Patent Application
20080148561
The present invention is directed to printed wiring boards. More particularly, the present invention is directed to methods for making multilayer, printed circuit boards.
Some conventional printed circuit boards are produced by a process that includes direct plate metallization. Blind vias improperly formed during the process sometimes cause electrical opens in the boards.
The geometry of laser-drilled vias in glass-reinforced epoxy circuit board materials such as FR-4 is not compatible with direct plate metallization. The advance of copper into the via during the metallization process is disrupted or blocked by glass fiber stubs in the barrel and/or copper foil overhang at the rim. The stubs result from incomplete laser ablation of the woven glass fabric in the laminate, since the epoxy resin ablates more readily. The overhang results from excessive epoxy ablation where glass content is low and the laser power and dwell time are thus too high. This defect is known as “barreling.”
Both of these problems are exacerbated by the desmear process typically performed after drilling and before plating, which attacks resin but not glass. Thus, the desmear process enhances the barreling and exposes more glass. The resulting vias are irregular and varied, and a uniform copper advance into the via is prevented. Accordingly, thin plating, plating folds, irregular via profiles, and/or field failures occur.
One conventional process for making printed circuit boards is described in U.S. Pat. No. 5,263,243. The disclosed process includes drilling prepreg prior to lamination. The process further includes conductive paste plugs for vertical interconnects. Such conductive paste plug processes are expensive and present their own reliability problems.
It may be desirable to provide a method for making multilayer, printed wiring boards including a low-cost and simple process for shaping blind vias to facilitate uniform direct plate metallization. It may be desirable to provide a method that accounts for some degree of mis-registration during the production process and/or prevents thin plating, plating folds, irregular via profiles, and/or field failures.
According to various aspects of the disclosure, a method of making a multilayer, printed wiring board may include pre-drilling a prepreg sheet to form a hole free of glass fibers. The hole has a first diameter. The method includes laminating the pre-drilled prepreg sheet between a copper foil and a substrate and allowing the hole to fill with resin from the prepreg sheet, patterning the copper foil to create an opening in a location associated with the resin-filled hole, drilling a via hole in the resin-filled hole, and metallizing the copper foil and the via hole. The via hole has a second diameter smaller than the first diameter.
In accordance with some aspects of the disclosure, a method of making a multilayer, printed wiring board may include removing glass fibers from a region of a prepreg sheet to form a region free of glass fibers. The region has a first diameter. The method includes laminating the prepreg sheet between a copper foil and a substrate and allowing the region to fill with resin from the prepreg sheet, patterning the copper foil to create an opening in a location associated with the resin-filled hole, drilling a via hole in the resin-filled region, and metallizing the copper foil and the via holes. The via hole has a second diameter smaller than the first diameter.
An exemplary method of making a printed wiring board is described with respect to the illustrations of
According to various aspects of the disclosure, a prepreg sheet 102 is prepared and sandwiched between an entry material 104 and a backer material 106 for drilling, as shown in
The prepreg sheet 102 is drilled with a drill bit 108 configured to form a bore 110 (
It should be appreciated that more than one through hole may be drilled in the prepreg sheet 102. The through hole(s) may be bored in prescribed positions of the prepreg sheet 102 (positions corresponding to the positions where connecting pads (not shown) of a substrate 43 are formed)
The drill speeds for drilling the B-stage prepreg sheet 102 differ significantly from those typically used to drill conventional C-stage printed circuit board laminate. The drill speeds typically used with C-stage laminate would melt the prepreg sheet 102, fouling the bit and preventing consistent drilling. The plunge and retract rates of the drill bit should be set at a maximum and plunge depth at a minimum to reduce the bit-to-prepreg contact time. Cut-back drill bits may also be used to reduce the bit-to-prepreg contact area.
It should be appreciated that the drill bit can be changed frequently to reduce the number of hits per bit. The appearance of the bit can be observed to determine if the bit becomes coated with resin, and the quality of the drilled holes can be observed so that the user can change the bit when appropriate. The used drill bits can be cleaned and reused.
The entry material 104 and backer material 106 are removed, and the drilled prepreg sheet 102 is sandwiched between a copper foil 116 and a substrate 118. According to various aspects, the substrate 118 may comprise a single sheet or a multilayer stack of laminated sheets. The sheet or stack may comprise, for example, one or more C-stage laminates of the same resin type as the prepreg sheet 102 or a different resin-type, and one or more copper foils or patterned copper foils.
Referring now to
According to various aspects, the substrate 118, the prepreg sheet 102, and the copper foil 116 may be integrally bonded by heating the resulting structure at a temperature of about 150° to about 250° C. when a resin such as epoxy, polyimide, phenol, or the like is used or at a temperature of about 400° C. when Teflon is used for the resin, while pressurizing the structure to a pressure of about 5 to about 40 kg/cm2, using a heat-pressure device. A heat-pressure device such as a hydraulic pressing machine, a hydraulic vacuum pressing machine or autoclave or the like may be used.
Next, as shown in
As shown in
As is known in the art, the necessary power of a laser used for drilling through resin is less than that of a laser used for drilling through glass. Since the laser drills through resin only, the wavelength of the laser may be adjusted to a setting ideal for drilling through resin without the need to drill through glass fibers, thus providing a well-defined and consistent via hole 126.
Referring now to
Thereafter, the metal plating layer 130, the copper foil 116, and the surface layer 122 of the substrate 118 may be patterned to thereby result in a completed multilayer printed wiring board or a sub-assembly thereof. The exemplary method of the disclosure may include a desmear process for removing resin from the copper surfaces before the metallization process.
It should be appreciated that the process of the disclosure may be used to produce any desired number of via holes in a prepreg sheet. The prepreg sheet may be combined with other laminates or substrate stacks to produce any desired printed circuit board configuration.
It will be apparent to those skilled in the art that various modifications and variations can be made in the devices and methods of the present disclosure without departing from the scope of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only.
