STENCIL AND METHOD FOR DEPOSITING A VISCOUS PRODUCT ABOUT SOLDER BALLS THEREWITH BACKGROUND OF THE INVENTION Field Of The Invention The present invention provides a stencil and method for depositing a viscous product about solder balls therewith.

Brief Description Of Related Technology Traditionally, stencil printing of electronic materials onto substrates involved the provision of a flood coat of electronic material through the stencil without using any effort to control the ultimate location of electronic material on the substrate.

Recent advances in that dispensing technique provide for stencils, the construction of which allows for a greater deal of precision regarding the location and height of viscous product dispensed onto substrates therewith. For instance, U. S. Patent No. 6,183, 839 (Bourrieres) describes and claims a stencil for the deposit and apportioning of more or less thick layers, based on points or in the form of points, of a viscous product on a substrate by passing a scraper over the stencil to bring the stencil into close contact with the substrate in the place where the scraper exerts a linear pressure. The stencil so described and claimed is constructed of a solid and uniformly thick sheet of a synthetic material with holes bored therethrough. The sheet thickness is determined according to the greatest height of deposit to be made on the substrate and the sheet has different boring diameters sized according to the height of the different deposits of the viscous product to be made and the thickness of the sheet. In this way, the viscous product proportions deposited and the deposit heights obtained by a passage of the scraper are different.

While such a stencil is useful for depositing a viscous material directly onto a substrate, this stencil was designed and developed for use with surface mount adhesives or solder pastes, not for use in dispensing an electronic material such as an underfill sealant.

With underfill sealant materials such a stencil is not as useful, because thinner coatings of such underfill sealant materials with more specific placement on and around solder balls attached to the substrate has resulted in refinements in the way those persons of ordinary skill in the art look to stencil print such underfill sealant materials. These thinner coatings are not the dispensed dots of viscous product, such as which results with use of the stencil as described above.

Other stencils, such as the so-called open stencil, also does not control placement of the electronic material but rather simply prevents it from flowing outward over the substrate itself.

Conventional wisdom has also directedimany users of electronic materials generally in the packaging and assembly of microelectronic devices to use a stencil, which blocks the provision of electronic material on the solder ball by way of stencil obstruction. That is, by choosing a stencil designed such that its openings appear in association with the solder ball attached in a pattern on the substrate, electronic materials would be dispensed only around the solder balls and not on the solder balls. It is perceived desirable at the end of the assembly process to ensure that little to no electronic material (other than a fluxing material) resides on the portion of the solder balls which is available for establishing electrical interconnection (the portion not attached to the substrate of the solder ball). It is believed that residual electronic material interferes with a solder reflow process and electrical disconnections are more likely to be observed.

Thus, it would be desirable to provide a stencil and method for depositing a viscous product about solder balls therewith that overcomes the processing issues addressed above.

SUMMARY OF THE INVENTION The present invention a stencil and method for depositing a viscous product about solder balls therewith.

The present invention has recognized the deficiencies in the conventional approaches. As such, the present invention uses a stencil, whose apertures are located in a pattern associated with the pattern of solder balls on the substrate. The openings are dimensioned to allow for the solder balls to protrude part way therethrough. Electronic material is dispensed onto the stencil and advanced over the stencil such that electronic material is deposited directly on the solder ball. And importantly each aperture is dimensioned and disposed to receive a single solder ball. This is in direct contrast to conventional efforts.

By choosing an electronic material with appropriate rheological properties, electronic material flows around the solder balls and in so doing creates a layer on the substrate at the solder ball/substrate interface to stabilize that interface against stresses. As the electronic material flows around and about the solder ball, void potential around the substrate and the solder ball is decreased because the rheology of electronic material allows for it to flow around and about the solder ball.

More specifically, the invention provides a process for depositing a viscous product onto a substrate to which is attached a plurality of solder balls. The steps include: providing a substrate to which is attached the plurality of solder balls; providing a stencil having a plurality of apertures, the plurality of apertures being dimensioned and positioned so as to accommodate the plurality of solder balls; positioning this stencil over the substrate such that the plurality of solder balls occupy the plurality of apertures, and the plurality of solder ball protrude through the stencil; and depositing the viscous product onto at least a portion of the plurality of solder balls that protrude through the stencil.

The viscous product has a rheology sufficient to allow the viscous product flow around and about the plurality of solder balls.

In addition, the present invention provides a stencil for use in depositing a viscous product onto a substrate to which is attached a plurality of solder balls, comprising a planar sheet having a viscous product receiving surface, a substrate abutting surface and a plurality of apertures through the planar sheet. The plurality of apertures are dimensioned and disposed to permit the plurality of solder balls to enter and pass part way through the plurality of apertures so that a portion of the plurality of solder balls may receive a viscous product dispensed thereon at a position beyond the receiving surface of the stencil.

BRIEF DESCRIPTION OF THE FIGURES FIG. la depicts in top view the inventive stencil.

FIG. 1b depicts in cross sectional view the inventive stencil of FIG. la.

FIG. 2 depicts in cross sectional view the application of viscous product on and around solder balls using the inventive stencil.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a stencil for use in depositing a viscous product onto a substrate to which is attached a plurality of solder balls, comprising a planar sheet having a viscous product receiving surface, a substrate abutting surface and a plurality of apertures through the planar sheet. The plurality of apertures are dimensioned and disposed to permit the plurality of solder balls to enter and pass part way through the plurality of apertures so that a portion of the plurality of solder balls may receive a viscous product dispensed thereon at a position beyond the receiving surface of the stencil.

With the inventive stencil, a plurality of apertures are located in the planar sheet in predetermined locations and are dimensioned such that a single solder ball on the substrate may part way extend through a single aperture. The apertures in the stencil as noted are dimensioned such that a solder ball may extend part way therethrough. That is, the apertures are larger in diameter than that of the solder ball themselves, though not so large so as to permit more than one solder ball through the aperture. The viscous product is dispensed onto the stencil and by way of a squeegee across the solder balls extending through the apertures of the stencil.

The thickness of the stencil may be adjusted accordingly such that a desired height of viscous product may be achieved around the solder balls. The stencil and method of the present invention have provided a thickness of underfill sealant material around the solder balls of less than twenty percent (20%) of the solder ball height. With a solder ball height of 0.5 mm, therefore, a coating thickness of 0.098 mm of underfill sealant material was measured. In this case, the solder balls were in a full array, with a 1.0 mm pitch, and the stencil thickness was 6 mil, with each aperture being circular and having a 0.85 mm diameter.

While the viscous product dispensed as an underfill sealant material had a viscosity of 2,132 cps, generally the viscosity should be lower than 10,000 cps to achieve the desired effect. Upon studying the present invention as so described herein, those with ordinary skill in the art will appreciate that the viscosity may vary depending on the thickness of the stencil and the shape and diameter of the apertures.

In contrast, use of the so-called open stencil was only able to achieve a thickness of about thirty eight percent (38%) of the solder ball height--nearly twice as thick. Clearly, where a reduction in the thickness of the underfill sealant material is an objective to be achieved, the method and stencil of the present invention provides almost a fifty percent (50%) reduction in thickness.

The viscous product is a flowable material, which may be rendered non-flowable, particularly when it has been positioned at the interface between the solder balls and the substrate. The viscous product is desirably an underfill sealant material, such as a thermosetting resin composition, the placement of which about the solder balls provides stress absorption at the interface between the solder balls and the substrate. The underfill sealant material may be alternatively a thermoplastic material or additionally include a thermoplastic material. The only requirement of the underfill sealant material is that it has a rheology appropriate to flow around and about the solder ball toward the substrate after it has been dispensed onto the solder ball. As noted above, a viscosity of 2132 cps provides an appropriate rheology when used with a stencil, the thickness of which was 6 mil, and whose apertures were circular and had a 0.85 mm diameter.

In contrast, a stencil with an open design (such that a plurality of solder balls may extend through the apertures in the planar sheet) does not provide the same low level of thickness Reference to FIG. la shows a top view of an example of the inventive stencil 1 with a plurality of apertures 2 therethrough. Reference to FIG. 1b shows a cross sectional view of the inventive stencil of FIG. la.

Reference to FIG. 2 shows in cross-sectional view the stencil 1 disposed over and about the solder balls 3 with viscous material 4 having been applied to the stencil 2 and a squeegee 5 applying and preparing to apply in the direction labeled A the viscous material 4 over the solder balls 3. The stencil 1 abuts substrate 8 with its substrate-abutting surface 7 and receives the viscous material 4 on its viscous product receiving surface6. Once the squeegee 5 applies the viscous material 4 over the solder balls 3, the viscous material 4 flows within the apertures 2 and about and around the solder balls 3 and flowing toward interface 9 of the solder balls 3 and the substrate 8. The rheology of the viscous material 4 allows for flow to occur and a gradient to build up at the solder ball/substrate interface. Once the viscous material gradient is rendered non-flowable (either by curing or solidification), stress absorption may be realized.

As may be seen with reference to the figures, the stencil used in depositing a viscous product onto the substrate (to which is attached a plurality of solder balls) includes a planar sheet having two surfaces and a plurality of apertures therethrough. The two surfaces may be referred to as a viscous product receiving surface and a substrate- abutting surface. The plurality of apertures in the planar sheet are dimensioned and disposed to permit the plurality of solder balls to enter and pass part way through the plurality of apertures so that a portion of the plurality of solder balls may receive a viscous product dispensed thereon at a position beyond the receiving surface of the stencil.

The plurality of apertures are aligned with the plurality of solder balls such that desirably a single solder ball is associated with a single aperture.

The planar sheet of which the stencil is comprised may be made of any material that is inert to both the substrate to which it will abut in application and the viscous product that it will receive during application, though stainless steel seems to be particularly desirable.