Method and apparatus for screen printing

A screen printing apparatus and method for forming printed wiring on a substrate, such as a wafer. The apparatus includes a reservoir disposed beneath a screen having a plurality of openings forming a pattern thereon. Disposed within the reservoir is a printing paste. The screen is pressed into the printing paste such that convex structures are formed on the upper surface of the screen, and are printable onto the substrate. A wiper is also movably disposed within the reservoir, cooperating with the screen to establish contact between the screen and the substrate.

FIELD OF THE INVENTION
 The present invention relates generally to a method and apparatus for
 screen printing. More particularly, it pertains to printing a screen paste
 through a medium onto a substrate.
 BACKGROUND OF THE INVENTION
 Circuit patterns are produced by a number of manners, including screen
 printing. Typically, in a screen printer a screen having a printing
 pattern, is disposed over an object to be printed. Printing ink is coated
 onto the screen, and then spread by the use of a wiper also disposed over
 a top surface of the screen. The wiper assists in the printing of the
 pattern onto the object to be printed. One example of a conventional
 screen printer is illustrated in U.S. Pat. No. 5,176,076 issued to Azuma
 on Jan. 5, 1993.
 Typical screen printers, however, can result in excessive material costs.
 The printing paste is disposed on the screen for each application and can
 dry out and solidify rapidly due to the repeated working of the paste, as
 well as its exposure to air. In addition, cleaning issues also occur as
 the paste dries on the screen in between uses. Other areas which challenge
 conventional screen printers is uniformity in the thickness of application
 of the printing medium. The printing paste undergoes changes such as
 solidification, oxidation, etc. from being left on the screen when not in
 operation, which results in an undesirable printing effect. Thus, the
 paste must be removed from the screen and replaced with new printing
 paste. Replacement of the paste requires additional material and
 additional labor. Furthermore, solidification of the paste can result in
 variations in thicknesses of the paste application leading to excessive
 use of the paste, thereby further adding to the overall cost of the unit.
 Accordingly, what is needed is a screen printer which overcomes the above
 waste and cleaning problems. What is further needed is a screen printer
 which assists in preventing the drying of printing paste between
 applications. What is also needed is a screen printer which provides a
 uniform application of material.
 SUMMARY OF THE INVENTION
 A screen printing apparatus is disclosed which includes a substrate which
 is adapted to receive a printing substance. The substrate can include
 devices such as a wafer or a printed circuit board. The apparatus further
 includes a printing screen having a top surface and a bottom surface,
 where the top surface is disposed toward the substrate. A retention device
 is also coupled with the substrate, and the retention device retains the
 substrate over the top surface of the printing screen. Further included is
 a reservoir for containing a printing paste, such as solder, where the
 reservoir has a plurality of side surfaces and a bottom surface. The
 reservoir is disposed below the bottom surface of the printing screen.
 In one embodiment, a wiper assembly is associated with the reservoir, where
 the wiper assembly includes a wiper and a reciprocating assembly. In
 another embodiment, the printing screen is flexed toward the substrate,
 and/or the top surface of the printing screen contacts the object to be
 printed. A retention device, in yet another embodiment, retains the
 substrate over the printing screen mechanically or by a vacuum.
 Alternatively, in another embodiment, a movement assembly is provided
 which is coupled with the printing screen. The movement assembly is
 adapted to provide paste on the printing screen, for example, in the form
 of convex bumps, or alternatively, to force paste through the printing
 screen. In another embodiment, a heater is thermally coupled with the
 reservoir.
 A screen printing apparatus includes a substrate which is adapted to
 receive a printing substance. The substrate can include devices such as a
 wafer or a printed circuit board. The apparatus further includes a
 printing screen having a top surface and a bottom surface, where the top
 surface is disposed toward the substrate. A retention device is also
 coupled with the substrate, and the retention device retains the substrate
 over the top surface of the printing screen. Further included is a
 reservoir for containing a printing paste, such as solder, where the
 reservoir has a plurality of side surfaces and a bottom surface. The
 reservoir is disposed below the bottom surface of the printing screen.
 Associated with the reservoir is a wiper assembly, where the wiper
 assembly includes a wiper and a reciprocating assembly. In another
 embodiment, the printing screen is flexed toward the substrate, and/or the
 top surface of the printing screen contacts the object to be printed. A
 retention device, in yet another embodiment, retains the substrate over
 the printing screen mechanically or by a vacuum.
 A further embodiment includes a method of screen printing onto a surface of
 a substrate. The method comprises disposing a bottom surface printing
 screen within a reservoir of a printing medium, contacting a top surface
 of the printing screen with a substrate; and translating a wiping
 mechanism over the printing screen, causing the printing medium to be
 disposed on the substrate. In another embodiment, the method further
 includes flexing the printing screen toward the substrate with the wiping
 mechanism.
 The printing apparatus advantageously aids in producing consistent bump
 heights, particularly when trying to obtain a bump higher than the width
 of the bump. The viscosity of the printing paste is also better
 controlled, in part, due to the use of the heater. In addition, by
 containing the print medium in a reservoir below the screen, automation of
 the screen printing operation is much easier since it is not necessary to
 dispense print medium onto the screen between applications.
 The printing apparatus provides a combination of pressure squeegee forces
 that is not currently available in standard screen-print equipment. This
 adds another control of the print medium, which would be useful to
 dispense accurate and consistent solder paste height and width
 characteristics. By utilizing the additional pressure control, it is
 possible to successfully use print medium with higher variations in
 viscosity and thixotropic index.
 The screen printing apparatus also dispenses a predetermined amount of
 printing paste having a predetermined temperature onto the printing screen
 in each printing cycle. The apparatus assists in preventing the printing
 paste from being solidified and maintains a desired degree of viscosity.
 In addition, the apparatus aids in preventing the printing paste from
 being excessively applied onto the surface of the screen, which would
 otherwise result in uneven printing. Furthermore, the below-described
 printing apparatus minimizes the amount of printing paste which is wasted.
 These and other embodiments, aspects, advantages, and features of the
 present invention will be set forth in part in the description which
 follows, and in part will become apparent to those skilled in the art by
 reference to the following description of the invention and referenced
 drawings or by practice of the invention. The aspects, advantages, and
 features of the invention are realized and attained by means of the
 instrumentalities, procedures, and combinations particularly pointed out
 in the appended claims and their equivalents.

DESCRIPTION OF THE EMBODIMENTS
 In the following detailed description, reference is made to the
 accompanying drawings which form a part hereof, and in which is shown by
 way of illustration specific embodiments in which the invention may be
 practiced. These embodiments are described in sufficient detail to enable
 those skilled in the art to practice the invention, and it is to be
 understood that other embodiments may be utilized and that structural
 changes may be made without departing from the scope of the present
 invention. Therefore, the following detailed description is not to be
 taken in a limiting sense, and the scope of the present invention is
 defined by the appended claims and their equivalents.
 A screen printing apparatus is illustrated in FIGS. 1-3. A screen printer
 130 is provided which generally includes a reservoir 132, a printing
 screen 144, and a substrate 170. The screen printer 130 allows for a
 printing medium, such as a printing paste, to be printed onto the
 substrate 170 in a predetermined pattern, as further discussed below.
 The reservoir 132 comprises a container for containing a printing paste
 142, therein, and includes side surfaces 133 and a bottom surface 134. In
 one embodiment, the reservoir 132 includes a heater 138 coupled therewith.
 The heater 138 is thermally coupled with the reservoir 132 and is adapted
 to control the temperature of the printing paste container therein.
 Controlling the temperature of the printing paste advantageously allows
 for better control of the height and width of the distribution of paste.
 The printing screen 144 comprises a template which contains a plurality of
 apertures 145 therein. The apertures 145 are disposed within the printing
 screen 144 such that they form a pattern thereon. Other shapes can be
 formed alternative to or in conjunction with the apertures 145 to achieve
 a multitude of patterns. The printing screen 144 is defined by an upper
 surface 146 and a lower surface 148.
 Coupled with the printing screen 144, in one embodiment, is a movement
 assembly 152. The movement assembly 152 is operatively coupled with the
 printing screen 144 and is adapted to translate the movement assembly 152
 upwards and downwards relative to the reservoir 132. The movement assembly
 152 comprises, in one embodiment, a servo motor. Alternatively, other
 devices for translating the printing screen 144 can also be used.
 The movement assembly 152, in another embodiment, controls the velocity and
 pressure of the printing screen 144 as it approaches the printing paste
 142 of the reservoir 132. By controlling these variables, as well as
 others, the amount, height, and width of the printing paste 142 disposed
 on the printing screen 144 can also be controlled. In yet another
 embodiment, the movement assembly 152 transfers the print screen 144 into
 the printing paste 142 such that convex paste bumps 154 are formed on the
 upper surface 146 of the printing screen 144, as shown in FIGS. 2 and 3.
 The movement assembly 152 controls the force of the printing screen 144 as
 it is being disposed within the paste 142, thereby controlling the rate of
 paste passed through the apertures 145 of the printing screen 144.
 As mentioned above, the substrate 170 provides a surface on which printing
 occurs. The substrate 170 is defined by an upper surface 172 and a lower
 surface 174, and in one embodiment, includes at least one pad 176 on the
 lower surface 174. The substrate 170 can include a wafer, a printed
 circuit board, or other alternatives. The substrate 170 is retained over
 the upper surface 146 of the printing screen 144, where the lower surface
 174 of the substrate 170 is disposed toward the upper surface 146 of the
 printing screen 144 in preparation for the printing process. In one
 embodiment, the substrate 170 is retained over the printing screen 144 by
 a retention device 150. The retention device 150 comprises, in one
 embodiment, a vacuum. In another embodiment, the retention device 150
 comprises a mechanical fastener which couples with a portion of the
 substrate 170. The retention device 150 allows for the printing process to
 be easily automated, as well as reducing time and cost of the printing
 process.
 During use, the substrate 170 travels to the screen printer 130, and
 arrives at a position above the printing screen 144, as illustrated in
 FIG. 1. In one embodiment, the movement assembly 152 transfers the
 printing screen 144 within the printing paste 142, thereby acquiring
 printing paste 142 on the printing screen 144. In one embodiment, the
 printing screen 144 is disposed within the printing paste 142, forcing
 printing paste 142 through the apertures 145 to form convex paste bumps
 154 thereon. The printing screen 144 is transferred from the printing
 paste 142 proximate to the substrate 170, until the printing paste 142
 contacts and adheres to the lower surface 174 of the substrate 170, as
 illustrated in FIG. 3. Alternatively, the upper surface 146 of the
 printing screen 144 contacts the lower surface 174 of the substrate 170 to
 cause the printing of the printing paste 142 on the substrate 170. Unused
 paste 142 falls back to the reservoir 132 to be reused before it dries
 out.
 In another embodiment, as shown in FIG. 4, a screen printer apparatus 230
 is provided. The apparatus 230 includes similar components as those
 discussed above. In addition, the screen printer 130 includes a wiper
 assembly 160. The wiper assembly 160, in one embodiment, is coupled with
 the reservoir 132.
 The wiper assembly 160, in one embodiment, includes a wiper 162 and a
 reciprocating assembly 164 operatively coupled with the wiper 162. The
 reciprocating assembly 164 provides a mechanism to transfer the wiper 162
 across the printing screen 144 and controls the velocity and pressure of
 the wiper 162, in one embodiment, using a servo motor, a hydraulic ram, or
 other alternatives. In one embodiment, the wiper 162 comprises an elongate
 structure which operates as a squeegee against the lower surface 148 of
 the printing screen 144. As the wiper 162 is forced over the lower surface
 148 of the printing screen 144, printing paste 142 is forced through the
 apertures 145, as shown in FIG. 5.
 During use, the substrate 170 travels to the screen printer 130, and
 arrives at a position above the printing screen 144. The wiper assembly
 160 actuates the wiper 162, and the wiper 162 is moved while kept in
 contact with the screen under a predetermined pressure and velocity. When
 the wiper 162 is moved across the printing screen 144, a printing paste
 142 is forced through the pattern of apertures 145 formed on the printing
 screen 144, resulting in a printing paste 142 being disposed through the
 printing screen 144 to the upper surface 146 of the printing screen 144
 (see FIG. 4). The substrate 170 is brought into contact with the printing
 paste 142 of the printing screen 144 to thereby form the pattern of
 printing paste 142 on the lower surface 174 of the substrate 170, as
 illustrated in FIG. 5.
 In one embodiment, the printing screen 144 contacts the lower surface 174
 of the substrate 170. This is achieved by raising the printing screen 144
 using a movement assembly 152 for the printing screen 144, as discussed
 above for FIGS. 1-3. Alternatively, in another embodiment, the printing
 screen 144 is brought into contact with the substrate 170 using the wiper
 assembly 160. For this embodiment, as shown in FIG. 6, the wiper 162 is
 forced against the lower surface 148 of the printing screen 144 and the
 printing screen 144 flexes toward the substrate 170. In yet another
 embodiment, the substrate 170 is lowered toward the printing screen 144 as
 the printing screen 144 is raised up from the reservoir 132. It should be
 noted that the above discussed embodiments can be combined to achieve the
 printing on the substrate 170.
 The screen printing apparatus provides several advantages in that a cleaner
 and more controllable printing process is achieved. Since the pressure and
 velocity of the printing screen movement is controlled, more consistent
 bump heights are achieved, particularly when trying to obtain a bump
 higher than the width of the bump. By utilizing the additional pressure
 control, it is possible to successfully use print medium with higher
 variations in viscosity and thixotropic index. In addition, since the
 printing paste is contained in the reservoir, it is also possible to use a
 material with higher viscosity. The heater coupled with the reservoir also
 allows for control of the viscosity, and the reservoir assists in
 preventing rapid drying and unnecessary waste of the printing paste.
 It is to be understood that the above description is intended to be
 illustrative, and not restrictive. Many other embodiments will be apparent
 to those of skill in the art upon reading and understanding the above
 description. The scope of the invention should, therefore, be determined
 with reference to the appended claims, along with the full scope of
 equivalents to which such claims are entitled.