Correct welding methods and guidelines for power modules
The following provides welding methods and guidelines for the Vicor product family, including full, small, micro, VE-200, VE-J00, VI BRICK and roughly identical filters and front-end modules. The following are guidelines for welding techniques to make the Vicor module and circuit board solder well. This paper points out some matters that need to be paid attention to, such as correct welding procedures, evaluation of solder joints, etc., in order to ensure good connection when users adopt Vicor module. It also inspects common bad welds and provides guidelines on detection and treatment.
The Vicor production department uses the IPC-A-610C standard as the basis for checking the quality of solder joints, and also recommends that users adopt the same standard in the production of their power components using Vicor power modules.
Standard for good solder joints
According to IPC-A-610C, the solder needs to fill at least 75% of the barrel surface to ensure that the interface can be firmly connected. The ideal is to be 100% full. For the solder joint to be fully on, the barrel surface and pins need to show that they have undergone a process called wetting, which is when the tin liquid on a surface reaches a temperature that causes the potential surface tension to be greatly reduced and the tin solution can adhere uniformly to the surface through capillary attraction (cohesive and intercohesive bonding).
During the welding process, whether the solder joint is sufficiently wet can be determined by whether the pin and the barrel surface are uniformly plated. In addition, during the bonding process of pins and barrel faces, the solder will gather at the joint of the two, forming a trailing profile on their respective surfaces. Once wetting has occurred and then solidifying, the two components are properly combined to form a high-quality connection. Solder joints should be smooth and shiny. This means that the solder joints have not been moved during solidification, and the circuit board has been cleaned properly before welding. Whether it is manual welding or wave soldering, the solder joints should have the above characteristics.
Welding procedure
Before soldering, ensure that the circuit board is clean and free of residual impurities, chemicals and solutions. It is also not recommended to add a cosolvent to the solder, which will become impurities left on the board and may damage the module when cleaned. In addition, if these impurities remain on the module, it may affect the normal operation of the module. The pin of the Vicor module is specially designed for low electrical impedance, and it is necessary to decide which installation scheme to use according to the application to reduce the mechanical stress of the pin and solder joint. When the module with heat sink or the module is used in an environment where it will be hit or shaken, the support should be used to reduce the pin stress. Lead plated pins should be used for direct welding modules, and gold plated pins should be used for supporting sockets (refer to SurfMate or InMate mounting system). Vicor does not recommend welding discrete component leads or connectors directly to the module.
One must consider is that the welded pins should protrude slightly from the board. If the pin length is shorter than the board thickness, it is impossible to weld the module. If the board is too thick and the pins are not long enough to feed through the board, a socket should be considered to ensure that the module is properly installed.
Before welding, the circuit board should be supported by a solid support to ensure that it will not be moved during welding. In this procedure, supports can also be used.
There are two types of pins in the Vicor module. The output pin (responsible for the output power to the load, the size of the pin depends on the output current) and the signal pin (with only a small amount of current, the same size of the pin in the same series). The larger the pin volume, the longer the welding time. In addition, the following conditions will affect the welding time:
1, circuit board thickness The thicker the circuit board, the more heat consumption, the longer the welding time.
2, copper-plated wire trace area Output pin requires a large copper-plated wire trace to reduce impedance and power consumption. Since copper is a good thermal conductivity material, the area of the copper-plated wire will affect the welding time.
3, the thickness of the copper plated wire trace is the same as the above, the thickness of the wire trace depends on the output current of the module. It also affects the time required for welding. The copper wire of the general circuit board is measured by weight per square foot. Two or three ounces of copper are commonly used.
4, the strength of the soldering iron (power) The greater the strength of the soldering iron, the shorter the heating time of the circuit board. As the soldering iron is heated at one point on the board, parts of the accessories, including the Vicor module, are also heated. If the copper wire is very large, due to the strong thermal conductivity of copper, the temperature inclination is low, such as the use of lower strength soldering iron, the entire wire need to be heated to a higher temperature in order to make the part close to the soldering iron hot enough to dissolve solder. Due to the heat loss of the stitch and circuit board, some soldering irons may not have enough heat to weld.
5, the iron head temperature is generally 63/37 solder melting point is 3960F (2000C). The higher the temperature of the soldering iron head, the faster the temperature of the pin and barrel surface reaches the melting point. However, the soldering iron head temperature is too high, can also damage the welding pad, circuit board or module pin.
6. Different solder types have different melting points, which will affect the temperature of the pin and welding gasket during reflow welding. Vicor recommends 63/37 solder solder for Vicor modules.