Ball planting problem in SMT proofing small batch processing

Small batch processing of planting balls in SMT proofing provides a flexible, fast, accurate and low-cost solution for secondary assembly, and large EMS enterprises have gradually entered this field. It is more and more necessary to apply ball planting technology in SMT proofing small batch processing industry, and EMS enterprises can only respond to OEM customers’ requirements if they master wafer-level and chip-level packaging technology. This paper mainly introduces the ball planting technology applied to PCBA circuit board. The whole process includes four steps: coating flux, ball planting, inspection and rework, and reflow soldering. Ball planting requires two online printers: one is a common screen printer for coating paste flux, and the other is used for ball planting. The two printing presses can be switched to ordinary printing presses for electronic assembly at any time. Coating flux Coating paste flux is the first step of the ball planting process, which is the key step to keep the ball positioned and well formed in reflow soldering. The specially designed screen plate is applied to the printing of paste flux, and the opening of the screen plate is determined based on the pad size of printed circuit board and the size of solder balls. In the step of printing paste flux, two types of scrapers are used at the same time, and the front scraper is a rubber scraper. A thin layer of flux is evenly coated on the screen board by the vertical scraper, and then the flux is printed on the PCBA pads by the rubber scraper. DOE is used to determine the best parameters of flux printing. After SMT proofing is printed in small batch, the coverage rate of flux on the pad is observed and calculated by microscope, and the DOE result is calculated. The flux coverage rate reflects the DOE experimental results.
In this DOE experiment, the optimal printing parameters can be set; Of course, different devices will have certain differences. In the process of SMT proofing small batch processing, the screen board is easily damaged, so it needs to be handled and moved carefully. In the process of flux printing, solid dust or other foreign substances can easily block the opening of the screen plate, so it can only be cleaned with an air gun. Cleaning agents such as isopropyl alcohol or alcohol can’t be used to clean the screen board, because it will dissolve and destroy the polymer materials on the screen board. Usually, it is wiped with a dust-free cloth dipped in deionized water and dried with an air gun after production. After the soldering flux printing of SMT proofing small batch processing is completed, it is necessary to check under the microscope for insufficient or misplaced printing. Usually flux is transparent, and it is difficult to detect defects by visual inspection. In the stage of ball planting, a specially designed template is also needed.
The opening design of the template is also based on the actual solder ball size and PCB pad size, which is based on two considerations: first, it is necessary to avoid flux polluting the template and solder balls; The second is how to make the solder ball pass through the template opening smoothly. The template structure has two layers: the main body is electroformed template, which has smoother hole wall than laser or chemical etching template, so that solder balls can pass through smoothly; The composite two layers have almost the same thickness as the diameter of solder balls, which can avoid the pollution of paste flux to electroforming template, and at the same time, the solder balls can smoothly reach the pad through the template and be stuck by the flux. AOI equipment is used for on-line detection after ball planting in SMT proofing small batch processing. The main defects are usually few balls and dislocation. After inspection, the circuit board with less balls should be reworked with off-line semi-automatic ball filling equipment; For dislocation defects, cleaning PCB and re-printing is the only way. The placement of few balls requires the use of an accurate image magnification system. First, one operating arm is used to coat paste soldering flux on the pad lacking balls, and then another operating arm is used to fill the balls on the pad. For lead-free products, the commonly used solder alloy is SAC105, whose melting point is slightly higher than that of the lead-free solder paste used for PCB circuit boards, so as to prevent defects from being caused again in secondary reflow. AOI inspection is required after reflow soldering.