Printing – Stenciling – Traveling-inker machines
Reexamination Certificate
2000-12-28
2002-12-17
Funk, Stephen R. (Department: 2854)
Printing
Stenciling
Traveling-inker machines
Reexamination Certificate
active
06494133
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a screen printing method and a screen printing machine each of which is for printing a print material on an object through pattern holes of a print mask.
2. Discussion of Related Art
A screen printing method and a screen printing machine are widely used in various industrial fields such as an electric-circuit production industry. For example, when an electric circuit is produced, solder paste is printed on a circuit substrate (e.g., a printed wiring board, PWB) on which the electric circuit is to be assembled. After one or more electric components (ECs) are mounted on the circuit substrate, the circuit substrate is heated to melt the solder paste printed thereon and thereby electrically connect the ECs to the circuit substrate. In this field, it is practiced to form solder bumps on a circuit substrate by first printing solder paste thereon and subsequently heating and melting the printed solder paste.
There are known two major methods of printing solder paste on circuit boards. One of them is an off-contact-type screen printing method, and the other is a contact-type screen printing method. In the off-contact method, a small space is left between a print mask and a circuit substrate, and a plate-like squeegee is moved on a surface of the print mask while locally pressing the mask against the circuit substrate and moving solder paste along the surface of the mask, so that while the mask contacts the substrate, the solder paste is printed on the substrate through pattern holes of the mask. Generally, a metal mask that is formed of a metal such as copper, stainless steel, or nickel is suitable for printing of a highly viscous fluid such as solder paste. However, the metal mask is too rigid to be used in the off-contact printing. Therefore, generally, a so-called combination mask including a metallic portion and an elastically deformable mesh portion surrounding the metallic portion is used in the off-contact printing. In the off-contact printing, the print mask, except for a local portion thereof that is currently pressed by the squeegee against the circuit substrate for printing the solder paste, is kept away from the substrate, and the local portion currently pressed by the squeegee is separated, little by little, from the substrate after the squeegee has passed over the local portion. Thus, the solder paste printed on the circuit substrate is advantageously separated from the pattern holes of the print mask.
However, when the solder paste placed on the upper surface of the print mask is forced into the pattern holes of the mask by the plate-like squeegee, so as to be printed on the circuit substrate through the holes of the mask, the. solder paste may enter some holes of an adjacent portion of the mask that is adjacent to the local portion currently pressed by the squeegee against the substrate and is currently away from the substrate, and may spread on the lower surface of the mask. In this case, the solder paste may be applied to other spots on the circuit substrate than prescribed print spots thereon, or may be. adhered to the lower surface of the mask to soil the same. These phenomena do not lead to accurate printing.
In this background, currently, the contact-type screen printing method is widely used. In the contact-type method, solder paste is printed in the state in which a print mask is held in close contact with a circuit substrate. This method is particularly suitable for printing of a highly accurate and fine pattern. Either a combination mask or a metal mask may be employed as the print mask. Generally, this method needs a special technique for separating the print mask and the circuit substrate from each other after the solder paste is printed on the substrate. There are known various separating techniques.
However, in. the contact-type method, the print mask and the circuit substrate are kept in close contact with each other while the solder paste is printed on the substrate. Therefore, the solder paste may enter, because of its capillary phenomenon and/or surface tension, the interface between the mask and the substrate. Meanwhile, in the case where the circuit substrate is sucked, by vacuum, against a support surface of a substrate supporting device, air present between the mask and the substrate are sucked by the vacuum via the through-holes of the substrate, so that the solder paste may be sucked into the interface between the mask and the substrate. Moreover, the solder paste may run on the lower surface of the print mask, because of small vibrations of the substrate supporting device. These phenomena do not result in accurate printing.
It is currently practiced to clean the upper and lower surfaces of the print mask, for solving the above-described problems. There are two cleaning methods, one is a wet method in which the solder paste is solved by a solvent and then is cleaned by a cleaning paper, and the other is a dry method in which the solder paste is just cleaned by a cleaning paper. In some cases, both the wet and dry methods are employed. In either method, it is difficult to clean up the solder paste completely. The most important problem with the cleaning methods is that the cleaning of the print mask results in increasing the cycle time and accordingly lowering the production efficiency. In addition, the expensive solder paste cleaned off the print mask cannot be recycled, which is against recent tendency toward environmental pollution control. Moreover, an exclusive cleaning device is needed, which leads to increasing the running cost.
Another problem with the contact-type printing is about the separation of print mask and circuit substrate. It is not easy to separate the print mask closely contacted with the circuit substrate, from the circuit substrate, without damaging the shape of the solder paste printed thereon. The degree of ease of separation of highly viscous fluid (i.e., solder paste) from pattern holes of print mask largely depends on an aspect ratio, h/f, as a ratio of an area, h, of side surface of each pattern hole to an area, f, of print pattern of the mask. Generally, it is preferred that the aspect ratio h/f be not greater than a solder-print critical aspect ratio (h/f=0.4 to 0.5). In addition, the printability of solder is influenced by various factors including the design or composition of solder paste, the machining or material of print mask, the circuit substrate, the printing machine, and the operator. It is almost impossible to perform printing most efficiently under the best conditions of those factors. Therefore, there is a need to obtain such a special technique which can compensate for some inevitable defects of those factors. For example, a servo system is used to separate the print mask and the circuit substrate from each other by moving them over a predetermined distance at empirically obtained speeds and accelerations. However, it is impossible to move, at a certain speed or acceleration, the circuit substrate with respect to its entirety, because of the elasticity of the print mask. That is, the central portion of the circuit substrate is moved away from the print mask, faster than the end portions of the same are. It is difficult to construct a reliable multidimensional space-analysis model to control drive shafts of the servo system used to separate the print mask and the circuit substrate from each other. Thus, currently, it is difficult for the contact-type printing method to perform a sufficiently stable printing for a long time. The properties of the solder paste change day by day. Generally, the basic data of the solder paste, such as viscosity, may change by ±15% a day. Now, a huge data base including data about various variable factors is under construction in order to establish useful conditions for the separation of print mask and circuit substrate, so that the data base is utilized by a computer. However, this function is not a basic function which is properly required for the printing machine. In addit
Fuji Machine Mfg. Co. Ltd.
Funk Stephen R.
Oliff & Berridg,e PLC
LandOfFree
Screen printing method and screen printing machine does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Screen printing method and screen printing machine, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Screen printing method and screen printing machine will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2954053