PCB Tech - PCB welding method and the reason for using high TG material

PCB circuit board welding method

1. Immersion tin effect

When hot liquid solder dissolves and penetrates the metal surface of the PCB being soldered, it is called tin in metal or tin in metal. The mixture of solder and copper molecules forms a new part of copper and part of solder alloy. This solvent is called tin. It forms intermolecular bonds between various parts of the PCB to generate a metal alloy compound. Good intermolecular bond formation is the core of the PCB welding process, which determines the strength and quality of the PCB welding joint. Only the copper surface is free from pollution and there is no oxide film formed due to exposure of the PCB to the air in order to contact the tin, and the solder and the working surface need to reach an appropriate temperature.

2. Surface tension

Everyone is familiar with the surface tension of water, which keeps the cold water droplets on the grease-coated PCB metal plate spherical, because in this case, the adhesion force of the liquid spreading on the solid surface is less than its cohesive force. Wash with warm water and detergent to reduce surface tension. Water will soak the grease-coated PCB metal plate and flow outwards to form a thin metal plate. This can happen if the adhesion force is greater than the cohesion force.

The cohesion of tin-lead solder is even greater than that of water, making the solder a sphere to reduce its surface area (for the same volume, compared with other geometric shapes, the sphere has a petite surface area to meet the requirements of low-energy states. Require).

The effect of flux is similar to the effect of detergent on greased PCB metal plates. In addition, the surface tension is highly dependent on the cleanliness and temperature of the PCB surface. Only when the adhesion energy is much greater than the surface energy (cohesion), PCB is the ideal immersion tin material.

3. Zhan Tin Corner

When the eutectic point temperature of the solder is about 35°C higher than the eutectic point temperature of the solder, a drop of solder is dropped on the surface of the hot-soldered PCB to form a meniscus. To a certain extent, the adhesion of the PCB metal surface to tin can be evaluated by the appearance of the meniscus. If the solder meniscus has a clear undercut, the shape is similar to a drop on a grease-coated PCB metal plate, or even tends to be spherical, the metal cannot be soldered. Only the length of the meniscus is less than 30. The angle is small and the weldability is good.

4. The production of metal alloys

The intermetallic bond between copper and tin forms crystal grains, and the shape and size of the crystal grains depend on the duration and strength of the soldering temperature. There is less heat during the soldering process, which can form a fine crystal structure, so that the PCB board can form an excellent solder joint with strength. Too long reaction time, whether due to too long PCB soldering time or too high temperature or both, will result in a rough crystal structure, which is sandy and brittle with low shear strength. Copper is used as the metal substrate of the PCB, and tin-lead is used as a solder alloy. Lead and copper do not form any metal alloy compounds, but tin can penetrate into copper. The intermolecular bond of tin and copper forms metal alloy compounds Cu3Sn and Cu6Sn5 on the joint surface of solder and metal.

The metal alloy layer (N phase + ε phase) must be very thin. In PCB laser welding, the thickness of the metal alloy layer is 0.1mm. In wave soldering and manual soldering, the thickness of the intermetallic bonding layer of the solder joints of the PCB circuit board is greater than 0.5μm. Since the shear strength of the PCB solder joints decreases with the increase of the thickness of the metal alloy layer, it is usually tried to pass the soldering time as short as possible The thickness of the metal alloy layer is controlled below 1 μm.

The thickness of the co-composite layer of the metal alloy depends on the temperature and time at which the solder joints are formed. Ideally, the welding should be completed within about 2 seconds at 220't. Under this condition, the chemical diffusion reaction of copper and tin will produce an appropriate amount of metal alloy connecting materials Cu3Sn and Cu6Sn5, with a thickness of about 0.5μm. Insufficient metal-to-metal connection usually occurs in cold welded joints or the temperature is not raised to the proper temperature during the welding process. It may cause the PCB soldering surface to be cut off. On the contrary, in solder joints that are overheated or soldered for too long, an excessively thick metal alloy layer will cause the tensile strength of the PCB solder joint to be very weak.

Reasons for using high TG materials

In addition to the basic FR-4 materials in PCB production, some customers also indicate that high TG materials are to be used in the materials, so why use high TG materials in PCB production?

The full name of TG used in PCB production is glasstransition temperature, which stands for glass transition temperature. The circuit board must be flame-resistant, cannot burn at a certain temperature, but can only be softened. The temperature point at this time is called the glass transition temperature (Tg point), and this value is related to the dimensional stability of the PCB board. The higher the TG value, the better the PCB temperature resistance. When the temperature rises to a certain area with high TgPCB, the substrate will change from "glassy" to "rubbery". This temperature is called the glass transition of the sheet (Tg) temperature. In other words, Tg is the temperature of the substrate to keep the high temperature (°C). That is to say, ordinary PCB substrate materials not only produce phenomena such as expansion, deformation, melting, but also a sharp decline in mechanical properties and electrical characteristics at high temperatures.

The increase of the Tg of the substrate will strengthen and improve the characteristics of heat resistance, moisture resistance, chemical resistance, and resistance stability for Shenzhen circuit board proofing. The higher the TG value, the better the temperature and other performance of the board, especially in the lead-free manufacturing process, there are more applications of high Tg.

High Tg refers to high heat resistance. With the rapid development of the electronics industry, especially the electronic products represented by computers, the development of high functionality and high multilayers requires higher heat resistance of PCB substrate materials as an important guarantee. The emergence and development of high-density mounting technologies represented by SMT and CMT have made PCBs more and more inseparable from the support of high heat resistance of substrates in terms of small aperture, fine wiring, and thinning. This is also a big reason why high TG materials are used in PCB production.

Therefore, in PCB production, the difference between general FR-4 and high Tg FR-4 is the mechanical strength, dimensional stability, adhesiveness, and water absorption of the material in the hot state, especially when heated after moisture absorption. There are differences in performance, thermal decomposition, thermal expansion and other conditions. High Tg products are obviously better than ordinary PCB substrate materials.