Solder Paste Printing There is an intrinsic relationship between printing speed, pressure, and solder paste type, and this relationship must be maintained to achieve acceptable printing results. Some solder pastes must be printed faster, while others must be slower to get better print results. If the blade is too light to sweep across the stencil, leaving a thin layer of solder paste or flux on the stencil, the pressure should be increased to scrape the surface of the stencil, but the upper limit of pressure increase must ensure the rolling of the solder paste. Requirements, because the rolling of solder paste during printing is one of the signs of good printing results. Printing too fast can result in incomplete filling of the openings, especially on the side of the pad that faces the direction of motion of the blade. Excessively sweeping the template can result in extreme tip and cover incompleteness because the solder paste is not completely released from the opening.
A problem with solder paste on the stencil is often mentioned. After the squeegee is scraped, the solder paste remains on the stencil. There are usually two reasons. First, although you may have used the correct pressure, the bottom dead center of the scraper or the distance the scraper is pressed into the stencil is still too small. Another reason why the solder paste remains on the stencil may be the lack of a proper supporting thimble below the substrate. With insufficient support, the substrate will sink under the pressure of the blade, so that the angle of the blade cannot scrape the solder paste on the screen. Insufficient substrate support causes the substrate to sink and can also cause variations in the pressure applied by the doctor blade to the substrate.
Through-hole devices Solder paste printing can also be used on via devices. Solder paste printing has become an acceptable process for the assembly of this type of board. This process of filling a PCB via with solder paste for a via assembly is commonly referred to as "intrusive soldering" or "pin-in-paste printing." The opening of the stencil must be designed to get the correct amount of solder paste to fill the vias, thus ensuring reliable solder joints. Usually, the solder paste will have a 50% shrinkage, so the first step is to calculate the amount of solder paste required. It is necessary to calculate the volume of solder paste required to fill the entire via and then subtract the value of the pin volume. The actual required solder paste volume should be twice the volume of solder required after curing. The size of the opening can be calculated by the thickness of the stencil and the available area around the via pad. Solder paste can be printed in an enlarged print area where the solder is pulled back to the solderable surface.
Rubber Scrapers Operators should know the different types of rubber blade hardness and know when to use them. For screen printing and stencil printing, there are several blade options of different hardness. Typically, when screen printing is used, a polyurethane scraper with a hardness of 60-80 shore A should be used. When using a softer material, the mesh of the screen should avoid scraping the glue layer from the substrate. A doctor blade with a hardness of 90-110 Shore A is usually used for stencil printing. However, when a polyurethane scraper is used on the stencil, the boring phenomenon on the large opening becomes a problem, and therefore, the metal squeegee is the first choice for stencil printing.
For fine-open or stepped stencils, the use of polyurethane squeegees results in a more consistent print and reduces the wear of the stencil. A stepped formwork is a template in which certain areas of the form are gently transitioned or stepped down to a thinner portion than the rest of the screen. This type of template is often used for boards that are mostly large openings and have only one or two fine pitch pin devices.
Towed blade scraper The drag angle of the dragged metal scraper is 60 degrees and the contact angle of the rubber scraper is 50 degrees without pressure application. Using the MPM's Balanced Control Programmable Print Head ("Prohead"), the contact angle can be adjusted within plus or minus 5 degrees of normal. The pressure applied to the blade must be sufficient to provide a clean scratch on the upper surface of the stencil, but not too large, which would cause indentation of the stencil and cause premature failure. The indentation is a phenomenon of permanent edge cracking at the bottom of the stencil when excessive pressure is applied to the stencil beyond the edge of the substrate.