Patent Application
20070295231
This invention generally relates to a screen printing apparatus for fabrication of substrates, circuit boards and other electronic circuit components, and more particularly relates to a method and apparatus for cleaning the mask, or stencil, utilized in such screen printing arrangements.
The complexity and compactness of present day electronic products requires increased packing density of various conductive circuit configurations on the surface of substrates, circuit boards, and other components. One method of accomplishing this is by a contact printing apparatus such as a 265 Infinity printer apparatus commercially available from DEK International of Flemington, N.J. Such machines print the desired pattern by depositing a conductive paste, such as solder paste, through a metal stencil directly onto the surface of the component. However, the precision of such printed patterns is often compromised by paste material and other debris accumulating on the stencil surface that contacts the surface of the component being printed.
At present, some such commercially available screen printers include various arrangements for facilitating the cleaning of the printing surface of the metal stencil after each printing step. For example, after a printing operation, one prior art printer automatically draws a paper strip across the printing surface of the stencil to wipe its operational surface. To enhance the cleaning of the stencil, selected chemical solvents are often disposed on to the paper strip just prior to, or during its engagement with the stencil surface. A subsequent additional step applies a vacuum through the cleaning paper to draw particles from the stencil to the paper as the paper wipes the stencil surface for a second time. Although these cleaning steps do remove some or even most of the paste residue from the printing face of the stencil they leave slight amounts of residue or debris on its printing surface of the stencil. After a number of substrates are printed it has been found such residue builds up and hardens on the printing surface of the stencil such that errors result in the pattern being printed resulting in a high substrate defect rate. To reduce this defect rate it is necessary, after printing twenty or so substrates to remove the stencil from the machine and clean the stencil face of the hardened material. This procedure results in increased machine downtime resulting in reduced production and increased cost of the component.
Additionally, as electronic assembles become smaller and denser, the printed patterns also become smaller, and cleaning of the stencil becomes more critical. In this instance the number of substrates that can be printed before of the stencils must be removed and cleaned of hardened material becomes reduced as even smaller amounts of such residues on the stencil face can cause undesirable increases in machine downtime and component scrap rate. Thus, a solution that would provide for a more complete cleaning of the printing stencils without extended machine downtime or reduced production has long been sought.
A stencil printing apparatus for printing paste material in a given pattern on a substrate in the fabrication of electronic components and assemblies comprises a paste dispenser configured for dispensing paste material through a stencil onto the substrate to define the given pattern thereon, and a cleaning module for passing beneath the stencil to clean the underside thereof having a blade extending at an angle therefrom such that its leading edge can be engaged against the underside for scraping any adherent or hardened debris from the under side of the stencil as the cleaning module passes beneath the stencil.
In the printing method of the invention, a substrate, to be printed with a paste material, is disposed beneath a stencil having a pattern defined therein. It should be noted that the substrate can be a module, wafer, fixture, board or other component Paste is then dispensed, through the stencil, onto the underlying substrate to create, on the substrate, the pattern defined by the stencil. The printed substrate is then removed from beneath the stencil and the bottom of the stencil is cleaned to remove any excess paste material from the bottom of the stencil. This cleaning of the bottom of the stencil can be done in several steps. The first step consists of wiping the lower surface of the stencil with a liquid impregnated paper; the second step is a vacuum cleaning of the substrate and the third requires raising the leading edge of a blade into engagement with the underside of the stencil and passing the blade across the underside of the stencil to remove any debris not removed by the prior cleaning steps.
Accordingly, it is an object of the present invention to provide an improved method of cleaning a printing stencil employed in printing apparatus designed for depositing precise patterns of solder paste, solder flux paste, or other metallic alloys and pastes thereof on such substrates.
Another object of the present invention is to provide a method of employing a cleaning blade for cleaning hardened materials from the printing face of a stencil employed in stencil printing apparatus.
A still further object of the invention is to provide a blade assembly operable in conjunction with other cleaning devices in a stencil printing apparatus for cleaning the printing surface of the stencil.
Another object of the invention is to provide a printing apparatus that deploys a blade at an appropriate time in a select cycle of the printing apparatus for scraping the printing face or surface of the stencil.
These and other objects and features of the present invention will become further apparent from the following description taken in conjunction with the drawings.
The preferred embodiment of the invention will now be described, with regard to the figures wherein:
The paste dispensing head 12 has an internal chamber 24, filled with a suitable viscous paste material, such as a solder paste 26, that is to be passed through the stencil 14 and deposited onto the upper surface 30 of the substrate 20 in a pattern defined by the stencil pattern, (not shown). It is, of course, to be understood that the stencil is designed to print the desired pattern required for of that particular substrate. A mesh, 32 is affixed to the bottom of the chamber 24 to aid in retaining the paste 26 in the otherwise open bottom of the chamber, until the paste is forced from the chamber 12 as will later be explained in detail with regard to operation of the printing apparatus 10.
The operation of the printing apparatus 10 is controlled by a computer system (not shown). This system, in conjunction with the monitoring of a variety of sensor signals from different areas and different operations of the apparatus, coordinates all steps in the process defined by the apparatus.
The process of printing a substrate consists of the steps of: selecting a stencil 14; filling the chamber 12 with a suitable solder paste 26; placing the stencil beneath and in contact with the under side of the chamber 12; placing the substrate 20 beneath and in contact with the underside 18 of the stencil; and pressurizing the chamber to force a select amount of the paste 26, from the chamber 12 through the stencil 14 where it is deposited on the surface 30 of the substrate 20.
The initial arrangement of the dispensing head 12, the stencil 14 and the substrate 20, in readiness for the actual printing step described above is shown in
Next, pressure, via input 28, is applied to piston 34, in chamber 24, to force a selected amount of the paste 26 from the chamber 24, through the underlying mesh 32 and stencil 14 to replicate a paste deposit of the stencil's pattern on the upper surface 30 of the substrate 20. Once the specified amount of paste is deposited onto the surface 30, the pressure on piston 34 is relieved to stop the flow of paste onto the surface 30.
When necessary, additional paste can be added to the chamber by any conventional means, known to those skilled in the art, so that chamber will always be ready for another dispensing cycle.
At this time, the dispensing head 12 is raised to its standby position, as illustrated in
The cleaning module 22, of the present invention, is illustrated, in section, in
In
Once the journals 70a and 70b are each secured to a respective axle 68a, 68b, they are attached to a support member 72 that holds the journals in a fixed, spaced relationship to each other, to confine the rod 66 there between, thus forming a blade support subassembly 73. This sub assembly 73 is then mounted on a support bracket 76, which carries a pair of vertical spaced apart mounting posts 78 each of which is proved with a respective spring 82. The distance between these posts is such that each post will engage a respective mounting hole 80 in a respective one of the journals 70a and 70b.
As shown in these figures, the support bracket 76 is under cut, i.e. provided with a notch 84, between the posts, to accommodate the subassembly 73 there between such that there is sufficient clearance for journals to rest on the springs 82 of the mounting posts 78 without bottoming out on the bracket 76. Accordingly, the springs will maintain a specified force between the blade's leading edge 62 and the lower face 18 of stencil 14 when the blade is engaged with the stencil as explained below.
The support bracket 76 also carries a rack arrangement 86, as well as a pair of upright slots 88 spaced evenly on either side of and from the rack 86 the use of which will be described below. The slots 88 are arranged to hold blade assembly 58 on the trailing edge 90 of the support bracket and to maintain the alignment of the blade assembly as it is raised to force the blade edge 62 against the bottom surface 18 of the stencil 14.
A stepper motor 92, or other conventional means, is also mounted on the support plate 56 adjacent the trailing end of the carriage 22. This motor is positioned so that its shaft, carrying a gear 96 fixed in engagement with the rack 86 on the subassembly 73, will raise and lower the assembly 73 at specified times during the cleaning of the lower face 18 of the stencil 14.
Referring now to
As noted previously the paper 42 passes over the tube 44 that is positioned between and higher than the rolls 38 and 40. The module 22 is raised, by the support plate 56 just as the tube 44 begins to pass beneath the leading edge of the stencil 14. This forces the paper, as it passes over the tube 44, into engagement with the lower face 18 to wipe the face 18 as the carriage traverses the stencil 14.
Simultaneously to enhance the cleaning of the face 18, the fluid dispensing section 46 of pipe 44 is filled with a paste dissolving solvent that wets the paper, as it passes over the tube 44. As the carriage 22 traverses beneath the face 18, fresh solvent wetted paper is drawn, from the roll 38, across the tube 44 and against the face 18, by the take-up rolls 40, such that the entire lower face 18 of stencil 14 is cleaned by fresh, solvent wetted paper. It is of course to be understood that the solvent dispensing pipe 46 is coupled to a controlled pressurized solvent source, (not shown) that will controllably dispense and deliver the solvent to the tube 44. Such an arrangement is within the skill of any competent engineer.
When the tube 44 reaches the trailing end of the stencil 14, the control system ceases delivery of solvent through tube 46, and lowers the support plate 56 so that the module 22 can return unhindered to its initial position and be ready to begin the second cleaning cycle. In the second cleaning cycle, the carriage 22 is again raised and caused to wipe, once again, the lower surface or face 18 of the stencil 14. Now however, instead of applying a solvent to the paper, a vacuum is applied to the paper as the paper passes over the tube 44, in contact with the lower surface or face 18 of stencil 14 to remove any loose particles or fibers from the lower stencil face 18. Again it is to be understood that that the vacuum is created by a suitable vacuum source coupled to the tube 44 that will apply and control the amount and delivery of the vacuum to the tube 44. Again such an arrangement is within the skill of any competent engineer. In this second cleaning cycle the vacuum, drawn through the paper 42 as it passes over the tube 44, pulls any loose or lightly held particles from the face 18 of stencil 14 on to the paper 42 being taken up on the take up roll 40.
Again, as the tube 44 reaches the far or trailing end of the stencil 14, the support plate 56 is lowered, the vacuum in the pipe 48 terminated and, at approximately the same time, the direction of travel of the cleaning module 22 is reversed to return the module 22 back to its start position.
Although the above-described procedure is adequate and removes the bulk of any paste deposited on the surface of the stencil, a residue remains and this residue can and does build up. This residue accumulates on the lower surface of the stencil especially around the openings in the stencil and hardens. In some instances, this hardened residue can clog an opening entirely or, at the very least, will reduce the size of an opening so that the amount of paste being passed on to the substrate is insufficient causing the created solder deposit, to vary and produce solder joints which are less reliable or anomalous. In other cases, such accumulations can cause the face of the stencil to fail to properly mate with the surface of the substrate being printed. This situation can cause too much solder to be deposited at selected points resulting in improper electrical characteristics or in undesired interconnections and/or short circuits in the circuit formed on the substrate surface again increasing substrate defect rate. Prior to the present invention the only way of avoiding these problems was to periodically shut down the machine on a regular basis in order to remove and replace the stencil. This increases the down time of the machine.
The present inventors have found, by introducing the present invention, i.e., the assembly 58 described in
The procedure that incorporates this third cleaning step and uses the above described blade assembly 58, shown in
The cleaning procedure of the invention begins this third cleaning step following completion of the first and second cleaning cycles described above. Immediately after the second cycle is completed, this third cycle begins. The support plate 56 again raises the cleaning module 22 just as the tube 44 begins to pass beneath the leading edge of the stencil 14. Again, this forces the paper, as it passes over the tube 44, into engagement with the lower face 18 to wipe the face 18 as the carriage traverses stencil. Again if desired the paper may be wetted with the paste dissolving solvent however this is generally unnecessary. Carriage 22 is now caused to begin to traverse beneath the face 18 and as the leading edge 62 of the blade 60 becomes positioned just beneath the leading edge of the stencil 14 the stepper motor 92 coupled to rack 86 is activated to raise bar 76, carrying the blade subassembly 73, to cause the leading edge 62 of the blade 60 to engage the lower surface 18 of stencil 14 and just compress the springs 82. Preferably, the edge 62 of blade 60 will form an angle approximate sixty degrees (60°) with respect to the stencil's lower surface 18. Because of the springs 82 the force with which the blade edge 62 contacts the lower face 18 of the stencil is constant. The module 22 now moves beneath the stencil 14 and the leading edge 62 of blade 60 scrapes the stencil's lower surface 18 clean of residue. When the cleaning module 22 reaches a point wherein the blade assembly 58 reaches the trailing end of the stencil 14, the stepper motor 92 is triggered and the blade assembly 58 is lowered. Simultaneously, the support plate 56 is lowered and the cleaning module 22 is returned to its start position.
It is to be noted that the stencil is typically formed of invar or stainless steel having a hardness between 70 and 97 HRB and the blade 60 is preferably formed of a similar material. A blade slightly harder than that of the material from which the stencil is formed has an improved ability to scrape residue off the lower side 18 of the stencil 30.
It should be noted that when the prior art, wet paper and vacuum cleaning cycles are regularly employed after printing of a substrate, the blade scraping step of the present invention may not need to be required after the printing of each substrate but should be employed once a selected number of substrates have been printed. The frequency of the scraping step of the invention is determined by the stencil and paste used for printing. However the present inventors have found that the typical stencil need only be scraped, in accordance with the present invention, only after twenty or more stencils have been printed. Additionally, whereas the blade cleaning step is described in the preferred embodiment in a third cycle following a wet paper and a vacuum cycle, it may be utilized alone, either before or after a respective one of these cycles or in conjunction with either or both of these cycles.
This completes the description of the preferred embodiment of the invention, and since changes may be made in the above construction without departing from the scope of the invention described herein, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not in a limiting sense. Thus, other alternatives and modifications will now become apparent to those skilled in the art without departing from the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2,490,665 | Dec 2004 | CA | national |
