The quality of solder paste printing on the surface mount product quality has a great impact, statistics show that 60% of the return to repair the board is due to poor solder paste printing caused by the failure of the screen printing basically determines the degree of good and bad SMT.
In the process of printing solder paste, the substrate is placed on the workbench, mechanically or vacuum clamping positioning, with positioning pins or vision to align. In manual or semi-automatic solder printer, the printing squeegee down on the template, so that the bottom surface of the template to contact the board surface. As the squeegee travels the entire length of the pattern area being eroded, solder paste is printed onto the pads through openings in the stencil screen. After the paste has been deposited, the screen snaps off immediately after the squeegee and returns to its original position. Snap off distance and squeegee pressure are two important equipment-related variables for achieving good print quality.
There is also the speed of the screen, the speed at which the squeegee travels over the screen stencil during the screen printing process is important because it takes time for the paste to roll and flow into the holes in the screen. If not enough time is allowed, then the solder paste will not be flat on the pads in the direction of the squeegee’s travel. Factors that determine the quality of printing are also: printing release delay, printing gap, etc., as long as a better grasp of the above factors, the general printing out of the solder paste graphic should be flat, the edge of the more flush.
Selection of Solder paste
Solder paste is a mixture of solder alloy and flux. The size of the solder beads in the solder paste should be selected appropriately and must be matched with the screen printing template. Viscosity is an important characteristic of solder paste, from the dynamic aspect, in the screen printing stroke, the lower the viscosity, the better the mobility, easy to flow into the screen printing hole, printed on the PCB pad. From the static point of view, after the screen printing scraping, the solder paste stays in the screen printing hole, and its viscosity is high, so it keeps its filling shape and does not collapse downward.
Types of Squeegee
There are two types of squeegee: rubber squeegee and metal squeegee. Squeegee wear, pressure and hardness determine print quality and should be carefully monitored. For acceptable print quality, squeegee edges should be sharp and straight. Low squeegee pressure results in missed and rough edges, while high squeegee pressure or a very soft squeegee will cause smeared printing and may even damage the squeegee and stencil or screen. Excessive pressure also tends to scoop paste from wide openings, causing insufficient solder rounding.
Types of Stencil
There are two types: metal and nylon wire. The process of making the openings controls the finish and accuracy of the opening walls, and the openings must be the right size. There are three common processes for making stencils: chemical etching, laser cutting and additive processes. In order to achieve good printing results, there must be a combination of the right solder paste material (viscosity, metal content, large powder size and lowest possible flux activity), the right tools (printer, stencil and squeegee) and the right process (good positioning, clean swabbing). Furthermore, post-press inspection is essential to greatly reduce the loss of rework due to poor printing in the later stages of the process.
Quick facts about NeoDen
① Established in 2010, 200+ employees, 8000+ Sq.m. factory.
② NeoDen products: Smart series PNP machine, NeoDen K1830, NeoDen4, NeoDen3V, NeoDen7, NeoDen6, TM220A, TM240A, TM245P, reflow oven IN6, IN12, Solder paste printer FP2636, PM3040.
③ Successful 10000+ customers across the globe.
④ 30+ Global Agents covered in Asia, Europe, America, Oceania and Africa.
⑤ R&D Center: 3 R&D departments with 25+ professional R&D engineers.
⑥ Listed with CE and got 50+ patents.
⑦ 30+ quality control and technical support engineers, 15+ senior international sales, timely customer responding within 8 hours, professional solutions providing within 24 hours.
Post time: Sep-23-2024