Laminate and method of manufacture thereof

A fabric material and method of its manufacture suitable for use in electronic packages including chip carriers. High insulation resistance is exhibited when subjected to high temperatures and humidity stress conditions.

TECHNICAL FIELD
 The invention relates to insulative laminate materials for use in
 electronic packages and more particularly to resin and fiber composites
 which exhibit a high insulation resistance when subjected to severe
 moisture, temperature, and pressure conditions.
 BACKGROUND OF THE INVENTION
 Insulative laminate materials are conventionally prepared by impregnating
 non-woven or woven fiber fabrics (typically glass) with thermosetting
 resin compositions such as epoxy or bismaleimide-triazine (BT) resins and
 then drying the fabric. The dried fabric is cut into sheets, referred to
 in the printed circuit industry as prepregs which are then laminated to
 solid or personalized electrically conductive layers (typically copper) in
 various combinations including other previously laminated structures to
 form single or multi layer substrates.
 Holes or vias having electrically conductive walls filled with electrically
 conductive materials may be formed in the substrate to provide electrical
 contact between the conductive layers. Numerous other variations to the
 art of substrate manufacture are well known. The substrates are used to
 hold electronic components including semiconductors, passive components
 other substrates, and miscellaneous components such as connectors,
 switches, or the like and to provide electrical interconnection between
 appropriate contacts on these components.
 The prepreg must exhibit a large number of properties to be used as a
 substrate material. U.S. Pat. No. 5,648,171 for example, lists high
 mechanical and thermal strength, good mechanical and thermal stability,
 thermal undeformabilty, and good aging resistance as requirements. Also
 listed are good adhesion to glass and copper, good machining properties
 (punchability, drillability), a low water uptake and high corrosion
 resistance.
 U.S. Pat. No. 5,376,453 notes that resistance to heat deformation is
 important because the substrate materials are exposed to high temperatures
 during processing. For example, printed circuit boards are exposed to a
 temperature of 270.degree. C. during flow soldering. Also temperatures of
 over 200.degree. C. can occur locally for brief periods during cutting and
 drilling. In U.S. Pat. No. 5,565,267 the electrically conductive layer is
 formed with an electrically conductive ink which for most applications
 would be cured at 200.degree.-230.degree. C. for 30 seconds to 1 hour.
 Materials with a high glass transition temperature (T.sub.G) exhibit
 favorable characteristics because if this glass transition temperature
 lies above the mentioned values, then inherent stability is guaranteed and
 damage such as warping is largely ruled out.
 U.S. Pat. No. 5,368,921 mentions that use of fibers which have been surface
 treated with a silane coupling agent or the like is preferred from the
 standpoint of water resistance. A silane coupling agent is used in U.S.
 Pat. No. 5,483,101 between an electrically conductive layer and a layer of
 resin (benzocyclobutene). U.S. Pat. No. 4,783,345 describes resin
 materials for the manufacture of prepregs which can be stored.
 The use of prepreg materials in the manufacture of chip carriers has become
 more attractive due to their cost advantage over conventional ceramic
 materials. Furthermore the electrically conductive layer (usually copper)
 is more conductive and can be processed more readily to very dense
 geometries than the paste materials typically applied to ceramic carriers.
 Chip carriers made with prepregs require many of the same characteristics
 of the prepregs as other substrates. All of the requirements listed above
 for substrates made with prepregs have been, at least to some extent,
 successfully addressed. However, chip carriers usually have an additional
 requirement for a high insulation resistance when subjected to humidity,
 temperature, and pressure.
 When this insulation resistance requirement is added to the requirements
 above, no satisfactory combination of these properties has previously been
 achieved.
 As a result, chip carriers made of prepregs must be handled in special
 moisture tight containers. Once a container is opened such a chip carrier
 must be soldered to a next level carrier such as a printed circuit board
 within a relatively short period of time. If such a chip carrier is not
 used within the time limit it must be slowly baked to remove moisture and
 again resealed in a moisture tight container. Industry wide specifications
 have been developed to allow component manufacturers to specify these time
 limits and handling procedures. Obviously such procedures are a burden to
 component assemblers because other components, e.g., ceramic, plastic
 encapsulated, passives, connectors, etc. have no such requirement.
 OBJECT OF THE INVENTION
 It is therefore an object of the invention to provide a prepreg material
 which can maintain a high insulation resistance when subjected to
 moisture, temperature, and pressure stresses.
 It is another object to provide such a material for use in chip carrier
 packages wherein no special handling for moisture susceptibility is
 needed.
 It is a further object to provide such a material in a relatively
 inexpensive manner particularly adapted to mass production.
 It is yet a further object to provide a manner for making such a material
 which can be accomplished in a facile manner.
 These and other objects are attained in accordance with one embodiment of
 the invention wherein there is provided a fabric material comprising a
 cloth member having a predetermined maximum thickness and a low percentage
 by weight of particulates as part thereof, and a hardened resin material
 substantially encasing the cloth member, including the particulates so
 that fabric material exhibits a relatively high insulation resistance.
 In accordance with another embodiment of the invention there is provided a
 method of making fabric material exhibiting a high insulation resistance
 comprising the steps of providing a cloth member having a predetermined
 maximum thickness and a low percentage by weight of particulates as part
 thereof, substantially encasing the cloth member, including the
 particulates with a resin material, and at least partially hardening the
 resin material.

BEST MODE FOR CARRYING OUT THE INVENTION
 For a better understanding of the present invention, together with other
 and further objects, advantages and capabilities thereof, reference is
 made to the following disclosure and the appended claims in connection
 with the above-described drawings.
 In FIG. 1 there is shown a magnified cross sectional view of a fabric
 material 10. The fabric material is composed of a woven cloth of strands
 12 and 14 each comprised of pluralities of fibers preferably of fiberglass
 filaments. Strands 12 run parallel to the paper and strands 14 run into
 the paper. The woven cloth has a maximum thickness "T" at positions where
 strands 12 and 14 overlap.
 The woven cloth is known to include a quantity of particulates, which term
 is meant to include dried film, excess coupler, broken filaments, and
 gross surface debris. For example it is normal to apply a sizing of
 polyvinyl alcohol, and corn starch and a lubricant of oil to the strands
 of fiber prior to weaving in order to improve the weaving process and
 minimize breakage of the strands. After weaving the sizing is removed by a
 firing step to clean the filaments of lubricants and other materials.
 However, some sizing is randomly left behind as particulates.
 A silane coupler is normally applied to the woven cloth to improve adhesion
 of resin materials. An excess quantity of coupler if left on the cloth
 will dry forming particulates scattered randomly throughout the cloth.
 Broken filaments can produce tiny pieces of filament material (usually
 glass) particulates. Finally, various airborne and other debris may find
 its way into the woven cloth and attach itself to the strands of
 filaments. The photograph of FIG. 3 shows particulates on the strands of
 woven glass fiber fabric.
 Encasing the woven cloth of FIG. 1 including the particulates which are not
 shown in FIG. 1, is a quantity of hardened resin material 16. (Resin
 material 16 is not shown with cross-sectioning for illustration purposes.)
 The resin may be an epoxy resin such as is currently used on a large scale
 worldwide for FR4 composites. A resin material based on
 bismaleimide-triazine (BT) is also acceptable. More preferably it is a
 phenolically hardenable resin material as is known in the art.
 As mentioned above the resin material should have a glass transition
 temperature TG greater than the expected temperature stresses which the
 fabric material will encounter in subsequent processing and use. Epoxy
 resins typically have TG of 130.degree. C. BT resins are known to have a
 T.sub.G of about 200.degree. C. A phenolically hardened resin material
 with a T.sub.G of about 145.degree. C. or higher is preferable for
 subsequent use in chip carrier manufacturing. As shown in FIG. 1 the
 quantity of resin material is sufficient to completely encase the woven
 cloth and particulates so the thickness of the resulting prepreg fabric
 material is greater than the maximum thickness of "T" of the woven cloth.
 The fabric material of FIG. 1 can be made using a conventional apparatus
 known as a treater tower wherein a cloth member including any particulates
 is substantially encased (usually by passing through a liquid bath) with a
 resin material and then the resin material is at least partially hardened
 (usually by exposure to heated air).
 FIG. 2 shows a fabric material 20 according to another embodiment of the
 present invention. Non woven cloth member 21 is composed of randomly
 arranged fibers 22, preferably glass filaments, and has a maximum
 thickness "T.sub.1 ". There are also particulates mixed with the fibers as
 in the embodiment of FIG. 1 discussed above. A quantity of hardened resin
 26 (not shown with cross-sectioning for illustration purposes) encases the
 fibers, including the particulates and is of sufficient quantity so the
 resulting prepreg fabric is thicker than the maximum thickness of "T.sub.1
 " of the non woven cloth 21.
 The fabric material of the present invention is particularly suitable for
 use in the manufacturing of chip carriers. Such electronic packages are
 particularly sensitive to moisture absorption which can cause damage when
 the chip carrier is subjected to high temperatures, for example during
 solder attachment of the chip carrier to a higher level package. The
 printed circuit industry has developed a set of standards known as
 Highly-Accelerated Temperature and Humidity Stress Test (HAST), a JEDEC
 standard No. 22-A110 and associated standards 22-A112 and 22-A113
 available from the Electronic Industries Association in Washington, D.C.
 to measure and specify this moisture susceptibility. For this test the
 insulation resistance of the fabric material is measured during and after
 exposure to a specified temperature, humidity, and pressure for a
 specified time. An insulation resistance below 1 million ohms is
 considered a failure, indicating a high level moisture absorption.
 Components which do not conform to the highest HAST level known as HAST
 level A may not be acceptable in high reliability chip carrier
 applications making these undesirable for use in high volume
 manufacturing. Currently known fabric materials do not reliably pass the
 HAST level A testing resulting in high yield loss or expensive special
 handling.
 Although all fabric material has some particulates, it has been discovered
 that fabric material made from a cloth member having a low enough content
 of particulates and a sufficient quantity of resin material to completely
 encase the cloth member including the particulates, so that the resin
 material extends beyond the highest protrusions of the cloth member (i.e.
 the fabric material is thicker than T of FIG. 1) will pass the HAST level
 A testing. The level of particulates is measured by examination under a
 microscope at up to 5000 times magnification.
 FIG. 3 is a portion of glass fiber fabric 30 of several glass filaments 32,
 gross surface debris 34, and dried film 36, at a magnification of 2000
 times. In the preferred embodiment, the level of particulates is less than
 0.05% by weight of the cloth member which is much less than found in
 typical cloth members currently used in industry today. Glass fiber woven
 cloth members suitable for use in the present invention, however, are
 available from the Asahi-Schwebel company in Japan as trade name AS-450
 and the Clark-Schwebel company in Anderson, S.C. as trade name CS-4370.
 Thus there has been shown and described a fabric material and method of
 making the fabric material exhibiting a relatively high insulation
 resistance. The fabric material has been shown to be particularly suitable
 for use in chip carrier manufacturing because of its low moisture
 susceptibility. The invention is thus deemed to constitute a significant
 advancement in the art.
 While there have been shown and described what are at present considered
 the preferred embodiments of the invention, it will be obvious to those
 skilled in the art that various changes and modifications may be made
 therein without departing from the scope of the invention as defined by
 the appended claims.