JPH04312891A - Screening mask, method of screening and method of formation of conductor pattern - Google Patents

Abstract

PURPOSE: To adhere a conductive paste in a selective thickness onto a substrate. CONSTITUTION: A screening mask 10 for forming an image pattern corresponding to a conductive line, via and pad to be adhered to an electronic substrate 31 can be thinned in a screening paste adhered onto a shape isolated at a distance of a predetermined distance or less by a selective disposition of a reinforcing tab 22. When a lamination of the substrates is executed, the paste is slightly developed in a thin part to other part. Accordingly, at the pattern part isolated at the distance of a predetermined distance or less, a short-circuit and an improper withstand voltage capability can be prevented.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for screening paste onto thin sheet materials. More particularly, in a preferred embodiment, the present invention is directed to an improved screening mask with selective tab placement to reduce shorts between screened conductors on a ceramic electronics packaging substrate.

0002

BACKGROUND OF THE INVENTION Multilayer ceramic for integrated circuit semiconductor package structures.
, MLC), a plurality of green ceramic sheets are formed by doctor blading a slurry containing a resin binder, particulate ceramic material, solvent, and plasticizer. The doctor bladed sheet is dried and cut into appropriately sized sheets. Via holes are then drilled to form electrical interconnections through the sheet. After a conductive paste is deposited in the holes and in the wiring pattern on the sheet surface and the sheets are stacked and laminated, the assembly is fired at a sintering temperature. An example of the final product is disclosed in US Pat. No. 4,245,273. Such substrates can have several dozen or more individual layers, each layer having a unique pattern. The pattern includes metallization filling individual via holes, thin conductive lines interconnecting the via holes, pads surrounding the via holes on external layers, and large conductive areas forming power or ground patterns.

[0003] Deposition of conductive paste on green ceramic sheets is typically accomplished by extruding the paste through a thin metal mask having a unique pattern of apertures for each layer. More specifically, metallization of the green sheet is accomplished by a screening nozzle that extrudes the metallization paste in contact with the green sheet and across a metal mask. This screening process can be automated in a machine such as that disclosed in US Pat. No. 4,362,486.

Prior art screening masks used selectively patterned mesh screening to deposit paste onto a substrate. As technology advances require denser substrate patterns and finer conductive lines, mesh screen masks are considered unsuitable for paste deposition due to the loss of fine conductive lines caused by relatively large screen meshes. Ta. This problem was solved by the development of a metal screening mask with an aperture pattern displaying the desired substrate pattern. However, such masks do not have sufficient structural support and need to be reinforced by the addition of tabs that bridge the pattern. The use of such tabs is described in Kutch et al.'s “IBM
Technical Disclosure Bullet
tin, Vol. 12, No. 12, 197
May 0, pp. 2073-2074” and Byrne (
Byrne)'s “IBM Technical Dis
Closure Bulletin, Vol. 18
, No. 4, September 1975, p. 1035”
has been disclosed. Methods for manufacturing such masks using etching techniques are described in Gibney et al., “IBM Technical Disclosure.
Bulletin, Vol. 19, No. 3
, August 1976, pp. 930-931" and Gibney et al., "IBM Techni.
cal Disclosure Bulletin,
Vol. 19, No. 5, October 1976, pp. 1790-1971. The use of reinforcing tabs in screening masks for the manufacture of thin film electroluminescent display panels is disclosed in U.S. Pat.
, 615,781 and 4,715,940. When the paste is screened across a mask of the type described in the above reference, the paste is forced into the pattern defined by the mask. The thickness of the tab is less than the thickness of the mask, and the paste is pressed under the tab to form a continuous conductive path on the substrate.

An unrecognized problem in the manufacture of multilayer substrates with close line and via spacing is the spreading of screening paste during the lamination and sintering processes, which reduces the already close spacing. As a result, the finished board will have unacceptable voltage withstanding capability between lines or between lines and vias. Also, in some cases, short circuits exist between these structures.

[0006]

SUMMARY OF THE INVENTION Unacknowledged problems in the prior art demonstrate the need for a screening mask and method that allows for the deposition of selective thicknesses of conductive paste. It is therefore an object of the present invention to provide a screening mask with selective tab placement for the deposition of conductive paste of selective thickness.

It is a further object of the present invention to provide a mask in which the selective tab placement also provides structural support for the mask.

A still further object of the present invention is to provide a mask that allows selective deposition of conductive paste without degrading the electrical performance of the resulting substrate.

Yet another object of the present invention is to provide a selective paste deposition method using the selective tab placement screening mask of the present invention.

0010

SUMMARY OF THE INVENTION In accordance with these and other objects of the invention, a screening mask is provided for transferring a pattern to a substrate. The mask includes a border (
Preferably, the edges) are included. The first screening surface is configured to contact a device containing a screening paste, and the second screening surface is adapted to contact a substrate. The mask has a first surface to a second surface.
It includes a pattern of aperture shapes that extend to the surface. The variable shape spacing of the mask includes shapes that are separated by a distance that is less than or equal to a predetermined distance. Defects with features separated by a distance less than a predetermined distance can be prevented by selectively applying thinner paste to areas of these features. In screened green ceramic sheets used in the manufacture of electronic boards, paste deposited through features separated by a distance less than a predetermined distance can spread during the lamination process and cause shorting problems on the finished board. arise. Tabs adjacent to the features separated by a distance less than or equal to a predetermined distance help bridge the features and selectively deposit thinner pastes.

The present invention also provides a screening mask for forming a pattern of conductors on an electronic substrate. Preferably, the peripheral area of the mask includes means for securing the mask to a screening device, such as an automatic screening machine. The first flat surface terminates in a peripheral area and is adapted to contact the paste nozzle of the screening machine. A second planar surface terminates in the peripheral region and is adapted to contact the screening substrate when the mask is placed in a screening position of the screening machine. 1st through the thickness of the mask
A pattern extending from the surface to the second surface defines a variably spaced conductor pattern to be formed on the substrate. The plurality of conductor image sections of the mask pattern have a distance that is less than or equal to the minimum center distance. Conductor image sections separated by a distance less than this minimum center distance can cause defective conductor deposition as the paste spreads during the lamination process and creates shorts in the final product. However, according to the present invention, the tabs on the first surface bridge the plurality of conductor image sections having a distance that is less than or equal to the minimum center distance. The thickness of the tab is less than the thickness of the mask, and at the screening location the tab and substrate define a screening paste aperture.

[0012] The present invention also provides a method for forming a mask on a substrate comprising: disposing a mask having a shape of variably spaced apertures defining a pattern over a portion of the substrate on which a pattern is to be screened; provides a method for screening patterns. The mask includes tabs that bridge the shapes, the tabs being adjacent to the shapes separated by a distance that is less than or equal to a predetermined distance. An opening is defined between the tab and the substrate by the mask and the substrate. The method then includes screening the paste onto the mask and into the aperture shapes and transferring the pattern to the substrate. The resulting substrate pattern is thinner in areas with tabs than in areas without tabs.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The drawings will be explained in more detail. Referring specifically to FIGS. 1A and 1B of the present invention, a mask 10 according to the present invention is shown. The border 11 of the mask peripheral area preferably includes means for securing the mask 10 to the screening device during screening operations. In the illustrated embodiment, the border 11 is fixed to the frame 15 by adhesive. The frame 15 is then secured to the screening device by placing the frame 15 over the taper pin of the screening device fitted into the hole 20 of the clamp 18. The alignment of this system is discussed further below. Masks having borders that incorporate features for directly securing the mask to a screening device are also within the scope of the invention. However, the use of frame 15 provides a more robust interface to the screening device when thin masks are used. The first planar surface 12 of the mask terminates in a border 11, as described in U.S. Pat. No. 4,362;
The present invention is configured to contact a device containing a screening paste, such as the paste nozzle of the screening apparatus disclosed in '486. A second flat surface 13 also terminates in the border 11 and is adapted to contact the substrate 31 when the mask is placed in the screening position of the screening device.

Pattern 14 is comprised of a plurality of aperture shapes extending through the thickness of mask 10 from first flat surface 12 to second flat surface 13 . As previously discussed, the pattern provides variably spaced conductor images to be transferred to the green sheet, such as vias and pads, variably spaced signal lines, or power and ground metallization. Define. The terms shape and conductor image are used interchangeably to describe the aperture shape of this pattern.

In addition to the electronic applications described, the invention also includes:
It can be used in any other application where pattern screening is performed and selective paste thickness is desired due to narrow feature spacing. In electronic substrate applications, the multiple mask shapes have sufficient spacing to prevent short circuit or voltage breakdown problems caused by paste spreading during lamination and sintering of multiple substrates. However, many sections of the conductor image are separated by smaller predetermined distances. Below this distance, conductor extension causes short circuit and voltage breakdown problems during lamination of screened substrates.

Examples of mask shapes are shown in FIGS. 2(A) to 2(D).
) is shown. Figures 2A and 2B illustrate the pattern to be transferred to the outer layer of the electronic board, including conductive lines and pads surrounding the vias. In this case, the features separated by a distance less than or equal to a predetermined distance define a conductive line and a line terminating in a pad. The predetermined distance is the distance between the openings that corresponds to the width of the intervening metal web. Effective spacing is also characterized as the center-to-center distance between the centers of adjacent aperture shapes. A center distance, which is a predetermined distance of the sum of half the widths of adjacent openings, is derived based on the allowed predetermined distance between shapes. It is often convenient to use this term when describing feature spacing. Figures 2(C) and 2(D) illustrate the pattern to be transferred to the inner layer of the electronic board, including conductive lines and vias. In this case, features separated by a distance less than or equal to a predetermined distance define conductive lines and vias.

For the conductor image section of the inner layer:
For example, the predetermined distance such as “a” in FIG. 2(D) is approximately 0.
．． 0040~0.0073 inch (0.10~0.19
millimeters). For shapes on the outer layer,
For example, a predetermined distance such as “c” in FIG. 2(B) is approximately 0.
．． 0025~0.0073 inch (0.06~0.19
millimeters). The preferred line width is 0
．． Within the range of 0.002 to 0.005 inches (0.05 to 0.13 mm). Stated another way, the minimum center-to-center distance, such as "b" in FIG.
20-0.30 mm). Also in the future, a reduction in line width will be achieved and will become significantly smaller.

As previously noted, bridging tabs are necessary to provide structural rigidity to masks currently used to produce closely spaced features. In the present invention, tabs bridging shapes are placed adjacent to these shapes separated by a distance that is less than or equal to a predetermined distance. The tab is disposed on the first surface 12 and extends towards the second surface 13, but has a thickness that is less than the entire mask thickness. If sufficient mask support is not provided by these tabs bridging conductor image sections having a distance of less than a predetermined distance, additional tabs may be provided.

Referring to FIGS. 2(A) to 2(D),
An example of tab placement on a conductor image section of a mask is shown. In FIG. 2(A), the line 23 is the pad 21
and this combined image runs parallel to line 35. Tabs 22 are selectively placed proximate these shapes. In FIG. 2B, line 26 terminates at pad 25, and a portion of this feature is located at a distance less than a predetermined distance from line 24. In FIG. The tabs 22 are placed adjacent to these features separated by a distance that is less than or equal to a predetermined distance. In FIG. 2C, line 28 and via 27 are separated by a distance that is less than or equal to a predetermined distance. Tabs 22 are placed in close proximity to these shapes. In FIG. 2(D), line 30
has a portion separated from line 29 by a distance less than or equal to a predetermined distance. The tabs 22 are placed where these lines are separated by a distance that is less than or equal to a predetermined distance.

Referring to FIG. 3, a cross-sectional view of the mask shape shown in FIG. 2A is shown. The mask 10 is
It is in a substrate contact position where it abuts the substrate 31. mask 10
defines a screening paste aperture 33 within the opening between tab 22 and substrate 31 . As can be seen from the drawing, the tab thickness "d" is thinner than the mask thickness "e". In the illustrated embodiment, opening 33 is approximately 0.0015 inches (
The total mask thickness is approximately 0.0035 inches (0.089 mm). During the screening operation, the conductive paste is forced through the opening 33, forming a thinner portion 34 of the paste 32 in the area below the tab. In a preferred embodiment, the typical thickness of the screened paste is 0.0015 inches (0.038 millimeters) and the thinner thickness below the tab is 0.0008 inches (0.02 millimeters). This thinner portion of paste thickness provides adequate resistance to the conductivity of electrical signals and does not degrade the operability of electronic device systems fabricated from substrates incorporating ceramic green sheets.

In the embodiment shown in FIG. 1A, the mask is preferably manufactured by etching from a metal selected from the group consisting of molybdenum or nickel-plated copper. Frame 15 has sufficient thickness to maintain structural integrity when attached to the screening machine. The thickness shown in FIG. 1B is greatly exaggerated, with the screening portion of the mask being much thinner than the frame.

When the mask 10 is bonded to the mask frame 15, the mask is attached to the fixture via holes 16 and frame holes 17 prior to bonding to align the mask with the frame. Final alignment adjustments are made using the diameter difference between the other frame holes 19 and clamp holes 20. Clamp holes 20 engage tapered pins of the screening machine to attach the mask to the screening position.

The first step in the screening operation is to provide an electronic substrate, such as a green sheet, to the screening position of the screening machine. A screening mask of the invention is then placed in contact with the substrate. A conductive paste is then screened across the mask, non-uniformly transferring the mask pattern to the substrate. The transfer is not uniform in that the screened conductor has a thinner cross-sectional area of thickness in the greensheet portions corresponding to the tabbed portions of the mask. As previously mentioned, the tabs are provided in close proximity to the features spaced apart by a predetermined distance or less and provide structural support for the mask in other areas as needed. Once the paste has been screened through the mask onto the substrate, the mask is removed from the substrate, the substrate is removed from the screening location, and another substrate is placed in the screening location. This operation can continue as long as necessary, making it a continuous flow operation.

[0024]

As explained above, when the screening mask of the present invention is used, the paste that adheres to the shapes separated by a predetermined distance or less can be made thinner by selectively arranging the reinforcing tabs. can. Therefore,
Since the paste spreads only a little during the lamination operation, it is possible to prevent short circuits and inadequate voltage-withstanding capability.

[Brief explanation of drawings]

FIG. 1 (A) is a plan view of a screening mask of the present invention. (B) is a side view of a screening mask arranged to screen a substrate.

2(A)-(D) show detailed views of various embodiments of patterns with selective tabs of the present invention; FIG.

FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. 2(A).

[Explanation of symbols]

10　　　　Mask 11 Border 12 First surface 13 Second surface 14 Pattern 15 Frame 18 Clamp 20 Hall 22 Tab 31　　　　　Substrate

Claims (33)

[Claims] 1. A screening mask for transferring a pattern to a substrate, comprising a first screening surface adapted to contact a device containing a screening paste and a second screening surface adapted to contact the substrate. a screening surface; a pattern of aperture shapes extending from the first surface to the second surface, the pattern having variable feature spacing including features separated by a distance less than or equal to a predetermined distance; 1. A screening mask comprising: a tab that bridges a shape, the tab being at least close to a shape separated by a distance equal to or less than a predetermined distance; 2. The screening mask of claim 1 further comprising a border having means for securing the mask during screening operations. 3. The screening mask of claim 2, further comprising a mask frame secured to the mask. 4. The tab extends from the first surface and has a thickness that is less than a distance from the first surface to the second surface, such that the mask is in contact with the substrate at a location where the tab contacts the substrate. Claim 1 defining an opening between the
Screening mask as described. 5. The screening mask of claim 4, wherein the aperture has a height of about 0.0015 inches (about 0.038 millimeters). 6. The screening mask according to claim 1, wherein the mask includes metal. 7. The screening mask of claim 6, wherein the metal is selected from the group consisting of molybdenum and nickel-plated copper. 8. The screening mask of claim 1, wherein the pattern defines lines and vias of an electronic board. 9. The screening mask of claim 8, wherein the shapes separated by a distance less than or equal to a predetermined distance define a line. 10. The screening mask of claim 9, wherein the predetermined distance is within a range of approximately 0.0040 to 0.0085 inches (approximately 0.10 to 0.22 mm). 11. The screening mask of claim 9, wherein the features separated by a distance less than or equal to a predetermined distance define lines and vias. 12. The screening mask of claim 11, wherein the predetermined distance is within a range of about 0.0040 to 0.0073 inches (about 0.10 to 0.19 millimeters). 13. The screening mask of claim 8, wherein the features separated by a distance less than or equal to a predetermined distance define a line and a line terminating in a pad. 14. The screening mask of claim 13, wherein the predetermined distance is in the range of about 0.0025 to 0.0073 inches (about 0.064 to 0.19 millimeters). 15. A screening mask for forming a pattern of conductors on an electronic substrate, comprising: a first planar surface adapted to contact a paste nozzle of a screening device; and a first planar surface adapted to contact a paste nozzle of a screening device; a second planar surface adapted to contact the substrate to be screened; and the second surface from the first surface defining a variably spaced conductive pattern to be formed on the substrate. a pattern extending through the mask thickness such that a plurality of conductor image sections of the pattern have a distance less than or equal to a minimum center distance; and a plurality of conductor image sections of the pattern separated by a distance less than or equal to a minimum center distance. A screening mask comprising: a tab on a first plane that is at least cross-linked and has a thickness less than the mask thickness, such that at the screening location the tab and the substrate define a screening paste aperture. 16. The screening mask of claim 15, further comprising a peripheral region including means for securing the mask to a screening device, and wherein the first and second surfaces terminate in the peripheral region. 17. The screening mask of claim 16, further comprising a mask frame secured to the mask. 18. The screening mask of claim 15, wherein the minimum center distance is within a range of about 0.008 to 0.012 inches (about 0.20 to 0.30 millimeters). 19. The screening mask of claim 15, wherein the image sections at a distance less than or equal to a minimum center distance define conductive lines. 20. The screening mask of claim 15, wherein the image sections at a distance less than or equal to a minimum center distance define conductive lines and vias. 21. The screening mask of claim 15, wherein the image section at a distance less than or equal to a minimum center distance defines a conductive line and a conductive line terminating in a pad. 22. The screening mask of claim 15, wherein the mask comprises a metal selected from the group consisting of molybdenum and nickel-plated copper. 23. A method for screening a pattern onto a substrate, the method comprising: positioning a mask having variably spaced aperture shapes defining a pattern over a portion of the substrate on which the pattern is to be formed. the mask includes tabs bridging the shapes, the tabs are at least proximate to the shapes separated by a distance less than or equal to a predetermined distance, and an opening is defined by the mask and the substrate between the tabs and the substrate. and screening the paste onto the mask and into the aperture shapes to transfer the pattern to the substrate, the substrate pattern having a thickness thinner in the tabbed areas than in the untabbed areas. and a screening method including. 24. The method of screening of claim 23, wherein the mask pattern defines conductive lines and vias in the electronic substrate. 25. The screening method of claim 24, wherein the features separated by a distance less than or equal to a predetermined distance define conductive lines. 26. The method of screening of claim 24, wherein the features separated by a distance less than or equal to a predetermined distance define conductive lines and vias. 27. The screening method of claim 24, wherein the features separated by a distance less than or equal to a predetermined distance define a conductive line and a conductive line terminating in a pad. 28. The screening method according to claim 23, wherein the screening step includes press-fitting the paste through the opening to form a thinner thickness portion of the substrate pattern. 29. A method for forming a pattern of conductors on an electronic substrate, the method comprising: providing an electronic substrate;
disposing a screening mask in contact with a substrate, the mask having a through pattern defining variably spaced conductors, and a plurality of conductor images of the through pattern having a distance less than or equal to a minimum center distance; screening a conductive paste across the mask to non-uniformly transfer the mask pattern to the substrate;
the screened conductors have a cross-sectional area less than other substrate conductor sections in a plurality of conductor sections corresponding to the plurality of conductor image sections of the mask at least less than or equal to a minimum center distance; and removing the mask from said substrate. A method for forming a conductor pattern, including a process. 30. An opening is provided between the tab and the substrate when the mask contacts the substrate, and the screening step press-fits paste through the opening to form a thinner cross-sectional area of the substrate conductor section. Claim 2 comprising
9. The method for forming a conductor pattern according to 9. 31. The method of forming a conductive pattern as recited in claim 29, wherein the image sections less than or equal to a minimum center distance define conductive lines. 32. Clause 29, wherein the image sections less than or equal to a minimum center distance define conductive lines and vias.
The method for forming the conductor pattern described above. 33. The method of forming a conductive pattern as recited in claim 29, wherein said image sections less than or equal to a minimum center distance define conductive lines and conductive lines terminating in pads.