Welding bridges are one of the frequently occurring defects in the assembly of circuit boards with wave soldering.
To solve this problem, in addition to improving the process parameters, a new type of selective de-bridging device can be added to the wave soldering equipment. Tests have shown that the use of such a system can greatly reduce the bridging of the circuit board.
Printed circuit board assembly encompasses a wide range of technologies, from single-sided vias to complex double-sided reflow assemblies, as well as 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 be expected in advance. Bridging is usually caused by the design of the board or the bracket used for welding. It is not easy to solve it completely, so it is only by repairing to eliminate the defect, thus prolonging the production cycle.
From the perspective of an electronic manufacturer (EMS), it is generally not possible to change the board design, only by craftsmanship. Experiments have shown that selective de-bridge technology can greatly improve the yield of assembly.
Welding bridge
The solder bridge is caused by the solder being in an unstable state of change, which occurs in the solder reflow region in the post-wave soldering stage. When the board leaves the peak, 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 can be easily changed by any slight disturbance, so the formation of solder bridging has a great relationship with the PCB and wave soldering processes.
Selective de-bridge
After the wave soldering, it is necessary to find the bridge and then carry out the repair, which will increase the cost and workload of the production, and it is generally believed that the repaired solder joint will reduce the reliability of the product. After years of research, engineers have developed a number of techniques for reducing bridging in the final stages of wave soldering, such as hot plate knives after welding (with a bundle of air or nitrogen blowing to the molten joint to remove the bridge) It is a standard technology that has been used for many years in wave soldering. However, this method has no selectivity for the soldered area, so that good solder joints are also affected. 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-bridging tools to be added to the wave soldering process. This new technology can be selected to be prone to soldering defects. The area is de-bridged without encountering other good solder joints.
Selective de-bringing adjusts the direction of the airflow so that it only aligns with the area where the bridge may occur to achieve the purpose of removing the bridge. The tool is used where the board has just exited the peak, where the solder is still molten, the spray range and other process parameters are programmed first and controlled by a computer in the welding system. The key to achieving selective de-bridging is to accurately align the airflow and keep the nozzle as close as possible to the PCB without contact. Before deciding to use the selective de-bridging system, let's compare it to the conventional hot air knife technology. It has been found that hot air knife technology cannot meet our requirements for various reasons, such as incompatibility with existing equipment and overall cost is too high. To further determine the effectiveness of the selective de-bridging system, we selected eight test panels for evaluation and referenced the bridging defect history data associated with the board design.
The situation of these templates is as follows:
• Samples A and B are approximately 12.7 & TImes; 7.5 cm, double-sided surface mount board. Both plates contain bridged perforated (PTH) connectors that are often bridged at the bottom after wave soldering. They are soldered without the use of a selectable bracket but with an adjustable bracket; except for the connector, all underside surfaces are attached. The components are wave soldered, but only the connectors need to be bridged.
• Samples C and D are approximately 30 & TImes; 15 cm. Both boards have different surface mount components, but have the same PTH components and multiple connectors, with bridges randomly occurring between the connectors and some surface mount components. Instead of using a selective welding bracket, the welding is carried out by the claws on the conveyor belt. The bridging device is not selectively used but is carried out on the entire board.
• Sample E and F dimensions of approximately 53.3 & TImes; 12.7 cm, double-sided surface mount board. Both models have multiple PTH connectors on one side and are wave soldered using a selective solder bracket with random bridges on the connector.
• The sample G size is approximately 38.1 & TImes; 35.6 cm. The board contains surface mount components requiring double-sided reflow and a selective solder bracket for PTH components. The PTH component contains multiple connectors that are located throughout the board and allow for a little warpage. Since the welding bracket is selected, the bridge device cannot be too close to the board and cannot be fully utilized.
· The size of the sample H is about 50.8×38.1cm. The other conditions of the board are exactly the same as the model G, but there is one more PGA socket. It also has a choice of solder brackets and allows for warpage. Tests have shown that the defect in the product is greatly reduced after the selective de-bridging system, but the small-sized sample being tested does not see the effect of the bridging tool, so we conducted a larger internal evaluation.
The test parameter selective de-bridge system was subjected to a three-month internal evaluation before the formal installation. After obtaining the customer's consent, the equipment supplier will install the bridge system on a wave soldering machine, and then we will finish the finished product. Comprehensive assessment of various data such as rate, downtime, operating costs, and maintenance.
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