CA1071747A - Method and apparatus for assembling electronic devices - Google Patents

Abstract

Abstract of the Disclosure Disclosed is a method of assembling electronic devices starting with providing a conductive substrate having first regions which support electronic components, second regions which electrically and mechanically interconnect various first regions and undesired third regions which provide temporary mechanical support for the first and second regions;
adding a mass of insulative molding material for encapsulating portions of the first and second regions, cutting away unde-sired third regions and mounting electronic components. A
substrate useful in carrying out the method of the invention is also disclosed. This substrate is particularly useful for assembling the components of an electronic digital watch.

Description

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:1~ Back~round of the Inventio~
1$ ¦ ~O Field of the Invention. The invention relates l9 Ito a.method and apparatus for assembling electronic devices, I
20 ¦and more particularly to a method and apparatus for

2~. 1assembling microminiature electronic apparatus involving 22 intagrated circuits or the like.
23 b. Prior Art. Previously, microminiature electronic 24 ~vices, such as calculators and watches, utilizing semiconductor ntegrated circuits, were constructed by moun.ting circuit com-~6 ponents on a printed circuit board, usually made of plas~ic 271 or ceramic material, and having conductive ipterconnections 28¦ ~etween components plated or printed o~ the circuit board.
291 Electronic co.-.,ponents were placed on the circuit board,.
301 aligned for correct positioning and then soldered or o.her-31¦ wise affixed in place. This form of assembling is labor 321 int~nsive and for this reason much of the assembly work in " 1071747 mounting electronic components on circuit boards is done in foreign countries where labor rates are less than in the United States.
It is an object of the invention to devise a method ; and apparatus Eor improving the assembling of electronic devices, especially microminiature devices involving integrated circuits mounted on circuit boards or holders. Another ob;ect of the invention is to provide an improved substrated for mounting ` components of microminiature electronic devices and to provide for stronger, more rugged mounting of electronic components, and which lends itself to automated assembly of electronic devices.
Yet another object of the invention is to provide for ease of mounting of elec,ronic components on a circuit board, especially for ease of placement of components in a very compact arrangement.
Summary of the Invention In its broadest form, therefore, the present invention provides in a circuit module having a conductive lead frame `
member partially encapsulated with a molding material, the lead frame member having on its obverse side a first exposed surface portion for providing an electrical and mechanical coupling to a circuit means and on its reverse side a second exposed surface portion disposed under the first exposed surface portion, wherein th`e improvement comprises the lead frame member including at the edge of its reverse side an integrally formed outwardly extend-ing taper means for locking the lead frame member into the formed molding material when the molding material hardens.
The present invention also provides the method of assembling an electronic module comprising: forming a conduc-tive lead frame member having an obverse and reverse surface

3~ to include an outwardly extending taper means at its reverse surface; positioning the lead frame member in a mold; applying pressure to the obverse and reverse surfaces of at least one selected portion of the lead frame; and partially encapsulating , . ~

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~071747 the lead frame member with a mass of insulative molding material ~ -- wherein the taper means at the reverse surface of the lead frame - member locks into the molding material when the molding material `
hardens.
:~ ' The invention will be understood more clearly with reference to the figures.
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Brief Description of the Drawings Figure la is a top plan view, obverse side or face, of substrates used in the method of the present invention, shown in a web.
Figure lb is a side view of the substrates of Figure la.
Figure 2a shows the obverse side of the substrates of Figure la with insulative molding material applied thereto in accord with the method of the present invention.
Figure 2b i~ a side cutaway view of the apparatus with the cut taken along lines 2b-2b in Figure 2a.
Figure 3a shows the reverse side of the substrates of Figure 2a, when the obverse side is turned top to bottom.
Figure 3b shows a side view of the apparatus of Figure 3a.

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~07~747 Figure 4a shows the obverse side of a molded sub-strate of Figures 2a and 3a which has been punched and die cut.
Figure 4b shows the reverse side of the substrate of Figure 4a when the obverse side is turned top to bottom.
Figure 5 shows the reverse side of punched and die ; cut substrate of Figure 4b with components added.
Figure 6a shows the obverse side of the substrate of Figure 5 with components added.
Figure 6b is a partial cutaway detailed view of the substrate shown in Figure 6a taken along the line 6b in Figure 6a.
Figure 7 is a side view of an alternate embodiment of the invention showing a pair of substrates in a single en-capsulation.
Figure 8 is a detailed side sectional view showing a preferred manner of anchoring substrates of the present inven-tion in encapsulation material.
Figure 9a is a detailed plan view of a zig-zag con-ductive path in the substrate of Figure 11.
Figure 9b is a side view taken along lines 9b-9b in Figure 9a sh-owing a manner of providing elevational relief in the zig-zag conductive path shown in Figure 9a.
Figure 10 is a partial side sectional view taken along lines 10-10 in Figure 6a showing a battery contact portion of the substrate of the present invention.
Figure 11 is a detailed view of ~he apparatus of Figure 1.

D~scription of the Preferred Embodimen While the present assembly method is illustrated by bm/~

-~ ~07~747 means of an example which is particularly apt for assembling the electronic components of a digital watch, it will be realized that the method of the present invention is applicable to a wide class of electronic devices, such as calculators and other devices and is not limited to electronic watches. In carrying out the method of the present invention, it is necessary to provide a substructure for assembling electronic `
circuits, without providing the underlying plastic or ceramic substructure which has characterized the prior art. More particularly, the method of the present invention is carried out by directly utilizing a substructure which comprises a unitary conductive substrate which is essentially planar and has first regions for supporting electronic components and second regions for making electrical connections. Portions of the first and second regions are::held together by undesired third regions, which will eventually be removed, but which provide mechanical support between the first and second regions thereby maintaining a unitary structure.
The substructure used for carrying out the method of the present invention is illustrated in Figure la and in more detail in Figure 11 which show~an elongated web of stamped or etched substructures which comprise conductive metal substrates ~ !
approximately 0.006 inches in thickness. This thickness is selected for ease of bending and formation by stamping or etching, but is not critical. The material of the substrate may be copper, aluminum or any metal having good conductivity, or an intermediate conductor plated with a good conductivity material. Since very thin interconnect members will be formed within the substrate, the substrate material should be readily bendable and one that lends itself to the formation of the bm~

107~747 interconnected by die stamping or chemical etching. It has been found that copper is a preferred material. A chemical etching process used for integrated circuit lead frame etching may be employed for formation of the present substrate.
Figure la shows a plurality of substrates 11, 13, 15 in a web 17, having holes within the web l9a, l9b, l9c, l9d, l9e for advancing the web. A plurality of substrates 11, 13, 15, etc. may be simultaneously processed in accord with the teach-ings of the present invention and thus the figures illustrate a plurality of identical substrates for that purpose.
In the plan view of Figure la the substrate 11 is indicated to have first desired regions, including 21, 22, 23, which serve to support electronic components. The support region is generally slightly larger than the portion of the electronic components supported.
A group of second desired regions are wire-like interconnections, including 31, 32, 33 between the first desired regions. In some cases these interconnections would correspond to the plating or printing of conductive paths on printed circuit boards. In other cases the second regions would correspond to hand-wired interconnections between components.
In still other instances, the second regions provide mechanical support between various first regions within the substrate.
If mechanical support is provided between desired first regions and electrical contact is not desired, the desired second ; region will usually be connected to an undesired third region which will ultimately be removed. The desired second region is usually continguous to an undesired third region and from all appearances would appear to be in all respects identical to a desired second region upon viewing the substrate at the bm~

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~07~747 outset.
The undesired temporary third regions are regions which are to be removed from the substrate before breaking electrical and mechanical contact between desired regions.
Electrical contact is broken because the two connected regions are not intended to be at the same electrical potential.
Mechanical contact is broken primarily to break electrical contact. However, the mechanical support which was provided by the link afforded by the undesired third region is replaced by another step in the method of the present invention sub-sequent to the formation of the substrate herein described, and described in more detail with reference to Figures 2a, 2b, 3a and 3b.
Holes are provided in desired portions of the sub-strate, usually in selected portions of a first desired region for ease of mounting of components. For example, the leads of a resistor may be accomodated in specified holes within the desired first regions. The electrical component, e.g. a resistor, is inserted and soldered in place within the appropriate holes when components are mounted, as described below.
In Figure la a side view of the substrate is shown.
It is to be noted that the substrate is at the outset planar in character. This allows the substrate to be manufactured in continuous lengths and then cut for use or storage and transportation.
The next step in the process, illustrated by products of the process, in Figures 2a, 2b, 3a and 3b, consists of adding a mass of insulative molding material, 27, 27', such as silicone or epoxy, for encapsulating defined portions of the ; bm~

1071~47 first and second regions to be held in place. The addition of molding material is usually accomplished by means of a mold consisting of two halves which come together with the substrate sandwich in the middle. The molds, not shown, have an internal cavity or core which is generally disc-shaped and contains a plurality of pins therein, extending from internal faces of the core.
There are generally two types of core pins including a first very thin, blunt pin which applies more pressure to selected segments of the first and second desired regions than to non-selected segments thereby providing for differences in elevation, i.e. elevational relief, between the selected and non-selected segments. Holes 31', 32', 33' left by these pins may be seen in Figure 3a. In the processing of substrates for the present invention, it may be desirable to have certain desired substrate portions higher or lower for the purpose of allowing components to be mounted at desired elevations or to allow interconnections to pass above or below components or other interconnections. By applying more pressure to selected segments of said first and second desired regions where elevational relief is desired, a contemporaneous addition of molding compound will freeze these elevational changes in place. When the mold is removed, the pins are withdrawn with ; the mold and the desired segments remain in the position where they were pushed by the pins and f~ozen in place by the molding compound.
A second type of pin is a larger blunt pin which comes into intimate contact with a desired portion which is shielded from molding compound and kept free of molding compound. Such a pin is intended to preserve the support bm~

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107~747 characteristics of a desired region for the subsequent mounting of a portion of an electronic component thereon.
For example, in assembling an electronic digital watch, a support region 22 in Figure 2a would be provided for support-ing an integrated circuit used for generating a reference frequency. The position or pad for mounting the integrated circuit or "watch chip" is one of the first desired regions and should be kept free of molding compound using the large blunt pins mentioned above. Accordingly, a pin corresponding to a dimension slightly larger than the surface area of the first desired portion for mounting the chip preserves a surface thereof from contamination with molding compound by contact with the surface during the injection of molding material into the mold. The same treatment would be provided for support ~
region 21 which holds the watch display components, component support portion 23 and battery tabs 24, 25.
In Figure 3a, molding compound is ~ept substantially out of the component depressions 26' wherein a frequency trimmer may be located, 29' wherein a crystal may be located, 28' wherein a resistor may be mounted, 30a' wherein a first battery may be mounted, 3Ob' wherein a second battery may be mounted, as well as another component region 34'. Each pin, large or small, protrudes from an inside surface of the mold in positions corresponding to the depressions shown in Figures 2a, 2b and 3a.
After the mold has been closed, molding material at an appropriate temperature is injected into the mold until the mold is filled. The molding material is allowed to harden and then the mold is separated, in a conventional manner. The resultant structure will have the appearance of the structure _9_ bm/~/

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shown in Figures 2a and 3a. The substrate 11 can be seen to include molding compound 27, 27' in Figures 2a, 2b, 3a and 3b, which is providing mechanical support for the desired first and second regions. In other words, the desired first regions which formerly relied upon some of the desired second regions and the undesired third regions for mechanical support, no longer rely on these regions for mechanical support but are now held in place by the molding material.
; The side view of Figures 2b and 3b show the added thickness of the substrate when encapsulated, in comparison to Figure lb.
- As mentioned previously, the mold which was used for application of the molding material is removed when the molding material has hardened. Figure 4a generally shows the next step in the process of the present invention, wherein undesired third regions which formerly provided support are now removed.
The undesired third regions include superfluous web material between the dashed lines 41, 43, which indicate punched cuts which break metallic interconnections which were previously provided for strength, but which are not desired from the electrical standpoint. Another undesired third region is along -~; the slit 44, on one side of the support region which holds the "watch chip" of the present apparatus and the slit 45, along one side of the support portion which holds the "driver chip"
for the watch display which is mounted on support portion 21.
In Figure 4b the lines 44', 45' correspond to the slits 44, 45 of Figure 4a. The superfluous undesired third regions are those which are removed in the punching operation described with respect to Figures 4a and 4b. After undesired third regions are removed, molding flash is chemically etched.

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Figures 5 and 6a illustrate the next step of the present method, the mounting of components on desired first regions. In Figure 5 the components include batteries 51, 52, a frequency trimmer 53, a crystal os~illator 54, a first resistor 55 and a circuit network 56.
In Figure 6a the components include time display members 61, 62, 63, 64, a first integrated circuit 65 usually referred to as a "watch chip" and a second integrated circuit 66, previously referred to as a "driver chip". Wire leads are used to connect the various chips to the display elements 61, 62, 63, 64 and to other circuit elements, in a known manner.
Contemporaneous to the mounting of components or at the time the third undesired elements are cut away, a bending tool may be used to form desired bends, such as for contact of the electronic device by an external switch. In Figure 6a, a switch tab 67 is shown which is bent in an upward direction as indicated by the arrow in Figure 6b. It is seen that the switch tab 67 is an extension of substrate 11.
It should be noted that~prior to the adding of molding material, some electronic components may be mounted in a sub-strate and molding compound poured over them. Generally it is preferable to add the electronic components after the completion of the molding operation for ease of repair of the components in the event they fail. However, in some instances more rugged construction may be achieved by inserting the components and adding molding material over them.
In Figure 2b it should be noted that molding material has been added to both sides of the substrate and not bm/,~

la7~747 necessarily in corresponding regions. Thus, it is seen that the insulative molding material encapsulates only defined portions of said first and second desired regions.
While the step of adding molding material has been described with reference to a single substrate, two or more substrates could be used utilizing-the pins described herein to provide for desired elevational relief and prevent short circuits from occurring. This is illustrated in Figure 7.
In Figure 7, a pair of substrates 71, 72 is mounted in a generally planar spaced relationship and corresponding desired regions of each substrate are brought into conductive contact and then molding compound is applied in the manner previously set forth. In this manner, the compact geometry of the substrate may be made even more compact laterally, at a slight expense of increased thickness. The manner of bring-ing the two s,ubstrates into contact is by maans of core pins in the mold which are used to make changes in elevational relief as previously described and more fully described below with reference to Figure 9a and 9b.
An improved method of locking each substrate shown in Figure 7 to the molding compound, as well as locking the substrates of Figure la to the molding compound shown in Figures 2a and 3a is indicated in Figure 8. Here it is seen that the upper edge of the substrate 11 has an outwardly extending taper 81 resembling a claw. Simllarly substrate 13 has an outwardly extending taper-at its edge, 82, with the same claw-like shape. Claws 81, 82 serve as spikes into the molding compound 83, once it hardens, thereby serving to lock a substrate in place.
Figure 9a shows a zig-zag portion of a desired second bm~

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~071747 region which is especially adapted for providing elevational relief in the vertical plane, such as for making contact between two parallel opposed substrates, such as illustrated in Figure 7.
In Figure 9b a core pin 91, projecting from an internal surface of a mold is shown projecting downwardly on the central portion 93 of the U-shaped zig-zag member 90 of Figure 9a. Pin 91 exerts a downward force indicated by the arrow A in the figure pushing against central portion 93 and forming bends at portions 92, 94 and again at regions 96, 97 shown in both Figures 9a and 9b. Once a bend is made, it may be frozen in place by the hardening of molding material over it. When molding material has hardened the core pin 91 is removed.
Figure 10 illustrates another bent region 101, a portion of substrate 11 which has been bent upwardly by a tool much like the switch 67, illustrated in Figure 6b. A battery, not shown, occupies the cavity defined by wall 102 and pushed down on the contact 101 for electrical connection therewith.
The bend region 101 has been bent by deforming it beyond its elastic limit with a core pin and there is no need to hold the bend in place with molding material.
Figure 11 is a more detailed view of the substrate of the present invention, corresponding to one of the substrates 11, 13, 15 shown in Figure la. The substrate is a unitary conductive member which is substantially planar before process-ing as described above. The substrate illustrated in Figure 11 is adapted to have a perimeter 120 which will fit inside a watch case. As previously mentioned, the substrate has first desired regions 21a, 21b, 21c, 21d, 22, 23, . . . having dimensions bm~;

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slightly larger than the portion of electronic components to be supported. For example, the region 22 accomodates a watch chip module, while the region 23 accomodates a driver chip.
A first desired region need not support an entire component, but merely a part of one. For example, a first desired region may support one end of a resistor, while another first desired region supports the other end.
A number of second desired regions 121, 122, 123, 124 . . . electrically connect some of the first regions. The structure further includes undesired third regions which are later removed and become void third regions. These regions appear as integral parts of the first and second regions shown -~
in Figure 11, but are defined as third regions in as much as they are the regions which must be cut away to achieve the proper electrical potentials. For example, the vertical lines 131, 132 indicate third regions formed by punching through the corresponding desired second regions 121, 123. The zig-zag region 90, previously described with reference to Figure 9a, is seen to be connected to the battery contact support 101'.
The exemplary substrate discussed with reference to a substrate for assembling a watch circuit is approximately .005 inches thick and is made of copper alloy, such as berrylium copper, although other materials, such as aluminum and steel have also been previously used with success. The method and apparatus of the present invention improves the assembly of electronic devices, especially microminiature devices, because less space is required for the mounting of components, a more rugged construction is achieved and increased automation of the assembly process is possible since indexing holes may be provided within the substrate for mounting of electronic bm~

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107~747 eomponents. Further, by eliminating the plastic and ceramie circuit boards of the prior art, a more eompaet arrangement in eleetronie device may be aehieved.

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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a circuit module having a conductive lead frame member partially encapsulated with a molding material, the lead frame member having on its obverse side a first exposed surface portion for providing an electrical and mechanical coupling to a circuit means and on its reverse side a second exposed surface portion disposed under the first exposed surface portion, wherein the improvement comprises the lead frame member including at the edge of its reverse side an integrally formed outwardly extend-ing taper means for locking the lead frame member into the formed molding material when the molding material hardens.

2. In a circuit module as in claim 1 wherein said circuit means includes an integrated circuit chip and the integrated circuit chip is electrically connected to the first surface of the lead frame member by wire bonding.

3. In the circuit module as in claim 1 further includ-ing an additional conductive lead frame member in a generally planar relationship to said lead frame member, both said lead frame members being partially encapsulated with the molding material in the circuit module.

4. In the circuit module as in claim 1 wherein the circuit module is formed to fit inside of a watch case and said circuit means includes an integrated circuit chip for an electronic watch.

5. The method of assembling an electronic module comprising:
forming a conductive lead frame member having an obverse and reverse surface to include an outwardly extending taper means at its reverse surface;
positioning the lead frame member in a mold;
applying pressure to the obverse and reverse surfaces of at least one selected portion of the lead frame; and partially encapsulating the lead frame member with a mass of insulative molding material wherein the taper means at the reverse surface of the lead frame member locks into the mold-ing material when the molding material hardens.

6. The method as in claim 5 including the step of mounting by wire bonding a circuit means to the selected obverse portion of the lead frame member.

7. The method as in claim 5 wherein the step of forming the lead frame member includes the step of chemically etching the lead frame.

8. The method as in claim 5 wherein the step of form-ing the lead frame member includes the step of die stamping.