formed on the lower surface of the PCB by way of the conductors 13 and the via holes 16. The upper surface of the PCB 10 within an encapsulation region is encapsulated with a molding compound, such as a thermosetting compound like epoxy resin, to form an encapsulant 23 which provides protection from external environmental stresses. The encapsulant 23 of the conventional COB package extends above the upper surface of the PCB and is usually formed using a transfer molding process.
In an exemplary memory chip configuration, the chip 20 on the chip bonding area 12 may be a flash memory chip having a storage capacity, for example, of 4 Megabytes (M), 16 M or 32 M. In general, the size of a flash memory chip increases with its storage capacity. For example, a 4 M flash memory chip has a size of about 116.1 mm.times.269.1 mm, where mm denotes a millimeter, a 16 M chip has a size of about 196.5 mm.times.444.7 mm, and a 32 M chip has a size of about 294.5 mm.times.514.7 mm. The dimension of the chip bonding area 12 varies according to the size of the chip 20 to be attached to it.
The COB package 100 retains the same pin name conventions among the versions using different flash memory chips so that an appliance using the COB package 100 can be easily expanded or upgraded by interchanging cards. Therefore, the structure of the base card on which the COB package 100 will be mounted must be uniform. Consequently, the shape or configuration of the PCB 10 also must be uniform, regardless of the size of the flash memory chip 20. Therefore it is necessary to be able to mount several kinds of flash memory chips, each with different capacity, onto a single sized and shaped PCB.
Since the shape of the COB package 100 should be uniform regardless of the size of the chip 20 mounted on the PCB 10 therein, the volume of molding compound such as epoxy resin employed for forming the encapsulant 23 of the COB package 100 becomes smaller as the chip size becomes larger. The COB package 100 having the smaller-sized 4 M chip, in which the largest volume of resin is incorporated to form the encapsulant, undergoes more warping than a COB package having the mid-sized 16 M chip. Similarly, the COB package 100 having the larger-sized 32 M chip, in which the smallest volume of resin is incorporated, also undergoes more warping than a COB package having the mid-sized 16 M chip--this time in an opposite direction.
Therefore, the volume of the molding resin must be balanced with the thermal expansion of the PCB, and with the other conditions of the molding process, in order to prevent failures of the COB package due to warping. However, it is not easy to balance the volumes and prevent such warping failures, because the encapsulant is constrained to have nearly uniform external dimensions while the chips within the encapsulant have different sizes. This situation forces the encapsulant to contain different volumes of molding compound.
Consequently, the COB package containing a 4 M flash memory chip usually undergoes warping in which the both ends of the PCB bend upward while the center part where the encapsulant is formed bends downward. By contrast, the COB package containing a 32 M flash memory chip undergoes warping in which the both ends of the PCB bend downward while the center part bends upward. Thus warping failures occur more often for COB packages with the smallest or largest memory chips, than for COB packages with a mid-sized memory chip.
Therefore there is a need for a COB package that reduces warping failures while varying memory chip sizes and simultaneously maintaining nearly uniform encapsulant external dimensions.