Introduction of Paste Printing Technology
First, the impact of print quality elements
Paste printing quality of the quality of surface-mount products a great impact, statistics show that 60% of the Reworked PCB is due to poor solder paste caused by failure, the good or bad silk screen basically determines the level of SMT good or bad. So in the surface mount solder paste printing strictly to the good. Even the best solder paste, equipment, and application methods are not necessarily guaranteed to yield acceptable results. The user must control the process and equipment variables, in order to achieve good print quality. In the process of printing solder paste, the substrate on the table, mechanical or vacuum clamping positioning, alignment pins or visual alignment. In manual or semi-automatic presses, the printing blade is pressed down on the template so that the bottom surface of the template contacts the top surface of the circuit board. As the scraper traverses the entire length of the etched pattern area, the solder paste is printed onto the pad through the apertures in the stencil / screen. After the solder paste has been deposited, the screen is snap-folded back into place immediately after the squeegee. The separation distance and squeegee pressure are two important device-dependent variables that achieve good print quality. There is also the speed of the screen printing. During the screen printing, the speed of the scraper on the screen printing template is important because the paste takes time to roll and flow into the wire hole. If the allowable time is not enough, then the direction of the scraper of the road, solder paste on the pad will be uneven. Deciding the quality of printing factors include: delay stripping printing, printing gap, as long as a good grasp of the above factors, the general printed out of the solder paste graphics should be flat, the edge is homogeneous.
Solder Past e Options:
Paste is a mixture of solder alloys and fluxes. Paste solder paste in the size of the selection should be appropriate, must be matched with the silk screen template. Viscosity is an important characteristic of solder paste, from the dynamic side, in the silk screen travel, its viscosity is lower, then the mobility of the better, easy flow into the silk screen hole, printed on the PCB pad. From a static point of view, after the scratch, the paste stuck in the silk screen hole, the high viscosity, then keep its filling shape, and will not collapse down.
Sub-rubber scraper and metal scraper two. Scraper wear, pressure and hardness determine the print quality, should be carefully monitored. For acceptable print quality, the scraper edges should be sharp and straight. Scraper pressure is low resulting in omission and rough edges, while scraper high or very soft scraper will cause sm eared printing, and may even damage scraper and stencil or screen. Excessive pressure also tends to cut out the solder paste from wide openings, causing insufficient fillet welds.
Template (stencil) Type:
Two kinds of metal and nylon wire. The process of making an opening controls the smoothness and precision of the perforated wall and the size of the opening must be appropriate. There are three common processes for making stencils: chemical etching, laser cutting and additive processes. In order to achieve good printing results, you must have the right solder paste materials (viscosity, metal content, maximum powder size and lowest possible flux activity), the right tools (presses, stencils and scraper) and the right process Good positioning, clean wipe). In addition, the post-printing inspection is also essential, it can greatly reduce the back process due to poor printing caused by the repair loss.