The invention relates to a method for carrying out solder ball placement and a stencil for use in such a method.

Semiconductor wafers contain small electronic devices that require connection to external circuitry. This external connection can be manufactured by forming an input/output connection at the surface of the wafer substrate and creating a connection point with a deposit of solder on the input/output connection.

There are two different approaches to depositing solder onto a substrate. The first is to apply a solder paste to a solder paste stencil and force the paste through the solder paste stencil using a squeegee. The following disclosure is concerned with a second approach, which involves the use of a ball placement stencil having apertures that allow accurate positioning of balls of solder upon a substrate. Typically, such methods would involve a first step of depositing flux onto the substrate at the connection points. The substrate, flux and solder are then heated to melt the solder to form the connection points.

Conventional ball placement stencils are often fouled by contact with the flux and require cleaning at significant cost. Cleaning also degrades the stencil and can result in separation of layers formed on the stencil. Accordingly, it is desirable to provide an improved stencil and/or tackle at least some of the problems associated with the prior art or, at least, to provide a commercially useful alternative thereto.

According to a first aspect of the invention, there is provided a method of placing solder balls on a substrate, the method comprising:

providing a substrate;

providing a plurality of balls comprising solder;

providing a ball placement stencil having an upper side and a lower side, the ball placement stencil comprising a plurality of apertures for receiving the balls and positioning the balls on the substrate, and having attached thereto a plurality of discrete spacing means on the lower side;

- l - positioning the ball placement stencil adjacent the substrate so that the ball placement stencil is spaced from the substrate by the spacing means; and

depositing the balls onto the substrate via the apertures of the stencil,

wherein the apertures reduce in width from the upper side towards the lower side.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

According to a second aspect of the invention, there is provided a ball placement stencil assembly for placing solder balls on a substrate, comprising.

an upper side and a lower side;

a plurality of apertures for receiving the balls and positioning the balls on a substrate;

a plurality of discrete spacing means attached to the lower side,

wherein the apertures reduce in width from the upper side towards the lower side.

Embodiments of the invention can provide methods or ball placement stencils in which close placement of balls of solder is possible without the ball placement stencil being fouled in the process of ball placement. The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 depicts a cross-sectional view of a ball placement stencil assembly in use on a substrate;

Figure 2 depicts a cross-sectional view of a flux deposition stencil in place on a substrate; and

Figure 3 depicts a plan view of the lower side of the ball placement stencil of Figure 1.

Figure 4 depicts a flowchart of the method according to claim 1. A first embodiment of the ball placement stencil assembly of the invention is shown in Figure 1. The ball placement stencil assembly is in place on a substrate 500. The substrate contains a plurality of semiconductor devices having input/output terminals 510 upon which a quantity of flux 300 has been deposited. A solder ball 400 has been placed onto each deposit of flux 300.

The ball placement stencil assembly comprises: a ball placement stencil 100 and a plurality of discrete spacing means 200. The ball placement stencil 100 has an upper side 110 (the free side) and a lower side 120 (the substrate 500 side) and a plurality of apertures 105. The position of the apertures 105 corresponds to the position on the substrate 500 of the input/output terminals 510 of the semiconductor devices. The apertures 105 are reverse tapered such that they are wider at the upper side 110 than the lower side 120. The apertures 05 may have a circular cross-section. The upper opening 115 of the aperture 105 at the upper side 110 has a first diameter. The lower opening 125 of the aperture 105 at the lower side 120 has a second diameter. The first diameter is greater than the second diameter. The solder ball 400 is generally spherical with a diameter 410. The diameter of a solder ball 400 falls in the range 130 microns to 300 microns. The opening 115 at the upper side 110 must be larger than the solder ball 400 to allow the solder ball 400 to be inserted into the aperture 105 and thereby prevent an aperture 105 being missed during solder ball 400 placement. However, if the aperture 105 is too large then accurate solder ball 400 placement is not possible. Advantageously, the reverse taper of the ball placement stencil 100 provides an upper opening 115 that readily accepts a solder ball 400, whilst providing a lower opening 125 that allows accurate positioning of the solder ball 400 on the substrate 500. Preferably, the upper opening 115 is up to 15% wider than the lower opening 125. More preferably, the upper opening 115 is between 5% and 15% wider than the lower opening 125. Most preferably, the upper opening 15 is 7% wider than the lower opening 125. Preferably, the upper opening 115 has a width of 160 microns to 380 microns, and the lower opening 125 has a width of 150 microns to 330 microns. Preferably, the lower opening 125 is at least 10% wider than the diameter of the solder ball 400. Preferably, the upper opening 115 is at least 10% wider than the diameter of the solder ball 400.

The ball placement stencil may be formed of a sheet of a material such as stainless steel, nickel, alloy 42, brass, Kapton, or a plastic. Of these, stainless steel or nickel are preferable.

The apertures 105 in the ball placement stencil 100 are preferably manufactured by a laser cutting process. A G6080 machine available from LPKF GMBH is suitable for forming the apertures 105.

Preferably, the laser cutting device will cut the sides of the aperture at an angle in the region of 5° to 25° from the vertical direction (i.e. the longitudinal axis of the aperture 105). Alternatively, the ball placement stencil 100 may be formed with the apertures 105 already present by electroforming.

The manufacture of a ball placement stencil 100 by laser cutting a sheet of material is more appropriate for the formation of apertures having a smaller ratio (e.g., in the range 5% to 7%) between the upper opening 115 widths and the lower opening 115 widths, while the electroforming process is more appropriate for the larger ratios (e.g., in the range 7% to 15%).

The discrete spacing means 200 are attached to the lower side 120 of the ball placement stencil 100. Preferably, each discrete spacing means 200 is a pillar. More preferably, the cross-section of each pillar is circular.

Preferably, the width of a discrete spacing means 200 is in the range 40 microns to 200 microns. More preferably, the width of a discrete spacing means 200 is approximately 100 microns. The discrete spacing means 200 may comprise a photo polymer material, such as dry film photo resist. Such discrete spacing means 200 are preferably formed on the lower side 120 of the ball placement stencil 100 by applying a layer of material and then, for example, photoetching the material using a mask.

Techniques for etching using masks are well known in the art. For example, a polymer may be applied to the surface and cured through a mask to form the discrete spacing means. Those areas protected by the mask do not cure and can then be removed from the surface to leave the discrete spacing means on the surface. For these techniques, the photo polymer preferably comprises a photoinitiator and a polymerisable polymer.

Alternatively, the discrete spacing means 200 may comprise metal, such as nickel. Such discrete spacing means 200 would be manufactured by electroforming.

The discrete spacing means 200 are arranged on the lower side 120 of the ball placement stencil 00 such that there is a clearance 210 (see Figure 3) between the edge of the discrete spacing means 200 and the lower opening 125 of each aperture 105. The clearance 210 has a minimum value of 40 microns.

The lower opening 125 of the aperture 05 is small and therefore located close to the flux deposit 300. The clearance 210 advantageously allows the discrete spacing means 200 to be located away from the flux deposit 300 thereby reducing the risk that flux 300 would adhere to the ball placement stencil assembly and increase the need for intensive cleaning. Intensive cleaning can degrade stencils, e.g. by separating the discrete spacing means 200 from the ball placement stencil 100.

As can be seen in Figure 1 , the ball placement stencil 100 has a thickness 130, while the discrete spacing means 200 have a height 230. The total height of the ball placement stencil assembly is equal to the sum of the thickness 130 of the ball placement stencil 00 and the height 230 of the discrete spacing means 200.

Preferably, the thickness 30 of the ball placement stencil 100 is in the range 50 microns to 300 microns. More preferably, the thickness 130 of the ball placement stencil 100 is in the range 100 microns to 200 microns. Preferably, the height 230 of the discrete spacing means 200 is in the range 25 microns to 300 microns. More preferably, the height 230 of the discrete spacing means 200 is in the range 50 microns to 75 microns. These ranges lead to a range for the total height of the ball placement stencil assembly of 75 microns to 600 microns. Preferably, the total height of the ball placement stencil assembly is in the range 100 microns to 350 microns. More preferably, the total height of the ball placement stencil assembly is in the range 150 microns to 250 microns.

Preferably, the total height of the ball placement stencil assembly is substantially equal to the diameter 410 of the solder ball 400. It has been found that matching these dimensions provides a ball placement stencil assembly that more readily accepts solder balls 400 when using either a squeegee or a ball placement machine (described further below) and prevents the solder balls 400 from being lost between the ball placement stencil 100 and the substrate 500.

Preferably, the thickness 130 of the ball placement stencil 100 and the height 230 of the discrete spacing means 200 are equal so that the lower opening 125 of the aperture 105 corresponds with the height of the solder ball 400 at its widest point, thereby providing accurate solder ball 400 placement.

Figure 2 shows a flux deposition stencil 600 in place on a substrate 500. The flux deposition stencil 600 has a plurality of holes 605 in locations corresponding to the input/output terminals 5 0 formed in the substrate 500.

Flux 300 can be forced into the holes 605 using known techniques to leave deposits of flux 300 upon the input/output terminals 510. The holes 605 have diameters in the range of 25% to 75% the width of the lower openings 125 of the apertures 105 in the ball placement stencil 100.

Figure 3 shows the arrangement of the lower openings 125 of the apertures 105 and the discrete spacing means 200 from the underside of the ball placement stencil assembly. Many semiconductor devices are formed in the wafer in an array. The input/output terminals 510 are therefore formed in a repeating pattern. Accordingly, the locations of apertures 105 in the ball placement stencil 100 also form a repeating pattern. This is illustrated in Figure 3 as a regular array of repeating units 800, but may be any repeating pattern. The apertures 105 are spaced apart so as to have a minimum separation between the upper openings 115 of 40 microns.

Figure 3 also shows (as a dotted line) the size of the openings 115 on the upper side 110 of the ball placement stencil 100. The discrete spacing means 200 positioned between four apertures 105 with a clearance 210 between the discrete spacing means 200 and the lower openings 125.

Since the apertures are reverse tapered, the lower openings 125 have a smaller cross sectional area than if the apertures were of constant cross section, as they are in conventional ball placement stencils. The size of the aperture openings in a conventional ball placement stencil must be equivalent to the size of the upper openings 115 in the embodiment in order to reliably receive the solder ball 400. There is therefore less area in a conventional ball placement stencil for discrete spacing means 200.

However, in the embodiments, since the lower openings 125 are smaller than the upper openings 115, the discrete spacing means 200 can be larger than if it were applied to a conventional ball placement stencil. Figure 3 depicts an arrangement in which four apertures 105 surround a single discrete spacing means 200 to form a repeating unit 800. However, other arrangements of apertures 105 and discrete spacing means 200 are envisaged, such as arrangements in which three or more apertures 05 surround each discrete spacing means 200 to form a repeating unit 800.

It is preferable that the discrete spacing means 200 is as large as possible to support the ball placement stencil 100. The size of the discrete spacing means 200 is therefore limited by the diameter of the apertures 05, the spacing between the apertures 105, and the clearance 210 between the apertures 105 and the discrete spacing means 200. The preferable parameters given above may result in a ratio between the area of the discrete spacing means 200 in each repeating unit 800, and the area of the apertures falling in the range 0.7 to 1.4. Figure 4 shows an exemplary flowchart of the method steps. Step A involves providing a substrate. Step C involves providing a plurality of balls comprising solder. Step B involves providing a ball placement stencil having an upper side and a lower side, the ball placement stencil comprising a plurality of apertures for receiving the balls and positioning the balls on the substrate, and having attached thereto a plurality of discrete spacing means on the lower side.

In step D, the ball placement stencil is positioned adjacent the substrate so that the ball placement stencil is spaced from the substrate by the spacing means. In step E the balls are deposited onto the substrate via the apertures of the stencil.

The following describes an embodiment of a method of placing solder balls on a substrate using the above described ball placement stencil assembly.

The above described flux deposition stencil 600 is positioned upon the substrate 500 so that the holes 605 are located above the input/output terminals 510. Flux is applied to the upper surface 610 of the flux deposition stencil 600 and a squeegee 700 is forced in a direction 710 across the upper surface 610 of the flux deposition stencil 600 to drive the flux 300 into the holes 605. The flux deposition stencil 600 is then removed from the substrate 500 to leave a plurality of flux 300 deposits upon the input/output terminals 510.

The ball placement stencil assembly is then positioned adjacent the substrate 500 so that the discrete spacing means 200 contact the substrate 500 and the apertures 105 are located above the flux 300 deposits on the input/output terminals 510.

Solder balls 400 are deposited into the apertures of the ball placement stencil 100 using a ball placement device. Alternatively, solder balls 400 are deposited onto the ball placement stencil 100 and forced into the apertures using a squeegee. The ball placement stencil assembly is then removed leaving a plurality of solder balls 400 in place upon flux deposits 300 on the semiconductor devices 500. Flux deposits 300 help to retain the solder balls 400 in position on the substrate 500.

The substrate 500, flux deposits 300 and solder balls 400 are then heated above the melting point of the solder balls 400. The solder balls 400 melt and form connection points on the surface of the input/output terminals 510. The wetting properties of the flux 300 helps to retain the solder in place above the input/output terminals 510.

Conventionally, when installing a ball placement stencil assembly correct registration of the ball placement stencil assembly is achieved by visually referencing fiducials. Fiducials are markings on the ball placement stencil assembly. Typically, fiducials are manufactured as holes machined through the ball placement stencil assembly. The inventors have discovered that such fiducials can often lead to balls of solder that have been deposited on the ball placement stencil assembly passing through the holes and reaching the substrate. This can lead to unwanted fouling of the substrate.

This problem can be overcome by adding material to the ball placement stencil assembly rather than machining holes.

A preferred embodiment of such a ball placement stencil assembly for placing solder balls on a substrate, comprises: an upper side and a lower side; a plurality of apertures for receiving the balls and positioning the balls on a substrate; a plurality of markers located on the lower and/or upper side. The lower side is especially preferred since the markers can be in the same step as the spacing means.

Preferably, the discrete markers are formed of a photo polymer material such as a dry film photo resist.

Preferably such a ball placement stencil assembly is manufactured by: providing an intermediate having an upper side and a lower side, the intermediate having a layer resist on the upper side; and selectively removing the resist to leave fiducials at a plurality of locations on the lower and/or upper side. Preferably, the resist is a photo polymer material such as a dry film photo resist, and the step of selectively removing the resist using a mask as discussed herein.

In one embodiment the ball placement stencil assembly further comprises a supporting frame around the periphery. Such supporting frames are well known in the art, such as the frame disclosed in WO2011/023964 which is incorporated herein by reference in its entirety.

According to a further aspect there is provided a kit comprising the ball placement stencil and a supporting frame for attachment around the periphery of the stencil.

According to a further aspect there is provided a kit comprising the ball placement stencil and a plurality of balls comprising solder and, optionally, a supporting frame for attachment around the periphery of the stencil.

Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims.