How to Solve Welding Bridge Defects in PCB Assembly?Posted on 2020-07-07
PCB Assembly involves a wide range of technologies. From single-sided through-hole mounting to complex double-sided reflow assembly, and ball grid array (BGA) devices that require selective wave soldering. When wave soldering these boards, it is often seen that bridges are formed in certain fixed areas (such as connectors), and these defects can actually be predicted in advance. The bridging is usually caused by the design of the board or the bracket used during welding. It is not easy to solve it completely. Therefore, only by repairing to eliminate defects, thus prolonging the production cycle.
In the process of using wave soldering for circuit board assembly, welding bridging is one of the defects that often occurs. To solve this problem, in addition to improving the process parameters, a new type of selective de-bridge device can be added to the wave soldering equipment. Tests have proved that the use of this system can greatly reduce the bridging of circuit boards.
Soldering bridging is caused by an unstable change of solder. It occurs in the solder reflow area after wave soldering. When the board leaves the wave crest, the solder suddenly retracts, causing the solder to reflow, and a large amount of molten solder in the reflow zone eventually forms a bridge. The solder in the molten state will easily change its wettability under any slight disturbance, so the formation of solder bridges has a lot to do with PCB assembly and wave soldering processes.
After wave soldering, it is necessary to find the bridge and then carry out rework, which will increase the production cost and workload. And it is generally believed that repaired solder joints will reduce the reliability of the product. After years of research, engineers have developed many techniques to reduce bridging in the final stage of wave soldering. For example, the hot air knife treatment of the entire board after welding (blowing a bunch of air or nitrogen to the molten solder joint to remove the bridge) is a standard technology that has been used in wave soldering for many years. However, this method has no selectivity for the welding area, so that good solder joints are also affected to a certain extent.
Recently, with the gradual maturity of process control and computer control and the application of circuit board tracking systems, it has been possible to develop practical selective de-bridge tools to be added to the wave soldering process. This new technology can select areas that are prone to welding defects to be de-bridged, without touching other good solder joints.
Selective de-bridging can be achieved by adjusting the direction of air flow so that it is only aimed at the area where bridging may occur. This tool is used where the circuit board has just exited the crest, when the solder is still in a molten state, the spray range and other process parameters are first set by the program and controlled by the computer in the soldering system. The key to achieving selective de-bridging is to accurately adjust the air flow and make the nozzle as close to the PCB as possible without contact.
Before deciding to use the selective de-bridge system, let’s compare it with the conventional hot air knife technology. After comparison, it was found that the hot air knife technology could not meet our requirements due to various reasons, such as incompatibility with existing equipment and high overall cost. In order to further determine the effect of the selective de-bridge system, we selected 8 test boards for evaluation and referenced the historical data of bridge defects related to the circuit board design.
The status of these models is as follows:
Samples A and B are about 12.7×7.5cm in size and are double-sided surface mount boards. Both boards contain a through-hole (PTH) connector that often produces a bridge at the bottom after wave soldering. Instead of using a selective soldering bracket, an adjustable bracket is used for soldering; all but the connector are surface-mounted The components must be wave soldered, but only the connectors need to be bridged.
The size of model C and D is about 30×15cm. The two boards have different surface mount components, but they have the same PTH component and multiple connectors. The bridging occurs randomly on the connector and in the middle of some surface mount components. Welding brackets are not used for welding, but are transferred by the claws on the conveyor belt. The de-bridge device is not used selectively but for the entire board.
Samples E and F are approximately 53.3×12.7cm in size, and are double-sided surface mount boards. Both models have multiple PTH connectors on one side, and use selective soldering brackets for wave soldering, and random bridging occurs on the connectors.
The size of the model G is about 38.1×35.6cm. The board contains surface mount components that need double-sided reflow soldering and selective soldering brackets for PTH components. The PTH component contains multiple connectors, which are located throughout the board, and the board can be warped a little. Due to the selective welding bracket, the de-bridge device cannot be too close to the board and cannot be fully utilized.
The size of the model H is about 50.8×38.1cm. The other conditions of the model are exactly the same as the model G, but there is an additional PGA socket. It also has optional welding brackets and allows warpage. The test shows that the defect of the product is greatly reduced after using the selective de-bridge system, but the small-size model tested cannot see the effect of the bridge tool, so we conducted a larger internal evaluation.
The selective de-bridge system was internally evaluated for a period of three months before the formal installation. After obtaining the customer’s consent, the equipment supplier installed the de-bridge system on a wave soldering machine. A comprehensive evaluation of various data such as downtime, operating costs and maintenance.
As mentioned earlier, eight different complex circuit boards were used in this internal test. The results show that the defect rate of each board is improved by an average of 84%. The effect of the selective de-bridge system mainly depends on the PCB assembly design, the optimization of process parameters, and the tools used. However, the system software does make the settings more systematic. And can be reused.
The reduction of bridging depends to some extent on the shape of the components, the degree of mixing, the type of bracket and the wave soldering equipment. For different situations, the estimated cost of repairs will be different.