Ball Mounting Problem in SMT Chip Processing

The application of ball mounting technology in the SMT chip processing industry is becoming more and more necessary, and EMS companies can only respond to the requirements of OEM customers if they master the wafer-level and chip-level packaging technology. Next, I will introduce the ball mounting technology applied to the PCB circuit board.

The entire process consists of four steps: fluxing, ball placement, inspection and rework, and reflow. Bumping requires two in-line printers: one is a common stencil printer for applying paste flux, and the other is used for bumping. Both presses can be switched at any time to regular presses for electronic assembly. Applying paste flux is the first step in the ball placement process and is a critical step in keeping the balls in place and forming well in reflow. A specially designed stencil is used for paste flux printing, and the opening of the stencil is determined based on the pad size and solder ball size of the printed circuit board. In the step of printing the paste flux, two types of squeegees are used at the same time, the front squeegee is a rubber squeegee. The vertical squeegee first uniformly coats a thin layer of flux on the stencil, and then the rubber squeegee prints the flux on the PCBA circuit board pads. The DOE was used to determine the optimal parameters for flux printing. After the printing of the SMT patch processing, the coverage of the flux on the pads was observed and calculated, and the results of the DOE were calculated. Flux coverage reflects DOE experimental results.

In this DOE test, the optimal printing parameter settings can be obtained. Of course, different equipment will have certain differences. The stencil is easily damaged during the SMT chip processing and production process, so it needs to be handled and moved carefully. During the flux printing process, solid dust or other foreign substances can easily block the opening of the stencil, and can only be cleaned with an air gun. Cleaning agents such as isopropyl alcohol or alcohol cannot be used to clean the stencil, as they will dissolve and destroy the polymer materials on the stencil. Usually, wipe with a clean cloth dipped in deionized water and dry it with an air gun after production. After the flux printing of SMT chip processing is completed, it is necessary to check under the microscope for insufficient printing or dislocation. Usually flux is transparent, and visual inspection is difficult to detect defects. In the balling stage, a specially designed formwork is also required.

The opening design of the stencil is also based on the actual solder ball size and PCB pad size. This is based on two considerations: one is to avoid flux contamination to the template and solder balls; the other is how to make the solder balls pass smoothly. Template opening. The stencil structure has two layers: the main body is an electroformed stencil with smoother hole walls than laser or chemically etched stencils, allowing solder balls to pass through smoothly; the composite two layers have almost the same thickness as the solder ball diameter, which is very good. Therefore, the contamination of the electroforming stencil by the paste flux is avoided, and the solder balls can smoothly pass through the stencil to reach the pad and be stuck by the flux. AOI equipment is used for on-line inspection after ball placement in SMT chip processing. The main defects are usually missing balls and misalignment. After inspection, the circuit board with fewer balls needs to be reworked with off-line semi-automatic ball filling equipment; for misalignment defects, cleaning the PCB circuit board and reprinting is the only way. Ball-less placement requires the use of an accurate image magnification system to apply paste flux to the missing pad with one manipulator arm and then fill the pad with the other manipulator arm. For lead-free products, the commonly used solder alloy is SAC105, which has a slightly higher melting point than the lead-free solder paste used for PCB circuit boards to prevent re-defects during secondary reflow. AOI inspection is required after reflow soldering.