The non-wetting of Pcb, also known as poor soldering, means that the solder coverage on the pcb circuit board substrate pads or device leads is less than the target solder wetted area, as shown in Figure 1. A large contact angle is typically formed between the solder and the base metal. The solder base metal does not form a metallurgical bond in the non-wetting place, and is in the base color.
1. Caused of the pcb circuit board pad is not wetted
The reasons for non-wetting are:
The metal has poor wettability. Poor metal wettability can be thought of as a result of the metal impurities or oxidation of the pads, pins, and the nature of the pads and pins themselves. For example, the plating process has phosphorus, pinholes, copper pad oxidation on the Ni/Au surface coating, 42 alloy is too thick at the pin exposed end, or the OSP coating is too thick, which are all causes of non-wetting.
(2) Plating properties. In general, the solder on the HASL pad is easily wetted sufficiently. For surfaces that are not HASL, such as OSP or Ni/Au, the pads cannot be sufficiently wetted around the edge of the pad, although a suitable weld seam structure has been formed.
(3) Solder alloys have the same problem, such as impurities such as aluminum, cadmium or arsenic in the solder. Irregular solder powder shapes reflect a large oxide content and therefore consume more flux and cause poor wetting. Obviously, poor wetting is caused by poor flux activity.
(4) Inappropriate reflow curves and atmosphere.
The preheating temperature, preheating time, peak welding temperature and reflow soldering atmosphere of the reflow soldering have a great influence on the wetting performance. On the one hand, if the heating time is too short or the temperature is too low, the flux reaction will be incomplete, resulting in poor wetting; on the other hand, the excess heat before the solder melts not only excessively oxidizes the metal of the pad and the lead, but also It will consume more flux and will also cause poor wetting. Re-flow with nitrogen will result in a significant improvement in wetting.
Whether wetting is a problem depends on whether the formed solder joint has sufficient bonding strength and fatigue resistance. For solder joints, if the wetting angle meets the requirements, even if some of the area on the pad is not wetted by the solder, the solder joint is generally considered reliable. For the application of fine pitch components, in order to ensure the sealing performance of the template and the pad during solder paste printing, the size of the template opening is usually smaller than the pad size. The result is generally non-wetting formed in the range of the edge of the pad without HASL.
The above is the analysis of the formation of the Pcb circuit board pad poor wetting.