WO1997019466A1

WO1997019466A1 - Method and apparatus for forming solder bumps on a substrate

Info

Publication number: WO1997019466A1
Application number: PCT/US1996/018734
Authority: WO
Inventors: Angelo Gulino; Thomas W. Edwards
Original assignee: Frys Metals Inc
Current assignee: Alpha Assembly Solutions Inc
Priority date: 1995-11-22
Filing date: 1996-11-22
Publication date: 1997-05-29
Anticipated expiration: 1998-05-22

Abstract

A method for forming solder bumps on solder pads (14) of a substrate (16) includes the steps of positioning a sheet of solder material (10) over the substrate, stamping the sheet in such a manner that a plurality of non-spherical solder preforms (18) are removed from the sheet, depositing the stamped solder preforms on the pads (14) of the substrate and heating the solder elements to a reflow temperature wherein the solder elements are transformed into a generally spherical solder bump. The foregoing method is achieved by an apparatus having a die (24) positioned between the sheet of solder material (10) and the substrate (16). The die has a plurality of openings (28) formed therein which correspond to and are in registry with the solder pads of the substrate. A plurality of punch elements (30) are provided for punching solder preforms from the sheet through the openings of the die. The arrangement is such that the punch elements extend into the openings of the die for punching solder preforms (18) from the sheet wherein the solder preforms are disposed upon the pads of the substrate.

Description

METHOD AND APPARATUS FOR FORMING SOLDER BUMPS ON A SUBSTRATE

Background and Summary of the Invention This invention relates generally to methods and apparatuses for forming and attaching spherical solder elements on a substrate, and more particularly to a method and apparatus for depositing non-spherical solder preforms onto pads ofthe substrate, and forming the non- spherical solder preforms into spherical solder bumps.

Methods and apparatuses for attaching spherical solder bumps to a printed circuit board substrate are well-known in the art. Representative of such prior art are U.S. Patent Nos.

3,436,818 to Merrin et al., 4,352,449 to Hall et al., 4,376,505 to Wojcik, 4,752,027 to Gschwend, 4,878,61 1 to LoVasco et al., 4,892,245 and 4,898,320 to Dunaway et al., 4,906,823 to Kushima et al., 5,024,372 to Altman et al., 5,090,609 to Nakao et al., 5,160,409 to Moore et al., 5,251,806 to Agarwala et al., and 5,307,983 to Dudderar et al. Of these patents, the patent to Altman et al. represents best some ofthe problems associated with current methods and apparatuses.

In attempting to attach solder "bumps", as referred to in Altman et al., a thick layer of solder resist material is applied to a printed circuit board substrate wherein some of the solder resist material is selectively removed to provide wells over metallized pads which are fixedly attached to the substrate. Next, solder paste is applied to the substrate in the wells and the excess is removed by a squeegee. The solder bumps are then placed within the wells over the metallized pads and reflowed (e.g., heated) for positively bonding the solder bump to the pad. After bonding the solder bumps to the pads ofthe substrate, the excess solder resist material is then removed from the substrate.

This method of attaching the solder contacts to the substrate is particularly representative of several of the known methods in that it is extremely time consuming to perform since the wells surrounding each metallized pad must be fabricated and removed from the substrate. There is presently a need in the industry for a method and an apparatus which conveniently and expeditiously forms spherical solder bumps onto pads of the substrate.

The instant invention provides an improved method for forming spherical solder bumps on solder pads of a substrate comprising the steps of positioning a sheet of solder material over the substrate, stamping the sheet in such a manner that a plurality of non-spherical solder preforms are removed from the sheet, depositing the stamped solder preforms on the pads of the substrate, and heating the non-spherical solder preforms to a reflow temperature wherein the non- spherical solder preforms automatically reflow into a generally spherical configuration due to surface tension forces ofthe solder.

The foregoing method is achieved by an apparatus comprising means for advancing a sheet of solder material over the substrate, and a die positioned between the sheet of solder material and the substrate. The die has a plurality of openings formed therein which correspond to and are in registry with the solder pads ofthe substrate. A plurality of punch elements are provided for punching solder preforms from the sheet through the openings ofthe die. The arrangement is such that the punch elements extend into the openings of the die for punching solder preforms from the sheet wherein the solder preforms are disposed upon the pads of the substrate.

Accordingly, among the several objects of the present invention are the provision of a simple method of forming a spherical solder bump on a solder pad; the provision of such a method wherein a non-spherical solder preform is transformed into a spherical solder bump by heating of the non-spherical solder preform; the provision of an improved method for depositing solder preforms onto pads of a substrate; the provision of such a method which is simple to follow and easy to preform; the provision of such a method wherein a relatively flat, disc-shaped solder preform is deposited onto a pad of the substrate and then heated to transform the solder preform into a substantially spherical solder bump; the provision of such a method which incorporates easy to use equipment; and the provision of such a method which produces high- quality results.

Also among the several objects of the present invention are the provision of an improved apparatus for depositing solder preforms onto pads of a substrate which is accurate in depositing the preforms onto the pads; the provision of such an apparatus which is simple in construction and easy to operate; and the provision of such an apparatus which produces high-quality results. Other objects, features and advantages ofthe invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.

Brief Description of the Drawings

In the drawings which illustrate the best mode presently contemplated for carrying out the present invention: Fig. 1 is a perspective view of a sheet of solder foil displaced over a substrate having metallized pads;

Fig. 2 is a partial cross section schematic view of an apparatus used for coining the foil onto the substrate; Fig. 3 is a side elevational view of a solder preform disposed on the substrate prior to its being soldered to the metallized pad of the substrate; and

Fig. 4 is side elevational view of the solder preform illustrated in Fig. 3 after it is soldered to the metallized pad of the substrate.

Corresponding reference numerals designate corresponding parts throughout the several views of the drawings.

Detailed Description of the Preferred Embodiment

Referring generally to the drawings, there is illustrated in Fig. 1 a sheet of solder material, generally indicated at 10, disposed over a printed circuit board substrate, generally indicated at 12, which has a plurality of metallized solder pads 14 attached to the body 16 ofthe substrate 12 in a grid pattern. As shown, the sheet of solder material 10 has a plurality of preforms 18 coined by a suitable stamping operation for enabling the easy removal of solder preforms 18 from the sheet 10. The sheet 10 is preferably fabricated from suitable solder material, such as a tin-lead composition and is very thin (otherwise referred to as a foil). The sheet of solder material 10 is preferably part of a roll 20 of solder material (see Fig. 2) which is readily available from any number of commercial sources.

The method of the present invention is directed to depositing the solder preforms 1 taken from the sheet of solder material 10 on the metallized solder pads 14 of the substrate 12. This method is achieved by an apparatus, generally indicated at 22, which is illustrated in Fig. 2 of the drawings. Referring now to Fig. 2, the apparatus 22 comprises a die, generally indicated at 24, positioned between the sheet of solder material 10 and the substrate 12. The die 24 has a body portion 26 having a plurality of bored openings 28 formed therein which correspond to and are in registry with the metallized solder pads 14 of the substrate 12. The die 24 is suitably supported by a housing (not shown), the construction of which can be readily developed by anyone skilled in the art.

A plurality of punch elements, each indicated at 30, are provided above the sheet of solder material 10 and are suitably attached to a support 32 for punching the solder preforms 1 from the sheet 10 through the openings 28 ofthe die 24. It should be noted that the sheet of solder material 10 can be preformed as illustrated in Fig. 1 or can be untreated and still fall within the scope ofthe present invention. However, one advantage to preforming the sheet 10 is that it is easier for the punch elements 30 to separate the solder preforms 18 from the sheet 10 than if the sheet 10 is not preformed. The punch elements 30 are also arranged so that the correspond to and are in registry with the openings 28 in the die 24 and the metallized solder pads 14 of the substrate 12. The arrangement is such that the punch elements 30 extend into the openings 28 of the die 24 for punching the preforms 18 from the sheet 10.

After being separated from the sheet 10 by the punch elements 30, the solder preforms 18 travel through the openings 28 ofthe die 24 and are deposited on the metallized solder pads 14 of the substrate 12. Preferably, each pad 14 has a layer of solder flux material (not shown) disposed thereon to initially hold the solder preforms 18 to the metallized pads 14 and to facilitate bonding. It is an important aspect ofthe present invention that the punch elements 30, openings 28 ofthe die 24 and the metallized solder pads 14 of the substrate 12 be in registry so that the solder preforms 18 are accurately deposited on the metallized solder pads 14.

For advancing the sheet of solder material 10 over the substrate 12, a first reel 34 is provided which has the roll 20 of wound solder material in sheet form thereon. A second, take- up reel 36 is attached to the free end of the roll 20 of wound sheet of solder material which is wound on the first reel 34. The take-up reel 36 can be driven by any suitable drive mechanism (not shown) for advancing unstamped portions of the sheet 10 over the substrate 12. Thus, after punching solder preforms 18 from the sheet of solder material 10, the take-up reel 36 is rotated by the drive mechanism so as to advance a fresh sheet of material for either another section of the printed circuit board substrate, or another printed circuit board substrate. Suitable means can also be provided for moving the substrate 12 or multiple substrates to the position where they are located below the die.

The method of the present invention is achieved by positioning the sheet of solder material 10 over the substrate 12, stamping or punching the sheet 10 with the punch elements 30 in such a manner that a plurality of solder preforms 18 are removed from the sheet 10, and depositing via openings 28 in the die 24 the stamped or removed solder preforms 18 on the metallized solder pads 14 of the substrate 12. As described above, the apparatus 22 of the present invention performs the method in a relatively uncomplicated and efficient manner. After the solder preforms 18 are deposited on the metallized pads 14 having a layer of solder flux thereon, the substrate 12 is heated for fixedly attaching the solder preforms 18 to the pads 14 of the substrate 12.

An extremely important aspect of this method is that the relatively flat and disc-shaped solder preforms 18, upon being subjected to heat, transform into a substantially spherical solder bump 38 (see Fig. 4). More specifically, it has been found that the surface tension ofthe solder material (e.g., 63 percent tin and 37 percent lead in a preferred embodiment) when in a liquified state transforms the relatively flat disc-shaped preform 18 as seen in Fig. 3 into a substantially spherically-shaped "bump" 38 configuration illustrated in Fig. 4. This spherical shape is especially suited for contacts which are often attached (as by soldering) to wire leads. In this regard, current methods of forming solder bumps rely on the preforming of spherical solder balls. Such spherical solder balls are quite difficult to manufacture and add significant costs to present BGA circuit packages. The instant method of depositing a non-spherical solder preform onto the pad and then heating the non-spherical solder element to its reflow temperature, thus forming a spherical bump, completely obviates the need to provide solder spheres. For this reason, the basic method of the instant invention is thought to represent a significant advancement in the art which will significantly reduce manufacturing costs.

It should be noted that the method and apparatus 22 ofthe present invention are especially suited for effectively and efficiently depositing solder preforms 18 onto a printed circuit board substrate 12. There are no extraneous steps which need to be performed, such as providing and removing a layer of solder resist material to the substrate as disclosed in the patent to Altman et al.

It should further be noted that while disc-shaped preforms are specifically illustrated in the drawings, other non-spherical shapes are equally as effective, and as such, any non-spherical shaped preform is suitable within the scope ofthe invention. While there is shown and described herein certain specific structure embodying the invention, it will be manifest to those skilled in the art that various modifications and rearrangements of the parts may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular forms herein shown and described except insofar as indicated by the scope of the appended claims.

Claims

1. A method for depositing solder preforms on solder pads of a substrate comprising the steps of: positioning a sheet of solder material over the substrate; stamping the sheet in such a manner that a plurality of solder preforms are removed from the sheet; and depositing the stamped solder preforms on the pads of the substrate.

2. A method as set forth in claim 1 , said step of stamping the sheet comprising the step of disposing a die below said sheet, said die having a plurality of openings formed therein which correspond to and are in registry with the solder pads of the substrate.

3. A method as set forth in claim 2, said step of stamping further comprising the step of providing a plurality of punch elements for punching solder preforms from the sheet through the openings of the die.

4. A method as set forth in claim 3, said openings ofthe die and punch elements being arranged in a grid array.

5. A method as set forth in claim 1 , said step of positioning the sheet over the substrate comprising the step of advancing the sheet relative to the substrate.

6. A method as set forth in claim 1 further comprising the step of heating the substrate having the solder preforms deposited thereon so as to fixedly attach the solder preforms to the pads ofthe substrate and to transform the relatively flat solder preforms into substantially spherical bumps.

7. An apparatus for depositing solder preforms on solder pads of a substrate comprising: means for advancing a sheet of solder material over the substrate; a die positioned between the sheet of solder material and the substrate, said die having a plurality of openings formed therein which correspond to and are in registry with the solder pads ofthe substrate; a plurality of punch elements for punching solder preforms from the sheet through the openings of the die, the arrangement being such that the punch elements extend into the openings ofthe die for punching solder preforms from the sheet, said solder preforms being disposed upon the pads ofthe substrate.

8. An apparatus as set forth in claim 7, said advancing means comprising a first reel having wound solder material in sheet form and a take-up reel, said take-up reel attached to the sheet of solder material wound on the first reel and being driven by drive means.

9. A method of forming a substantially spherical solder bump on a solder pad of a substrate comprising the steps of: depositing a non-spherical solder preform on the pad of the substrate; and heating the non-spherical solder preform to a melting temperature wherein said non-spherical solder preform automatically transforms to a substantially spherical shape due to the surface tension of the heated solder material, said solder preform simultaneously being bonded to said pad.

10. A method of forming a spherical solder bump, the method comprising the steps of:

(a) disposing a solder preform above a surface of a solder pad; and

(b) heating the solder preform to form the spherical solder bump.

1 1. The method according to claim 10, wherein step (a) includes stamping a solder foil to form the solder preform.

12. The method according to claim 1 1, wherein step (a) includes disposing the solder foil between a punch element and the solder pad. - 8 -

13. The method according to claim 12, wherein step (b) includes melting the solder preform.

14. The method according to claim 10, wherein step (b) includes melting the solder preform.

15. The method according to claim 10, wherein step (a) includes disposing the solder preform on a layer of solder flux disposed on the surface of the solder pad.

16. An apparatus, comprising: a die having at least one hole disposed therethrough; a support including at least one punch element in registry with the at least one hole; and a solder foil disposed between the die and the support so that the at least one punch element can contact a portion of he solder foil and pass at least partially into the at least one hole to form at least one solder preform from the portion ofthe solder foil.

17. The apparatus according to claim 16, further comprising a substrate disposed below the die.

18. The apparatus according to claim 17, wherein the substrate includes at least one solder pad in registry with the at least one hole.

19. The apparatus according to claim 18, further comprising: a first reel, a first segment ofthe solder foil being wound onto the first reel; and a second reel, a second segment ofthe solder foil being wound onto the second reel, the second segment being different than the first segment.

20. The apparatus according to claim 16, further comprising: a first reel, a first segment of the solder foil being wound onto the first reel; and a second reel, a second segment ofthe solder foil being wound onto the second reel, the second segment being different than the first segment.