Solder beading was found on boards that had been reflowed. It is a large solder ball embedded in a pool of flux that is positioned next to a discrete components. These components have a very low ground clearance, such as chip resistors and capacitors, thin small outline packages (TSOPs), small outline transistors (SOTs), D-PAK transistors, and resistor assemblies. Because of its location in relation to these components, solder beadings are often referred to as “satellites.”
Solder beading not only affects the appearance of the product, but more importantly, due to the dense components on the printed circuit board, there is a danger of short circuit during use, thus affecting the quality of electronic products. There are many reasons for the occurrence of solder beading, so it is necessary to prevent and improve them one by one in order to better control them. The following article will discuss the factors that cause solder beading and the countermeasures to reduce the generation of solder beading.
Why does solder beading happen?
Solder beading is usually related to excessive solder paste deposition. The increase in its occurrence can be attributed to the increased use of no-clean solder paste due to its lack of “body” that is squeezed under discrete components to form solder beads. When chip components are mounted in no-clean solder paste, it is easier for the solder paste to squeeze under the component. Extrusion tends to occur when too much solder paste is deposited.
When the PCB circuit board leaves the liquid solder, it is very easy to form solder beading. This is because when the PCB board and the tin wave are separated, they will be pulled into a tin column, and when the tin column is broken and dropped back to the tin cylinder, the solder will be sputtered out, and the solder will fall on the PCB board to form solder beading. Therefore, when designing tin wave generators and tin cylinders, attention should be paid to reducing the drop height of tin. The small drop height helps reduce dross and spatter.
Factors that cause solder beading
The main factors affecting the generation of solder beading are:
(1) Stencil opening and pad pattern design
(2) Stencil cleaning
(3) Repeatability of the machine
(4) Reflow oven temperature curve
(5) Chip pressure
(6) The amount of solder paste outside the pad
(7) Falling height of tin
(8) Outgassing of volatile substances in the circuit board and solder mask
(9)Flux
Ways to prevent solder beading
(1) Select the appropriate land pattern and size design. In the actual pad design, the PC should be combined, and then the corresponding pad size should be designed according to the actual component package size and solder terminal size.
(2) Pay attention to the production of steel mesh. It is necessary to properly adjust the opening size according to the specific component layout of the PCBA board to control the amount of solder paste printed.
(3) It is recommended to take strict baking action on PCB bare board with BGA, QFN and dense pin components on the board, so as to ensure the removal of moisture from the upper surface of the solder board to maximize solderability.
(4) Improve the quality of stencil cleaning. If the stencil is not clean, the solder paste remaining at the bottom of the stencil opening will gather near the stencil opening to form too much solder paste and cause solder beads.
(5) Ensure the repeatability of the equipment. When the solder paste is printed, due to the offset between the template and the pad, if the offset is too large, the solder paste will flow out of the pad, and solder beads will easily appear after heating.
(6) Control the placement pressure of the placement machine. Regardless of whether it is the placement pressure control method or the component thickness control, the settings need to be adjusted to prevent solder balls.
(7)Optimize the temperature profile. Control the temperature of reflow soldering so that most of the solvent can evaporate on a better platform.
Although tin beading is tiny, it can have serious effects. When it comes to electronics, the details matter. Therefore, in addition to the attention of process production personnel, relevant departments must also actively cooperate, and communicate with process personnel in a timely manner for material changes and replacements to prevent adverse events caused by changes in process parameters caused by material changes. Designers responsible for PCB design should also communicate more with craftsmen, refer to problems or suggestions fed back by craftsmen and make improvements as much as possible.
