Coating apparatus – Immersion or work-confined pool type – Mask or stencil
Reexamination Certificate
1997-10-14
2001-01-09
Parker, Fred J. (Department: 1762)
Coating apparatus
Immersion or work-confined pool type
Mask or stencil
C118S410000, C118S504000
Reexamination Certificate
active
06171399
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for carrying out the deposition of a viscous material on a substrate through the apertures of a stencil or screen, and may be used in a screen printing machine for the deposition of solder paste on a printed circuit board.
BACKGROUND
As is known, a substrate such as a printed circuit board on which electronic components are placed requires that the components be soldered to the board. A viscous material, such as a non-conductive or conductive adhesive solder paste or another silicon type viscous material is often deposited on the substrate before the component is placed on the substrate. Screen printing machines that are commercially available from a number of sources are used to automatically deposit the viscous materials through a screen or stencil onto the substrate. The screen or stencil can be either of the traditional type, that is to say a mesh or metallic stencil, or of the type which is the subject-matter of WO 96/01743 of the assignee of the present invention, relating to a stencil allowing the deposition of the materials to be applied.
Examples of the implementation of the present invention provided herein are within the area of deposition by screen printing of solder pastes such as are used in the electronics industry to produce printed circuit boards by soldering of components onto the boards. Nevertheless, the present invention can find application in other technical fields where viscous materials are used.
Generally, the solder pastes used in screen printing in the electronics industry are heterogeneous materials, the components of which have different densities, and are composed of metallic materials and organic or flux materials. The mass of the metallic portion of the solder paste represents approximately 85% to 90% of the total mass, with a density of 8 to 12 according to the metals used. It is understood that the term density means the weight as compared to 1 liter of water. In volume, the metallic portion represents only approximately 50% of the total volume. The organic material, also called flux, has a density of approximately 1.
The solder pastes described above are made up of metallic microspheres joined by the flux or organic material. This viscous flux comprises rheologic agents, adhesive agents and cleaning agents which affect the process of assembling components on printed circuit boards. The process, which is well known, involves:
deposition by screen printing of solder paste contacts onto selected portions of the substrate;
placing of component leads on the paste contacts, the adhesive agent of which holds the components to the board; and
reflowing the solder paste in a furnace or oven, which causes the coalescence of the metallic microspheres, and, when cooled, results in the component being fixed on the board at the proper location.
The function of the alloy included in the solder paste is to provide the supply of metal necessary to ensure electrical interconnection between the leads of the components and the printed circuit by soldering. The organic materials in the paste must disappear at the conclusion of the soldering operation. Nevertheless, there is normally a residue of the organic materials which must be cleaned with water or with solvent, which is both costly and polluting.
Solder pastes reportedly having low residue have been developed. In these pastes, the organic part has substantially the same value in volume terms as in the previous pastes described above. Light solvents with low boiling points can be introduced to provide proper rheology or flow characteristics. Because of this, these solvents become volatile more rapidly during a pre-heating step, which generally precedes the reflow step described above during the assembly of components onto printed circuit boards. At the end of the reflow step there thus remains little residue. In order to provide a satisfactory adhesive capacity, the light solvents described above are combined with adhesive resins, which become volatile or sublimate in the reflow step.
The cleaning agents for preparation of a surface which is suitable for obtaining satisfactory inter-metal connections occupy a very small part of the total volume of the paste, in order to reduce the residue. As the overall efficiency of the cleaning must not change, the volume efficiency of the active cleaning constituent has to increase in the same proportions as the total amount of residue diminishes.
These developments in low residue solder pastes result, on the one hand, in a greater dilution of the active cleaning constituents in the paste and, on the other hand, in a greater volatility of the additional solvents used. It is therefore necessary that an extremely homogeneous distribution of the active cleaning constituent be obtained within the volume of each deposit when the solder paste is applied to the substrate. If this is not achieved, deposits of the solder paste will be obtained in which the efficacy of cleaning, for example, will not be identical for adjacent areas. Certain areas will have too much cleaning constituent applied, resulting in cleaning and residue problems. Other areas will not have sufficient cleaning constituents, and therefore the soldering obtained will be of poor quality.
The high degree of solvency and solubility of the additional solvents used results in the evaporation thereof while printed circuit boards are produced. In prior art modes of deposition, the material is dragged by means of an inclined wiper (see the prior art system shown in
FIG. 1
) in the open air and therefore the evaporation problem is not solved. The evaporation results in a change in the rheology of the solder paste during production, which leads to the parameters for control of the machine being modified. In extreme cases, the solder paste may become too dry and no longer pass properly through the apertures in the stencil.
These problems are exacerbated when production requirements necessitate high-speed screen printing, for example at 200 millimeters per second as opposed to 20 to 50 millimeters per second. To counteract these problems, thixotropic additives are introduced and combined with the other solvents. Evaporation of the base solvents therefore modifies the possible speed of deposition. By way of example, an evaporation of 1% from the volume of solder paste completely changes the rheology and makes screen printing very difficult, if not impossible.
Another of the problems caused by the known technology is the control of the wear and tear on the wiper system. Progressive erosion of the active edge of the wiper by rubbing alters the intrinsic qualities of the paste applied, and that of the depositions, because of the uncontrolled and random retrieval of a certain quantity of microspheres of the metallic portion of the solder with each wipe. In fact, the wipers are normally only changed when the poor quality of the deposition is a noticeable consequence of their wear.
In the prior art, two types of wipers have been commonly used. The first type of wiper is a rubber or polyurethane type. The hardness of this type of wiper varies generally between 70 to 90 Shore. This wiper has the advantage of good deformation by virtue of its low degree of hardness and its flexibility, and therefore good sealing is produced. It has the inconvenience of deforming during passage over the apertures in the stencil. For apertures where the dimension parallel to the wiper is less than 0.5 mm, this is not a major problem. However, where the apertures have dimensions parallel to the wiper greater than this value, the deposit is hollowed out. Where deposits are larger than 3 mm, they are completely dragged off again.
The second type of wiper is a metallic type. The advantage of this type of wiper is its ability to maintain rigidity and therefore not allow the deposit to be hollowed out. The hardness of this type of wiper, however, despite its flexibility, does not allow for perfect sealing with the stencil. The hardness of the metallic wiper sometimes exceeds th
Bourrieres Francis
Kaiser Clement
Novatec S.A.
Parker Fred J.
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