Abstract:
A laser beam ( 102 ) cuts through a component carrier mask ( 96 ) made of thin elastomeric material such as silicone rubber to form slots ( 98 ) having slot openings of a desired shape. In a preferred embodiment, a light absorptivity enhancement material such as iron oxide introduced into the silicone rubber causes formation of a flexible support blank that operationally adequately absorbs light within a light absorption wavelength range. A beam positioner ( 106 ) receiving commands from a programmed controller causes a UV laser beam of a wavelength that is within the light absorption wavelength range to cut into the mask multiple slots with repeatable, precise dimensions. Each of the slots cut has opposed side margins that define between them a slot opening of suitable shape to receive a miniature component ( 10 ) and to exert on it optimal holding and release forces.

Description:
RELATED APPLICATIONS 
     This patent application derives priority from U.S. Provisional Application No. 60/416,311 filed Oct. 4, 2002. 
    
    
     COPYRIGHT NOTICE 
     © 2003 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR § 1.71(d). 
     TECHNICAL FIELD 
     This invention relates to carriers for miniature components and, in particular, to a laser-based method of forming in a miniature component carrier dimensionally precise slots shaped to grip a miniature component and hold it in a controlled orientation. 
     BACKGROUND OF THE INVENTION 
     Computers and other electronic equipment are becoming more powerful and can perform a wider range of tasks. To prevent growth in the sizes of the computers and other electronic equipment and operate them at higher speeds, the electronic circuits use miniature electronic components in high density packing arrangements. One such miniature electronic component, a solid state capacitor, is a tiny rectangular “chip” that is smaller than a grain of rice.  FIG. 1  shows a capacitor chip  10  that has a solid enclosed body  12  of square or rectangular cross section and made of ceramic or other dielectric material. Capacitor chip  10  contains within body  12  multiple spaced-apart metal plates (not shown). One terminal end of each of alternate metal plates is connected to the exterior of body  12  and is adapted by a metallizing process to form a pair of spaced-apart mutually opposed electronic contact surfaces or ends  14 . One or more of the contact surfaces  14  of chip capacitor  10  are striped with a solderable paste that is dried and then fired to produce surfaces that later can be soldered directly onto a circuit board. U.S. Pat. No. 5,226,382 describes a machine for placing a stripe or trace of solderable paste on surfaces of a chip and drying the paste so that the paste can later be fired. This machine uses a metal carrier belt or tape in which slotted rubber masks are formed. The slots in the masks receive chips in position for processing, such as covering opposed ends of the chips with solderable paste. 
     A relatively new electronic circuit chip is composed of multiple circuit components fit into a single array chip that is simultaneously solderable to one of a number of different electronic circuits. This device is called an Integrated Passive Component (IPC) or array chip because it comprises a plurality or array of circuit components, such as four or five separate capacitors stacked together in a single chip. 
       FIG. 2A  shows a typical IPC or array chip  20  with its side wall surfaces covered with stripes  22  of solderable paste. Array chip  20  has overall dimensions such as 3.2 mm (0.125 in) long and 1.5 mm (0.060 in) wide top and bottom surfaces  24 , 1.5 mm (0.060 in) wide and 1.0 mm (0.040 in) high opposed end surfaces  26 , and 1.0 mm (0.040 in) high and 3.2 mm (0.125 in) long opposed side surfaces  28 .  FIG. 2B  shows that installing array chip  20  into an electronic circuit entails placing separate solderable paste stripes  22  along opposite wall surfaces, such as side surfaces  28  (as shown) or end surfaces  26  (not shown), and soldering paste stripes  22  to copper traces  30  formed on a circuit board  32 . The width of each stripe  22  is typically set at 0.38±0.18 mm (0.015±0.007 in), with a 0.3±0.18 mm (0.012±0.007 in) turn-down edge at the end of each stripe along the adjacent wall as shown on top and bottom surfaces  24  in FIG.  2 A. As with other chip components, after the paste is applied, it is subjected to a heat-drying cycle to set the paste and thereafter to a firing cycle to fuse the paste on array chip  20 . 
     The small size of a chip and the small differences between its width and height dimensions raise the importance of handling the chip and its insertion into the mask of a carrier belt or tape. The multiple stripes are placed on only the appropriate circuit board surfaces, and their placement is accomplished with extreme accuracy. Splashing of the paste onto other surfaces of the chip would provide a site for a short circuit and thereby significantly degrade electronic equipment function. Accordingly, a feed device places the chip onto the carrier tape in a correct position and location, and the chip is handled correctly so that the appropriate surface is exposed in proper orientation to receive the stripes of paste within a specified accuracy. 
     There are two principal types of miniature component carriers that transport the components and present them for processing. A first type of carrier is an endless belt or tape that is typically used to carry single component chips, such as capacitor chips  10 , which are larger than array chips. The endless tape is formed with a plurality of transversely oriented elongated apertures arranged centrally between and uniformly spaced apart along the marginal edges of the tape. Each of the apertures is adapted to receive in coplanar fixed registration a thin, resilient mask having at least one orifice and preferably a series of orifices of sizes and shapes to compliantly receive the chips in specific orientation so that their end surfaces intended for termination extend outwardly from the masks. 
     A second type of carrier is an endless belt that is typically used to carry array chips such as array chips  20 . The belt has a core typically made of stainless steel with multiple apertures spaced apart along the belt length. A thin elastomeric material, such as silicone rubber, is molded over the stainless steel core to form a resilient mask. A slot is formed during the molding process in the resilient mask at locations where the over-molded elastomeric material covers the apertures. 
     Silicone rubber is difficult to aver mold onto the belt because silicone rubber flows well through small cracks in the mold. For this reason, slot openings with precise dimensions are difficult to form. The array chip component is held in the slot under compression by an interference fit. A 0.05 mm (0.002 in) desired interference fit nominally requires a ±0.025 mm (±0.001 in) slot opening tolerance range. For example, a 5.1 mm (0.20 in) thick array chip component typically requires a 0.43-0.48 mm (0.017-0.019 in) slot opening. A less than a −0.025 mm (−0.001 in) slot opening width tolerance results in a slot that is too tight, causing the silicone rubber nubs of the slot opening to deflect (rather than compress) and thereby cant the array chip component held in the slot. A slot opening width of greater than 0.025 mm (0.001 in) lets the component fall out of the belt. 
     The use of precision moldings typically provides a 25 percent initial yield in dimensionally accurate slot openings, and the belt needs to be reworked to increase the yield to a 65-80 percent nominal yield benchmark. Yield represents the number of chip components that remain in their associated slot openings during processing. Variations in the thickness dimension of the chip components also contribute to the relatively low yield achieved with the 0.05 mm (0.002 in) interference fit. 
     What is needed, therefore, is a method of accurately forming with high initial yields component slots in a miniature component carrier belt to precise dimensions and close tolerances. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a technique for cutting into a resilient mask of a miniature component carrier multiple slots of repeatable, precise dimensions and of shapes that compliantly grip miniature components and hold them in a controlled orientation. 
     The invention is preferably implemented with an ultraviolet (UV) laser beam that is directed to cut through a component carrier mask made of thin elastomeric material to form slots having slot openings of a desired shape. Silicone rubber is a preferred conventional elastomeric material used in the production of miniature component carriers. The formation of slots in the carrier belts is currently accomplished by injection molding techniques. A preferred embodiment of the present invention uses a UV laser beam to form by UV ablation a slotted resilient mask made of elastomeric material. UV ablation of the elastomeric material, which is preferably silicone rubber, ensures formation of slots of the required shape and dimensional quality. The absorption of conventional elastomeric materials, including silicone rubber, at the UV laser ablation wavelength region (shorter than about 400 nm) is insufficiently strong to cut slots at commercially acceptable throughput rates. To overcome this drawback, the method entails introducing a light absorptivity enhancement material into the silicone rubber to form a flexible support blank that operationally adequately absorbs light within a laser ablation wavelength region and using a UV laser beam to cut the slots. Iron oxide or titanium dioxide is a preferred dye dopant functioning as a light absorptivity enhancement material. 
     A beam positioner receives commands from a controller programmed to cause the laser beam to cut into the mask multiple slots with repeatable, precise dimensions. Each of the slots cut has opposed side margins that define between them a slot opening of suitable shape to receive a miniature component and to exert on it optimal holding and release forces. Slot openings of different shapes are used to accommodate miniature components having different configurations. A programmable controller can improve yield by processing slot opening dimensions customized to accept particular lots of chip components having known dimensions. Tailoring the slot openings to specific chip component sizes increases the likelihood of compliance with the tight interference fit dimension tolerance range. 
     A laser emitting light of wavelengths shorter than 550 nanometers and preferably light of ultraviolet (UV) wavelengths, is a preferred source of light emissions for constructing a slotted component carrier made of silicon rubber doped with iron oxide or titanium dioxide. 
     Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings. 
     Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an enlarged isometric pictorial view of a capacitor chip. 
         FIG. 2A  is an enlarged isometric pictorial view of a typical integrated passive component or array chip coated with solderable paste. 
         FIG. 2B  is an enlarged isometric pictorial view of the array chip of  FIG. 2A  mounted on a surface of a circuit board. 
         FIG. 3  is a fragmentary top plan view of a component carrier tape in which a series of apertures carries masks that hold chip components. 
         FIG. 4  is a fragmentary plan view showing a carrier tape having a variety of different apertures. 
         FIG. 5  is a fragmentary isometric view of a carrier tape carrying masks that are formed over the apertures. 
         FIG. 6  is a sectional view of the carrier tape and masks taken along lines  6 — 6  in FIG.  5 . 
         FIG. 7  is a fragmentary isometric view of an exemplary pattern of apertures formed through the masks of  FIG. 6  for carrying chip components. 
         FIG. 8A  is a plan view of an alternative type of carrier tape to that of the carrier tape of  FIG. 3 , and  FIG. 8B  is an enlarged fragmentary view of the component holding apertures in the mask strips of the carrier tape of FIG.  8 A. 
         FIG. 9A  is a plan view of an over-mold or over-coat type of carrier tape, and  FIG. 9B  is an enlarged sectional view taken along lines  9 B— 9 B of FIG.  9 A. 
         FIGS. 10A and 10B  are, respectively, a simplified pictorial diagram of a laser-based system shown cutting a slot in a carrier tape and a pictorial view of an endless carrier tape routed around two spaced-apart sets of stacked spools and made in accordance with the present invention. 
         FIGS. 11 and 12  show as a function of wavelength the optical transmission curves for, respectively, silicone rubber and silicone rubber doped with iron oxide. 
         FIGS. 13A and 13B , are, respectively, a diagram of a carrier tape in which slots of a dog bone or bow tie shape are cut and pictorial view of portions of the carrier tape and its slots. 
         FIG. 14A  shows a carrier tape in which slots of a sawtooth shape are cut, and  FIG. 14B  is an enlarged view of one of the slots in the carrier tape of FIG.  14 A. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 3  shows an endless component carrier in the form of a flexible metal tape of stainless steel or other high strength metal that is approximately 0.13 mm (0.005 in) thick and about 5.1 cm (2.0 in) wide. Tape  50  is of an “endless” variety in that it has no beginning or end but is maneuvered about a series of pulleys and sprocket wheels between various processing stations such as described in U.S. Pat. No. 5,226,382. Tape  50  is defined by spaced-apart mutually parallel side margins  52  and  54  and includes a series of pilot or sprocket holes  56  that serve as drive perforations to receive drive stubs of drive sprocket wheels (not shown). Sprocket holes  56  are disposed adjacent at least one and preferably both of side margins  52  and  54  and are uniformly spaced along the length of tape  50 .  FIG. 4  shows tape  50  formed with a variety of apertures of different shapes into which a mask can be inserted. Multiple first apertures  60  formed in discrete patterns are spaced uniformly along the length of tape  50 , preferably positioned intermediate of side margins  52  and  54 . Apertures  60  may be a series of closely spaced round holes as shown in  FIG. 3 , a series of elongated rectangular openings as shown in the end portions of  FIG. 4 , or a series of elongated openings in repeated patterns in a side-by-side arrangement as shown in the center portion of FIG.  4 . In a configuration of other than round holes, apertures  60  are generally defined by a pair of spaced-apart elongated side edges  62  terminated by a pair of short-end edges  64 . Each of apertures  60  receives a mask  66  that is of a size and shape to remain fixed to tape  50  and carry multiple chip components. “Mask” is the term used in the art to define an element made of silicone rubber or other resilient material that surrounds and partly encloses a chip component during some stage of its fabrication process. The purpose of mask  66  is to provide a generally elongated resilient-walled holder in which a chip component may be temporarily held during the process of metallizing its opposite ends. An example of a chip capacitor of a type amenable to transport by tape  50  is shown in FIG.  1 . 
       FIG. 7  shows that mask  66  is defined by a pair of spaced-apart top and bottom exterior surfaces  70  and  72  that, when mask  66  is fixed in place on tape  50 , lie, respectively, above and below and coplanar with the surfaces at tape  50 . In its simplest form, shown in  FIG. 3 , each mask  66  is cast in place about an aperture  60  so that a plurality of masks  66  may be arranged in a pattern parallel or transverse to the longitudinal axis of tape  50 . One or more second apertures  74  of a size smaller than that of first aperture  60  are formed in each mask  66  to keep the metal core of tape  50  out of contact with the chip component. The size of second apertures  74  is slightly smaller than that of the chip component in at least one direction so that the chip component can be positionally accepted and resistively grasped during advancement of the chip component from one processing stage to another. Mask  66  is defined, in addition to top and bottom surfaces  70  and  72 , by a pair of opposed elongated slots  76  positioned intermediate of top and bottom surfaces  70  and  72  for receipt of first aperture elongated side edges  62  formed in tape  50 . The length of a removable mask  66  is less than the width of tape  50  and is preferably less than the distance between adjacent sprocket holes  56 . 
       FIGS. 8A and 8B  are respective plan and enlarged fragmentary views of an alternative component carrier tape  50 ′ that is similar to tape  50  with the exception that silicone rubber mask strips  66 ′ are molded into or coated over apertures  60 ′ of generally rectangular shape with curved ends in a core portion. Apertures  74 ′ are formed in a single row in each mask strip  66 ′ along the width of carrier tape  50 ′. 
       FIGS. 9A and 9B  are respective plan and enlarged sectional views of a component carrier tape  90  of an over-mold or over-coat type in which a core portion  92  having multiple apertures  94  spaced apart along the tape length is covered by a support blank of thin elastomeric material to form a mask  96 . A single slot  98  is cut in mask  96  at each location where the elastomeric material covers an aperture  94 . 
       FIG. 10A  is a simplified pictorial diagram of a laser-based system  100  showing by way of example the cutting of a slot  98  in a carrier tape  90  in accordance with the present invention. Light energy propagating in the form of a shaped beam  102  from a laser  104  is incident on and controllably guided by a beam positioner  106  about the surface of mask  96  at a location of an aperture  94  to cut a slot  98  of a desired shape with precise dimensions. Beam positioner  106  guides beam  102  in response to signals produced by a programmable controller  108 , such as the controller installed as part of a Model 5320 Via Drilling System sold by Electro Scientific Industries of Portland, Oreg., the assignee of this patent application. A tape feed mechanism (not shown) moves carrier tape  90  to align its apertures  94  with beam  102  to cut slots  98 . Carrier tape  90  is preferably in the form of an endless tape shown in  FIG. 10B , and a suitable tape feed mechanism is the tape feed mechanism installed as part of a Model 750 Belt Termination System, also sold by Electro Scientific Industries. The cutting of slots  98  in mask  96  is achieved by providing a mask  96  having light absorption wavelength range with which the wavelengths of laser beam  102  operationally overlap. 
     The support blank forming mask  96  includes a composition of elastomeric material and a light absorptivity enhancement material. The elastomeric material imparts elastic properties to the support blank to make it flexible. Because the elastomeric material, such as silicone rubber, operationally inadequately absorbs light energy to cut a slot, the light absorptivity enhancement material, such as iron oxide or titanium dioxide, imparts light absorptivity properties to the support blank to make it operationally adequately absorb light energy included within a light absorption wavelength range to cut the slots but not to change the elastic properties imparted by the elastomeric material. The light emission wavelengths of beam  102  are preferably shorter than 550 nm, and 266 nm is a preferred wavelength for cutting slots in a mask  96  formed of silicone rubber doped with iron oxide. 
     In a preferred embodiment, the support blank forming mask  96  is prepared from a liquid formulation of silicone rubber (99 percent by weight) and iron oxide (1 percent by weight) to provide silicone rubber with a brown color without changing its elastic properties.  FIGS. 11 and 12  show as a function of wavelength the optical transmission (absorptivity) curves for 0.356 mm (0.014 in) thick samples of, respectively, silicone rubber and silicone rubber doped with iron oxide (1 percent weight).  FIG. 11  shows for undoped silicone rubber about 3 percent and about 17 percent light transmission at 266 nm and 550 nm, respectively.  FIG. 12  shows for silicone rubber doped with iron oxide about 0.01 percent light transmission at 266 nm and 550 nm. The 3 percent light transmission by the undoped silicone rubber is sufficient to render operationally inadequate light absorption to cut slots in a silicone rubber support blank. 
     As indicated in  FIGS. 10A and 10B , carrier tape  90  formed with a rubber surface without surface features is placed on processing equipment of laser-based system  100 . Laser beam  102  of sufficient energy cuts into mask  96  slots or pockets  98  into the rubber at the locations of apertures  94 . Programmable controller  108  of system  100  is implemented with software to control the geometry of the slots, the shapes and sizes of which are determined by the end use application (i.e., sizes and shapes of chip components). 
     The power and wavelength of laser beam  102  are controlled to keep the heat generated during cutting below the point of damage to the silicone rubber. An example of laser pulse parameters is a 355 nm UV laser operating with 2.85 watt emissions at a 15 KHz pulse repetition rate. Cutting into a 0.356 mm (0.014 in) thick carrier tape  90  a rectangular bar of 3.05 mm (0.12 in)×0.711 mm (0.028 in) dimensions at a 75 mm/sec feed speed and a 5 μm laser beam bite size takes 35 pulse repetitions and 3.51 seconds to complete. 
       FIGS. 13A and 13B  and  FIGS. 14A and 14B  show tapes having slots of different geometries.  FIGS. 13A and 13B  show a tape  90   1  in which each of tapered slots  98   1  is of a “dog bone” or “bow tie” shape. Tapered slots  98   1  are produced by cuts that form in tape  90   1  opposed slot side margins  110  separated along their lengths by longer slot distances at opposite ends  112  and a shorter distance at a medial location  114  between ends  112 . In a preferred embodiment, the slot distances become gradually smaller from opposite ends  112  to medial location  114 .  FIGS. 14A and 14B  show a tape  90   2  in which each of slots  98   2  is of a “sawtooth” shape. Slots  98   2  are produced by cuts that form in tape  90   2  opposed slot side margins  116  separated along their lengths by alternating shorter and longer distances that form corresponding alternating narrower and wider openings. In a preferred embodiment, the narrower openings are in the general form of concave tapered segments  118  and the wider openings are in the general form of parallel straight line segments  120 . 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.