GB2042774A

GB2042774A - Fabricating method of electronic watch module

Info

Publication number: GB2042774A
Application number: GB8001694A
Authority: GB
Original assignee: Citizen Watch Co Ltd
Current assignee: Citizen Watch Co Ltd
Priority date: 1979-01-18
Filing date: 1980-01-18
Publication date: 1980-09-24
Also published as: GB2042774B; CH641626B; CH641626GA3

Abstract

A method of fabricating an electronic watch module in which a flexible printed circuit board (12) having a printed circuit pattern and a plurality of leads (42) extending therefrom is prepared and an IC chip (14) is wireless bonded to the leads (42) of said flexible printed circuit board (12), which is then inserted into a cavity of a molding die unit (50) and at least a part of one of said flexible printed circuit board (12) and said IC chip (14) is fixedly retained within the cavity of said die unit (50). A thermoplastic resin is supplied into the cavity of said molding die unit (50) for thereby encapsulating said flexible printed circuit board (12) and said IC chip (14), without distortion of the leads (42). <IMAGE>

Description

SPECIFICATION Fabricating method of electronic watch module This invention relates to a method of fabricating an electronic watch module in which an IC chip is wireless bonded to a flexible printed circuit board and is encapsulated in a thermoplastic resin.

Resins for the encapsulation of IC chips are conventionally of the heat-cured type, there being no examples of thermoplastic resins employed for this purpose. The heat-cured resins are almost all epoxy resins, and the encapsulation is accomplished by low-pressure encapsulation systems that rely upon transfer molding machines.

Low-pressure encapsulation using the transfer moulding machine requires a molding time of from 1.5 to 2 minutes and an increase in labor. As a method of reducing this labor it has been proposed that the curing time of the epoxy molding material be shortened or that the number of molds be greatly increased in number. Although a rapid-curing type epoxy resin is now under development, curing time will still be approximately one minute even with this new resin. And adopting a large number of molds leads to considerable expense.

When encapsulation is conducted by inserting into a mold a flexible circuit board having an IC chip wireless-bonded thereto with use being made of of IC chip encapsulation grade thermoplastic resin, positioning of the IC chip cannot be stabilized since the circuit board bends under the molding pressure and the IC chip itself can be swept away owing to the flow of resin if molding is attempted freely without subjecting the IC chip and circuit board to some form of control.

Moreover, when leads are bent due to the molding pressure they may contact the edge portion near a pad of the IC chip for the IC connection, giving rise to a so-called shoulder-touch phenomenon which can be fatal to an electronic watch module.

According to the present invention, there is provided a method of fabricating an electronic watch module, comprising: preparing a flexible printed circuit board having a printed circuit pattern and a plurality of leads extending from said printed circuit pattern; wireless bonding an IC chip to the leads of said flexible printed circuit board; inserting said flexible printed circuit board with said IC chip into a cavity of a molding die unit; fixedly retaining at least a part of one of said flexible printed circuit board and said iC chip within the cavity of said molding die unit; and supplying a thermoplastic resin into the cavity of said molding die unit for thereby encapsulating said flexible printed circuit board and said IC chip.

In the accompanying drawings, in which: Fig. 1 is a plan view of an electronic watch module fabricated by a method of the present invention; Fig. 2 is a cross sectional view of the watch module taken along line 2-2 of Fig. 1; Fig. 3 is a view illustrating a shoulder-touch phenomenon encountered in a prior art watch module fabricating method; Fig. 4 is a block diagram illustrating an electronic watch module fabricating method of the present invention; Fig. 5 is a fragmentary cross sectional view of a molding die unit to be used for carrying out the method shown in Fig. 4; Fig. 6 is a fragmentary cross sectional view of the watch module with its recessed portion being potted with a resin of IC encapsulation grade; Fig. 7 is a fragmentary cross sectional view of the watch module with its recessed portion being press fitted with a plastic molded article;; Fig. 8 is a fragmentary cross sectional view of a modification of the molding die unit shown in Fig.

5; Fig. 9 is a fragmentary cross sectional view of the molding die unit shown in Fig. 8; Fig. 10 is a fragmentary cross sectional view of another preferred example of a molding die unit to carry out the fabricating method of the present invention; Fig. 11 is an enlarged cross sectional view showing a modification of the molding die unit shown in Fig. 10; Fig. 12A is a plan view of another example of an electronic watch module fabricated by the method of the present invention; Fig. 1 2B is a cross sectional view of the watch module taken on line Y-Y of Fig. 1 2A; Fig. 13 is a plan view of another example of the watch module with a flexible printed circuit board being shown by a phantom line; Fig. 14 is a cross sectional view of the watch module taken on line 14-14 of Fig. 13;; Fig. 1 5 is a cross sectional view of a conventional watch module; Fig. 16 is a cross sectional view of a watch module improved over the structure shown in Fig.

15; Fig. 17 is a fragmentary cross sectional view of a molding die unit to carry out another preferred example of a method of the present invention; and Fig. 18 is a cross sectional view of a watch module fabricated by the molding die unit of Fig.

17.

A fabricating method of an electronic watch module will now be described in conjunction with the accompanying drawings, in which Fig. 1 is a plan view of the electronic watch module fabricated by a method of the present invention, Fig. 2 a cross-sectional view of Fig. 1 taken along the line 2-2 and showing a display cell, battery and connector are incorporated in the circuit substrate of Fig. 1, Fig. 3 a diagram showing the occurrence of the shoulder-touch phenomenon when a lead is bent by molding pressure, Fig. 5 a diagram showing the positional relationship between a flexible printed circuit board and a molding die unit into which the flexible printed circuit board has been inserted, Fig. 6 a diagram showing the circuit board after insert-molding, the circuit board being potted in a recess of a molded article on the wiring pattern side of the IC chip, and Fig. 7 a diagram of another modification of the present invention in which a plastic molded article has been fit into the recess of Fig. 6.

Referring to Figs. 1 and 2, an electronic watch module designated generally at 10 comprises a flexible printed circuit board 12 having an IC chip 14 mounted thereon by the flip-chip bonding or the mini-mod method, the circuit board having been inserted into a molding die unit and the IC chip 14 encapsulated in a thermoplastic resin 16 which can be used for low-pressure encapsulation.

The watch module 10 is provided with portions for accommodating other elements, namely, connector accommodating recesses 18 for accommodating connectors 20 that provides connection to a display cell 22, a battery accommodating recess 24 for accommodating a battery 26, a crystal vibrator accommodating recess 28 for accommodating a crystal vibrator, chip capacitor accommodating portions 30 for accommodating chip capacitors used for boosting voltage, as well as accommodating portions 32, 34 for accommodating other elements.

The present electronic watch module fabricating method makes use of an IC chip encapsulation grade PPS (polyphenylene sulfide) resin as the thermoplastic resin for low-pressure encapsulation. This resin allows a high level of productivity since the required molding time is on the order of 20 seconds. As the IC chip is bonded by the mini-mod method, the height of the portion at which the IC chip is mounted can be made lower than that possible with a wire bonding method, thereby enabling the production of a module which is thinner than that heretofore available. Furthermore, since the lead has a higher strength than a wire, and because the bonding strength is high, breakage and peeling do not occur and reliability is enhanced.

Designated at 36 is a recess provided on the wiring pattern side of the IC chip, the recess being used for potting or for receiving a plastic molded article which is to be fit therein, as will be subsequently described in a later molding step.

Reference numeral 38 denotes a voltage-boosting chip capacitor, and 40 a battery connection spring implanted in the module 10.

Illustrated in Fig. 3 is an IC chip bonding finger or lead 42 which is obtained by extending the wiring pattern on the flexible printed circuit board 12 to a device hole. The portion B depicts the shoulder-touch phenomenon in which the lead 42 touches the shoulder of the IC chip 14 because of molding pressure.

Fig. 4 shows a method of fabricating an electronic watch module according to the present invention. The method comprises the steps of preparing a flexible printed circuit board, wireless bonding an IC chip to the printed circuit board, locating the printed circuit board in a molding die unit, retaining a predetermined portion of the printed circuit board and the IC chip in a fixed place in the molding die unit, and supplying a flow of thermoplastic resin into the molding die unit so as to encapsulate the flexible printed circuit board and the IC chip. In one preferred aspect, the molding die unit has a bump supporting die projection such that a bump side of the IC chip and lead or finger portions of the flexible printed circuit board are backed by the bump supporting die projection of the molding die unit.The thermoplastic resin is composed of a polyphenylene sulfide-type thermoplastic resin.

The molding die unit is heated at a temperature of about 1 500C and the thermoplastic resin is injection molded at a mold temperature of about 280--3800C and a low injection pressure of from 75 to 200kg/cm2.

The flow of the resin is indicated by the arrows in Fig. 5. It can be appreciated that the resin flows in such a manner that the IC chip 14 and lead portions 42 are pressed against the tapered wall 50' a of the projection 50a of the molding die unit 50 as encapsulation progresses. This prevents the lead portions 42 from bending and completely eliminates the shoulder touch-phenomenon. While allowing the wiring pattern side of the IC chip 14 to be received by the molding die may seem to cast some doubt about reliability, no problems arise because it is the bonding portion 14a of the IC chip 14 that is received and not its wiring pattern itself. The state illustrated in Fig. 5 is one in which the molding die unit 50 has been closed after inserting the flexible printed circuit board 12.

Shown by the hatched lines 52 in Fig. 6 is an article which has been molded by introducing PPS resin of IC encapsulation grade into the arrangement of Fig. 5 which is then used to effect low-pressure molding. The potting portion 54 both protects the IC chip 14 and enhances reliability.

Fig. 7 illustrates an example in which the potting step is replaced by press fitting a plastic molded article 56 into the recess.

Fig. 8 shows another preferred example of a method of fabricating an electronic watch module according to the present invention, with like parts bearing the same reference numerals as those used in Fig. 5. The molding die unit comprises, in addition to the bump supporting die projection 50a, a chip supporting projection 60 having its top end engaging the back side of the IC chip 1 4. the chip supporting projection 60 is slidably disposed in a bore 61 of the lower die 50 and has its bottom end resiliently supported by some suitable cushioning means such as a compression spring 62 disposed in a recess 64 of the lower die 50.

Thus, the projection 60 is resiliently biased upward to restrict the horizontal movement of the flexible printed circuit board 12 during the encapsulating step by resiliently sandwiching the IC chip 14 by the action of the prqjections 50a and 60. After the I C chip 14 has been sandwiched between the die projections 50a and 60, a liquid of thermoplastic resin is supplied into the molding die unit in a direction to flow from the back side of the iC chip 14 toward the die projection 50a. In this case, the lead portions 42 of the flexible printed circuit board 12 are urged against the tapered wall 50' a of the die projection 50a so that bending of the lead portions of the flexible printed circuit board can be avoided to completely prevent the shoulder-touch phenomenon.Further, since the IC chip 14 is resiliently held by the die projections 50a and 60, the lateral displacement of the IC chip 14 is prevented during the encapsulating step.

Fig. 9 shows a modification in which the bump supporting die projection is replaced with a plurality of pins 66, each pin 66 having a bottom planer surface 66a engaging the bonding portion 14a, i.e., the bump side of the IC chip 14, and a tapered wall portion 66b engaging an end portion of each lead portion 42 of the flexible printed circuit board 12. With this arrangement, the IC chip 14 is resiliently supported by the pins 66 and the die projection 60. When the liquid of the thermoplastic resin is supplied into the molding die unit, the liquid of the thermoplastic resin flows into a region between the pins 66 and the IC chip 14 and the flexible printed circuit board 12 are encapsulated in the thermoplastic resin except at areas which are contacted with the pins 66 and the die projection 60.This eliminates the necessity of subsequent potting step or press fitting of a plastic molded article.

Fig. 10 is a cross-sectional view of another example of a molding die unit for manufacturing the electronic watch module and illustrates a flexible printed circuit board in an inserted state.

Specifically, a flexible printed circuit board 90 comprises a flexible sheet 92, a patterned copper foil 94 and a finger portion 94a formed at one end of the patterned copper foil 94, these elements being plated with Sn at portions which so require.

An IC chip 96 has a gold (Au) bump 98 which is bonded to the finger portion 94a by a wireless bonding technique. A mold 100 comprises an upper die 100a and a lower die 1 00b which define a cavity 102 into which is inserted the flexible printed circuit board 90 having the IC chip 96 bonded thereto, an IC chip holder 104 for receiving at least a portion of the bottom and side surfaces of IC chip 96, a plurality of retaining projections 1 00'a and 1 00'b for fixing the flexible printed circuit board 90 in the vicinity of the IC chip 96, a plurality of retaining pins 106 each having a recessed portion for accommodating cell connectors for the connection of an electro-optic display device, a gate 108 for the inflow of thermoplastic resin, and a plurality of second retaining pins 110 for embracing and securing the flexible printed circuit board 90 in the vicinity of the gate 108. Since the molding pressure and temperature are high in the vicinity of the gate 108, a flexible circuit board would experience violent deformation if the retaining pins were absent. This in turn would cause irregular flow in either the upper or lower side of the flexible printed circuit board 90 and the direction of the flow would differ with each molding operation, as would the fluidity of the molding resin inside the mold. This would make it impossible to produce molded articles of a constant quality. However, molding can be stably conducted if the plurality of retaining pins embrace the flexible circuit board from above and below in the vicinity of the gate as described above.The gate 108 comprises upper and low molds 1 08a, 1 08b which are movable molds that are separately adjustable in height to facilitate the flow of the resin.

Fig. 11 is an enlarged view of a modification of the movable gate 108 shown in Fig. 10. In Fig. 11, retaining pins 110 are disposed inside the upper and lower gates 108'a,108'b and are adapted to place the leading edge of the flexible printed circuit board 90 inwardly of the gate 35'. The upper and lower gates 35'a, 35'b are adapted such that they can be slid along the retaining pins 36.

The molds shown in Figs. 10 and 11 makes it possible to obtain molded articles of regular quality by restraining the violent deformation of the thermoplastic circuit board particularly in the vicinity of the gate, such deformation being attributable to the pressure produced when the thermoplastic resin is molded. Moreover, the provision of movable gates allows fine adjustment of the flow of molding resin, making possible the manufacture of the electronic watch module at a high yield and of uniform quality without movement of the IC chip or breakage of fingers or leads. Since the molding operation relies upon thermoplasticity the molding cycle can be shortened and burrs reduced. Molded articles have a high degree of resiliency and components can be fixed by heat caulking.The fact that the recesses for accommodating these components can be simultaneously molded into a unitary body allows a reduction in the number of parts which must be assembled and facilitates automation of the assembly operation.

Fig. 1 2A and 12B show an example of an electronic watch module fabricated by another method of the present invention.

The watch module is formed by first using an etching or similar process to form into a pattern a metal foil such as copper which has been attached to a flexible sheet 11 5 provided with throughholes 115a, each located between adjacent electrodes that form a display electrode group, and with a device hole 11 sub. This provides a lead pattern 11 6. A wireless bonding method such as mini-mod bonding is then employed to electrically connect the electrodes of the IC chip 11 3 to lead electrodes 11 6 for bonding. The thermoplastic resin 114 are molded at low pressure to seal the circuit pattern and IC chip 113 and form a watch module. It has been verified that the IC chip 113 and metal foil pattern are not affected by the molding process and that a sufficiently high timepiece quality can be obtained.

Unlike the conventional low-pressure molding technique which requires a cycle time of approximately 3 minutes, injection molding as practiced in the present embodiment can be accomplished in a cycle time of 15 to 20 seconds.

There is no formation of fins so that a fin removal treatment is unnecessary. The result is a profitable reduction in time and labor.

The flexible sheet 11 5 employed in the present invention is subjected to press machining or the like to form the holes 11 spa between the electrodes 11 6b forming the group of closely adjacent display electrodes. The through-holes 11 5a are shaped so as to be filled by the thermoplastic resin 114 at the time of molding.

The resin, as shown in Fig. 1 2B, fills the spaces between the patterns and the spaces in the flexible sheet and therefore obviates the problem of electrical leakage between patterns while at the same time providing a mechanical strength which is equivalent to or greater than that cf the conventional arrangement which uses the lead frame. In addition, the exposed portions of the display electrode group are formed at the interface of the sealing resin and the foil electrodes so that the demand for reliability can be met owing to excellent adhesion and the highly effective sealing of the IC chip.

Adopting the wireless bonding technique such .as mini-mod bonding in place of the conventional wire bonding step allows the bonding strength to be increased, as previously described, by a factor of 4 or 5 so that the injection molding pressure can be withstood. Bonding cycle time, though dependent upon the number of electrodes, can be shortened to from 1/10 to 1/1 5 that of the prior art in a case where the number of electrodes is 40.

The present invention as described above with reference to Figs. 1 2A and 1 2B affords a high reliability owing to the effective IC seal, and is also advantageous in the reduction of labor that can be achieved. Moreover, the fact that the seal for the IC chip and the circuit substrate are molded into a unitary body allows a reduction in the number and types of parts and contributes to a reduction in overall thickness.

Fig.13 is a plan view of a flexible printed circuit board for an electronic watch module, and Fig. 14 is a cross-sectional view of a watch module incorporating the circuit board of Fig.13.

In Figs. 13 and 14, reference numeral 121 denotes a circuit board for the watch module and reference numeral 122 å flexible printed sheet comprising a thin film of polyimide or the like to the surface of which is bonded a metal foil 122a such as of copper that is formed into a prescribed circuit pattern by etching or a like method.An IC chip 130 is electrically connected to a lead 122b on the circuit pattern by gang bonding via a wireless bonding method, after which the flexible printed sheet 122 is insert-molded in thermoplastic resin to seal the IC chip 130 and simultaneously mold portions for accommodating the other elements that constitute an electronic watch module, namely a display element accommodating portion 134, connector accommodating portion 123, battery accommodating portion 124, crystal vibrator accommodating portion 125, capacitor accommodating portion 126, as well as portions 127, 128 for accommodating other elements. This fabricates the circuit board 121.The problem of the poor bonding strength between the flexible printed sheet 122 and thermoplastic resin is solved by punching holes indicated by the two-dot chain line around the outer portion of the flexible printed sheet 122 leaving connecting portions 122c, and then carrying out molding so as to cover this outer portion from the outside at the time of insert molding. This permits the majority of the outer portion of flexible printed circuit board 121 to be covered by the molding resin and therefore prevents the invasion of water of the like from the interfacial boundary of the molding resin so that sufficient reliability can be obtained.

Moreover, spaces provided around the IC chip 130 facilitate the flow of resin so that the strength of this region can be reinforced. They also permit the back surface of the IC chip to be backed up and stabilized by the mold. The timepiece module shown in Fig.14 is completed by installing such elements as the connector 131, display element 132, battery 133 and capacitor 135 in the circuit board 121 molded as described above. This provides a number of advantages such as fewer parts for assembly and a reduction in labor required for the assembly operation. Another is a further increase in productivity owing to the continuous tape-like configuration of the flexible printed sheet 122 which lends itself to treatment in a conveyor-like manner so that each processing step can be carried out in continuous fashion.

Fig.15 is a cross-sectional view of a circuit substrate employed in the conventional technique.

A first circuit substrate 201 which mounts an IC chip 211 composed of a flexible printed circuit board 212, a reinforcing metal frame 215 to which is adhered a patterned copper foil and an insulating sheet 214, and an IC molding resin 216.

A second circuit substrate 202 serves to mount externally circuit elements or to provide a conductive path. More specifically, the second circuit substrate 202 comprises a lead frame 221 consisting of a sheet of Kovar (Registered Trade Mark) or phosphor bronze or the like which is shaped into a prescribed circuit pattern by etching or a similar process, and a circuit board 222 for supporting and accommodating the lead frame 221, a circuit element 223, etc. The circuit board 222 is made of a thermoplastic resin and is molded together with a projection 222a for heat caulking, the lead frame 221 being heat-caulked to the projection 222a to be supported and fixed thereby. A connecting pin 203 is caulked or welded to the lead frame 221 and serves to provide an electrical connection between the first circuit substrate 201 and second circuit substrate 202.

The circuit substrate employed in this prior art -technique makes use of the flexible sheet since a mini-mod mounting technique is adopted because of the possibility of automatic assembly and the fewer mounting steps. To reinforce the flexible sheet it is necessary that the metal frame, having an insulating sheet attached to both its surfaces by a bonding agent, be positioned with respect to the flexible sheet and fixed thereto. In addition, molding the IC chip requires the use of an epoxy or silicone resin which in turn necessitates a long tunnel drier to cure the resin and a curing oven for post-curing.

There is also the requirement fr the connecting pin to electrically connect the first and second circuit substrates and, when assembling the watch module, at least two pin tubes and screws are necessary to position and secure a liquid crystal cell supporting frame and the first and second circuit substrates.

Thus it is apparent that a large number of parts is involved in the assembly of the timepiece module, and that the assembly operation is complicated and inclusive of a large number of steps. Problems are even encountered in automatic assembly since the number of parts remains high, the apparatus is complicated, manufacturing costs are high, and apparatus maintenance is difficult.

The reason why the aforesaid two circuit substrates cannot be united into a single body resides in using highly fluid heat-cured resin to seal the IC chip. Such a resin does not permit the heat-caulking of the lead frame. The watch module fabricated by the method of the present invention eliminates the aforementioned disadvantages and provide an electronic watch module which is low in cost, highly reliable and capable of being made extremely thin. To this end the watch module is formed by supporting and fixing a second circuit substrate by means of the thermal deformation of a first substrate, the first substrate being made by directly sealing an IC chip in a thermoplastic resin which can be molded at low pressure.

Fig.16 is a cross-sectional view of another example of a watch module fabricated by the method of the present invention. A first circuit substrate 204 is molded from a thermoplastic resin 241 which is capable of low-pressure injection molding. First circuit substrate 204 is formed by the insert-molding of an IC chip 242 and a flexible printed circuit board 243 which comprises a first circuit board, and includes recesses for accommodating circuit elements other than the IC chip, such a capacitor 205 and battery 206, and also has a projection 241 a which fixes a second circuit board 207 by heat caulking.The second circuit board 207 is formed by using an etching process or the like to shape a Koval or phosphor bronze sheet into a prescribed circuit pattern, and which is adapted to externally mount circuit components such as a crystal vibrator (not shown) which is welded thereto. Circuit board 207 is also adapted to be brought into electrical contact with capacitor 205 which is forced into contact with circuit board 207 by a mechanical spring. The electrical connection between the first circuit substrate 204 and second circuit board 207 can be provided by bending the leading edge of the lead frame of second circuit board 207 to form a connecting portion 207a that serves the desired purpose. The IC chip 242 and fingers 245 are not affected by the molding process, and a sufficiently high timepiece quality can be obtained.Low pressure moldable thermoplastic resins other than polyphenylene sulfide which provide the same excellent results include ordinary engineering plastics such as polybutylene terephthalate resins and polyamide resins which have a high degree of fluidity. Hence, any thermoplastic resin having a high degree of fluidity is suitable as the molding resin. The first circuit board that mounts the IC chip and constitutes the first circuit substrate 204 is not limited to the flexible printed circuit board but can instead comprise a printed circuit board made of a glass epoxy substrate or the like which will serve as the lead frame.

In the watch module of Fig.16, a circuit board can be supported and fixed by direct heat caulking as in the manner of a lead frame by forming the circuit board of a resin which molds an IC. This reduces the number of parts, facilitates automated assembly, decreases the number of assembly states, and makes possible a futher reduction in thickness and size.

White various examples of a method of fabricating an electronic watch module have been described above, it should be noted that various other changes or modifications may be made. For example, a flexible printed circuit board may be encapsulated by two molding steps, i.e., by firstly covering one surface of the circuit board by a thermoplastic resin and secondly covering the other surface of the circuit board by the thermoplastic resin.

Fig. 17 shows an upper mold 314 and lower mold 315, with P.L. designating the parting line which forms the boundary between the molds.

Fig.18 shows that the flexible printed circuit board 302 will be affected by molding pressure, and hence deformed at portions which are not clamped between the upper and lower molds, if molding is conducted by the customary one-step operation, the flexible printed circuit board being inserted in the mold before an article is injection molded from a thermoplastic (which in the present embodiment is an IC chip encapsulation grade PPS, or polyphenylene sulfide, resin). This deformation will cause wiring patterns to crack and will give rise to the shoulder-touch phenomenon.

The molding method will be more clearly described with reference to Fig.17. The flexible printed circuit board is inserted into the lower (movable) mold 31 5, after which the first molding operation is performed. The structure of the mold is such that the lower mold 31 5 has a large number of cavities while the upper mold 314 merely has a cavity defining a portion for encapsulation of the IC chip 309, the remainder of this mold surface being flat. Thus, when the molds are closed, the flexible printed circuit board is embraced between them with its entire upper surface contacting the surface of the upper mold 314. When molding is conducted the plastic is caused to flow in from the lower mold 315 owing to the location of the gate, so that the flexible printed circuit board 302 is pressed against the upper mold 314 by the inflowing plastic.The flexible printed circuit board 302 experiences no deformation and remains flat owing to the flatness of the upper mold 314. This first molding operation molds approximately one-half of the watch module 301, as shown in Fig. 17. The lower mold 315 is then moved together with the flexible printed circuit board and is brought into alignment with another upper mold (not shown) having cavities for forming on the flexible printed circuit the remaining portions of the substrate. A second molding operation is now carried out to completely cover with plastic the surface of the circuit board that has been left exposed by the first molding operation. This completes the formation of the watch module 301.

In the molding method described above, a watch module is fabricated through a logical process of insert-molding a flexible printed circuit board by making use of an IC chip encapsulation grade thermoplastic, with portions for accommodating various elements being molded while encapsulating the IC chip. According to a feature of the method described above, the circuit board serves as a boundary with the molding operation being divided into two steps, one step for each side of the printed circuit board (i.e., a two-step molding operation). As a result, the printed circuit board does not experience any deformation, an advantage which is not available with the conventional one-step molding operation, so that the breakage of wiring patterns and the shoulder touch phenomenon can be eliminated.

Claims

1. A method of fabricating an electronic watch module, comprising; preparing a flexible printed circuit board having a printed circuit pattern and a plurality of leads extending from said printed circuit pattern; wireless bonding an IC chip to the leads of said flexible printed circuit board: inserting said flexible printed circuit board with said IC chip into a cavity of a molding die unit; fixedly retaining at least a part of one of said flexible printed circuit board and said IC chip within the cavity of said molding die unit; and supplying a thermoplastic resin into the cavity of said molding die unit for thereby encapsulating said flexible printed circuit board and said IC chip.

2. A method according to claim 1, in which said thermoplastic resin essentially consists of a polyphenylene sulfide-type thermoplastic resin.

3. A method according to claim 1 or 2, in which said molding die unit is heated at a temperature of about 1 500C during said resin supplying step and said thermoplastic resin is molded at a temperature of about 2800C-3800C and an injection pressure of from 75 to 200 kg/cm2.

4. A method according to claim 1, 2 or 3, in which said molding die unit comprises a stationary die projection extending into the cavity of said molding die unit, said die projection engaging with and supporting a bump side of said IC chip.

5. A method according to claim 4, in which said thermoplastic resin is caused to flow from a back side of said IC chip toward the bump side of said IC chip.

6. A method according to claim 4 or 5, in which said molding die unit also comprises a movable die projection engaging with and resiliently supporting the back side of said IC chip to prevent transverse movement of said IC chip during the molding step.

7. A method according to claim 6, in which said stationary die projection comprises a plurality of pins engaging connecting portions between the leads of said flexible printed circuit board and the bumps of the IC chip.

8. A method according to claim 1,2 or 3, in which said molding die unit has a gate through which said thermoplastic resin is supplied into the cavity of said molding die unit, and in which an edge portion of said flexible printed circuit board is retained by a pair of projecting pins to assume a predetermined position relative to said gate to control the flow of said thermoplastic resin passing to the both sides of said flexible printed circuit board.

9. A method according to claim 1, 2 or 3, in which said flexible printed circuit board has at least one row of perforations each formed the adjacent connecting lead terminals of said flexible printed circuit board, said perforations being molded with said thermoplastic resin.

10. A method according to claim 1, 2 or 3, in which said flexible printed circuit board has an outer periphery a major portion of which is surrounded by said thermoplastic resin to prevent entry of moisture into said flexible printed circuit board.

11. A method according to claim 1, in which said electronic watch module has a plurality of outwardly extending projections for fixedly retaining associated components after heat caulking said projections.

12. A method according to claim 1, in which one surface of said flexible printed circuit board is initially covered by said thermoplastic resin and the other surface of said flexible printed circuit board is subsequently covered by said thermoplastic resin.

13. A method according to claim 1, in which said module has a plurality of recesses formed during said resin supplying step to accommodate therein a plurality of electronic components, respectively.

14. A method substantially as shown and described with reference to the accompanying drawings.