In PCB design, through hole (via) is a common connection method that can connect different layers of lines, or provide welding locations for components. There are several types of holes, one of which is a via in pad (via in pad), in which the hole is punched into the SMD or BGA pad, then the hole is blocked with resin and a layer of copper is plated on the surface, so that the hole is not visible in the pad.
Advantages and disadvantages of disk holes
Vid in Pad is the basic architecture of HDI (high-density interconnection) board, and its advantages are obvious: reduce the distance between boards and signal transmission distance, which is beneficial to reduce signal interference and loss; Electroplated copper filled solid holes can effectively reduce the parasitic signal and parasitic inductance in the hole, which is conducive to high-frequency signal transmission; Effectively reduce the thickness of the finished plate and improve the wiring density per unit area.
However, the disc hole also has some disadvantages, such as high cost, long production cycle, easy to produce bubbles and so on. One of the most serious problems is voids, which are bubbles or voids that form inside the solder joint. The void will affect the mechanical strength and heat conductivity of the solder joint, resulting in poor welding or failure. Therefore, when designing and manufacturing disk holes, it is necessary to pay attention to avoid or reduce the generation of holes.
The cause of the void
1. Insufficient or uneven resin plugs. Resin plug is one of the key processes for disc holes, it needs to fill the hole completely and cure to prevent the solder paste from flowing into the hole. If the resin plug hole is not sufficient or uniform, it will lead to residual air inside the hole or the gap caused by the shrinkage of the resin, these Spaces will be filled by the solder paste during welding, and the formation of a void.
2. The electroplating cap is not smooth or firm. The electroplated cap is plated with a layer of copper on the surface of the pad after the resin plug to increase the welding area and wettability. If the plating cap is not smooth or firm, there will be gaps or separation between the solder paste and the copper layer, and these gaps or separation will also form voids.
3. The solder paste itself contains oxygen or other impurities. Solder paste is a metal material used to connect components and PCBS, and it usually contains a certain proportion of flux and other additives. These fluxes and additives react or volatilize at high temperatures, producing oxygen or other gases.
4. The welding temperature is inappropriate. Welding temperature is an important factor affecting solder paste fluidity and wettability. If the temperature is too high or the heating time is too long, it will lead to excessive oxidation or volatilization of the solder paste, resulting in a large amount of gas; If the temperature is too low or the heating time is too short, it will lead to poor fluidity of the solder paste or too fast solidification, preventing gas discharge.
How to avoid the problem of holes in the disc
1. Optimize the resin plugging process. Resin plugging process needs to control the type of resin, viscosity, temperature, pressure, time and other parameters to ensure that the resin can be fully and evenly filled through the hole, and does not shrink or crack after curing.
2. Optimize the plating cap process. The plating cap process needs to control the composition, temperature, concentration, current, time and other parameters of the plating solution to ensure that the plating can be smoothly and firmly covered on the pad, and has good adhesion with the resin and the substrate. At the same time, it is necessary to choose a suitable copper reduction method to remove excess copper and keep the surface flat.
3. Select and use appropriate solder paste. The selection and use of solder paste need to consider its matching with components and PCB, wettability, oxidation, volatility and other characteristics to reduce the production and residue of gas.
4. Optimize welding temperature and time. Select the appropriate reflux temperature and time to ensure that the solder paste is fully wetted and flowing.
PCSPCB is capable of producing high quality, high performance, high reliability solder paste for a variety of PCB design and manufacturing processes, including disc holes, microholes, etc.
English
Chinese