GB2495761A

GB2495761A - Solder Ball Placement Method and Apparatus

Info

Publication number: GB2495761A
Application number: GB1118160.9A
Authority: GB
Inventors: Hans Korsse; Leo Mertens; Theo Van Mechelen
Original assignee: Alpha Fry Ltd
Current assignee: Fernox Ltd
Priority date: 2011-10-20
Filing date: 2011-10-20
Publication date: 2013-04-24
Also published as: GB201118160D0; WO2013057465A3; WO2013057465A2; TW201322395A

Abstract

The present invention relates to a method of placing solder balls 400 on a substrate, the method comprising: providing a substrate; providing a plurality of balls comprising solder; providing a ball placement stencil 100 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; 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.

Description

Solder Ball Placement Method and Apparatus 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 the 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.

According to a first aspect of the invention, there is provided a method defined by claim 1.

According to a second aspect of the invention, there is provided a ball placement stencil assembly defined by claim 31.

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.

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) -2.-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 105 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 151 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 115 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, Icapton, 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 06080 machine available from IJPKF 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 50 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 51 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 photoetching the material using a mask.

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 100 such that there is a clearance 210 (see Figure 3) between the edge of the discrete spacing means 200 and the lower opening of each aperture 105. The clearance 210 has a minimum value of 40 microns.

The lower opening 125 of the aperture 105 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 from the ball placement stencil 100.

As can be seen in Figure 1, the ball placement stencil has a thickness 130, while the discrete spacing means 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 100 and the height 230 of the discrete spacing means 200.

Preferably, the thickness 130 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 is in the range 25 microns to 300 microns. More preferably, the height 230 of the discrete spacing means 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 510 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 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.

1-lowever, 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 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 105 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 105, the spacing between the apertures 1051 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.

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. -.9-

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.

-10 -

Claims

<claim-text>Claims: 1. 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; 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.</claim-text> <claim-text>2. The method of claim 1, wherein the plurality of apertures form a regular array across the stencil.</claim-text> <claim-text>3. The method at claim 1 or claim 2, wherein the discrete spacing means are located on the lower side of the stencil between three or more apertures.</claim-text> <claim-text>4. The method of any of the preceding claims, wherein the combined thickness of tile ball placement stencil and the discrete spacing means is substantially equal to the diameter of the balls.</claim-text> <claim-text>5. The method of any preceding claim, wherein the combined thickness of the ball placement stencil and the -11 -discrete spacing means is in the range 100 microns to 350 microns.</claim-text> <claim-text>6. The method of claim 5, wherein the combined thickness of the ball placement stencil and the discrete spacing means is in the range 150 microns to 250 microns.</claim-text> <claim-text>7. The method of any preceding claim, wherein the thickness of the ball placement stencil is in the range microns to 300 microns.</claim-text> <claim-text>8. The method of any preceding claim, wherein the thickness of the ball placement stencil is in the range microns to 200 microns.</claim-text> <claim-text>9. The method of any preceding claim, wherein the thickness of the discrete spacing means is in the range microns to 300 microns.</claim-text> <claim-text>10. The method of any preceding claim, wherein the thickness of the discrete spacing means is in the range microns to 75 microns.</claim-text> <claim-text>11. The method of any preceding claims, wherein the aperture on the upper side has a diameter greater than the diameter of the corresponding aperture on the lower side by up to 15%.</claim-text> <claim-text>12. The method of any preceding claims, wherein the aperture on the upper side has a diameter greater than the diameter of the corresponding aperture on the lower side by 5% to 15%.</claim-text> <claim-text>-12 - 13. The method of any preceding claims, wherein the aperture on the upper side has a diameter in the range microns to 380 microns.</claim-text> <claim-text>14. The method of any preceding claims, wherein the aperture on the lower side has a diameter in the range microns to 330 microns.</claim-text> <claim-text>15. The method of any preceding claim, wherein the narrowest width of the apertures is greater than the diameter of the balls by at least 101.</claim-text> <claim-text>16. The method of any preceding claim, wherein the greatest width of the apertures is greater than the diameter of the balls by at least 101.</claim-text> <claim-text>17. The method of any preceding claim, wherein the stencil is formed from a sheet of stainless steel, nickel, alloy 42, brass, Kapton, or plastic.</claim-text> <claim-text>18. The method of any preceding claim, wherein the stencil is formed from a sheet of stainless steel or nickel.</claim-text> <claim-text>19. The method of any preceding claim, wherein the pillars comprise a photo polymer material.</claim-text> <claim-text>20. The method of any preceding claim, wherein the pillars comprise nickel.</claim-text> <claim-text>21. The method of any preceding claim, wherein the pillars comprise dry film photo resist.</claim-text> <claim-text>-13 - 22. The method of any preceding claim,wherein the clearance in the plane of the stencil between each discrete spacing means and its closest aperture is at least 40 microns.</claim-text> <claim-text>23. The method of any preceding claim, wherein the smallest separation between two apertures is at least 40 microns.</claim-text> <claim-text>24. The method according to any of the preceding claims, wherein each discrete spacing means is a pillar.</claim-text> <claim-text>25. The method of any preceding claim, wherein each discrete spacing means has a substantially circular cross-section.</claim-text> <claim-text>26. The method of claim 25, wherein each pillar has a diameter of from 40 microns to 200 microns.</claim-text> <claim-text>27. The method of claim 26, wherein each pillar has a diameter of 100 microns.</claim-text> <claim-text>28. The method of any preceding claim, wherein: the apertures are arranged on the ball placement stencil in an array of repeating units; and in each unit the ratio of the area of the lower surface covered by pillars to the area of the apertures at the lower surface is in the range 0.7 to 1.4.</claim-text> <claim-text>29. The method according to any of the preceding claims wherein, before depositing the balls onto the substrate, flux is applied to a plurality of locations on the substrate so that the balls are then deposited on the flux.</claim-text> <claim-text>-14 - 30. The method according to claim 29, wherein the flux is deposited to form a regular array on the substrate.</claim-text> <claim-text>31. 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.</claim-text> <claim-text>32. The ball placement stencil assembly of claim 31, wherein the plurality of apertures form a regular array across the stencil.</claim-text> <claim-text>33. The ball placement stencil assembly of claim 31 or claim 32, wherein the discrete spacing means are located on the lower side of the stencil between three or more apertures.</claim-text> <claim-text>34. The ball placement stencil assembly of any one of claims 31 to 33, wherein the combined thickness of the ball placement stencil and the discrete spacing means is substantially equal to the diameter of the balls.</claim-text> <claim-text>35. The ball placement stencil assembly of any one of claims 31 to 34, wherein the combined thickness of the ball placement stencil and the discrete spacing means is in the range 100 microns to 350 microns.</claim-text> <claim-text>-15 - 36. The ball placement stencil assembly of claim 35, wherein the combined thickness of the ball placement stencil and the discrete spacing means is in the range microns to 250 microns.</claim-text> <claim-text>37. The ball placement stencil assembly of any one of claims 31 to 36, wherein the thickness of the ball placement stencil is in the range 50 microns to 300 microns.</claim-text> <claim-text>38. The ball placement stencil assembly of any one of claims 31 to 37, wherein the thickness of the ball placement stencil is in the range 100 microns to 200 microns.</claim-text> <claim-text>39. The ball placement stencil assembly of any one of claims 31 to 38, wherein the thickness of the discrete spacing means is in the range 25 microns to 300 microns.</claim-text> <claim-text>40. The ball placement stencil assembly of any one of claims 31 to 39, wherein the thickness of the discrete spacing means is in the range 50 microns to 75 microns.</claim-text> <claim-text>41. The ball placement stencil assembly of any one of claims 31 to 40, wherein the aperture on the upper side has a diameter greater than the diameter of the corresponding aperture on the lower side by up to 15%.</claim-text> <claim-text>42. The ball placement stencil assembly of any one of claims 31 to 41, wherein the aperture on the upper side has a diameter greater than the diameter of the corresponding aperture on the lower side by 5% to 15%.</claim-text> <claim-text>-16 - 43. The ball placement stencil assembly of any one of claims 31 to 42, wherein the aperture on the upper side has a diameter in the range 160 microns to 380 micronS.</claim-text> <claim-text>44. The ball placement stencil assembly of any one of claims 31 to 43, wherein the aperture on the lower side has a diameter in the range 150 microns to 330 microns.</claim-text> <claim-text>45. The ball placement stencil assembly of any one of claims 31 to 44, wherein the narrowest width of the apertures is greater than the diameter of the balls by at least 10%.</claim-text> <claim-text>46. The ball placement stencil assembly of any one of claims 31 to 45, wherein the greatest width of the apertures is greater than the diameter of the balls by at least 10%.</claim-text> <claim-text>47. The ball placement stencil assembly of any one of claims 31 to 46, wherein the stencil is formed from a sheet of stainless steel, nickel, alloy 42, brass, Kapton, or plastic.</claim-text> <claim-text>48. The ball placement stencil assembly of any one of claims 31 to 33, wherein the stencil is formed from a sheet of stainless steel or nickel.</claim-text> <claim-text>49. The ball placement stencil assembly of any one of claims 31 to 48, wherein the pillars comprise a photo polymer material.</claim-text> <claim-text>50. The ball placement stencil assembly of any erie of claims 31 to 49, wherein the pillars comprise nickel.</claim-text> <claim-text>-17 - 51. The ball placement stencil assembly of any one of claims 31 to 50, wherein the pillars comprise dry film photo resist.</claim-text> <claim-text>52. The ball placement stencil assembly of any one of claims 31 to 51, wherein the clearance in the plane of the stencil between each discrete spacing means and its closest aperture is at least 40 microns.</claim-text> <claim-text>53. The ball placement stencil assembly of any one of claims 31 to 52, wherein the smallest separation between two apertures is at least 40 microns.</claim-text> <claim-text>54. The ball placement stencil assembly of any one of claims 31 to 53, wherein each discrete spacing means is a pillar.</claim-text> <claim-text>55. The ball placement stencil assembly of any one of claims 31 to 54, wherein each discrete spacing means has a substantially circular cross-section.</claim-text> <claim-text>56. The ball placement stencil assembly of claim 55, wherein each pillar has a diameter of from 40 microns to microns.</claim-text> <claim-text>57. The ball placement stencil assembly of claim 56 wherein each pillar has a diameter of 100 microns.</claim-text> <claim-text>58. The ball placement stencil assembly of any one of claims 31 to 57, wherein: the apertures are arranged on the ball placement stencil in an array of repeating units; and -18 -in each unit the ratio of the area of the lower surface covered by pillars to the area of the apertures at the lower surface is in the range 0.7 to 1.4.</claim-text> <claim-text>59. The ball placement stencil assembly of any one of claims 31 to 58, before depositing the balls onto the substrate, flux is applied to a plurality of locations on the substrate so that the balls are then deposited on the flux.</claim-text> <claim-text>60. The ball placement stencil assembly of claim 59, wherein the flux is deposited to form a regular array on the substrate. *l9</claim-text>