In PCB design, vias are a common way to connect lines on different layers, or to provide soldering locations for components. There are various types of vias, one of which is via in pad, which is to punch the via on an SMD or BGA pad, then plug the hole with resin and plating a layer of copper on the surface so that the hole is not visible on the pad.
Figure 1BGA pad in-tray holes.
Advantages and disadvantages of holes in the dish
Vid in Pad (HDI) is the basic architecture of HDI (High Density Interconnect) boards, and its advantages are obvious: reducing the distance between boards and signal transmission distance, which is beneficial to reducing signal interference and loss; Electroplating and filling copper solid holes can effectively reduce the parasitic signal and parasitic inductance in the hole, which is conducive to the transmission of high-frequency signals. Effectively reduce the thickness of the finished board and improve the wiring density per unit area.
However, there are also some disadvantages of holes in the plate, such as high cost, long production cycle, easy to produce bubbles, etc. One of the most serious problems is voids, which are bubbles or voids that form inside the solder joints. Voids can affect the mechanical strength and thermal conductivity of solder joints, resulting in poor welds or failure. Therefore, care needs to be taken to avoid or reduce the occurrence of voids when designing and manufacturing holes in the disc.
Causes of voids
1.Inadequate or uneven resin plug holes. Resin plugging holes is one of the key processes for holes in the disc, which requires the vias to be completely filled and cured to prevent solder paste from flowing into the vias. If the resin plug pores are inadequate or uneven, it can lead to residual air inside the vias or gaps caused by resin shrinkage, which can be filled by solder paste during soldering and create voids.
2.The plating cap is uneven or not secure. Plating caps are a layer of copper plated on the surface of the pad after the resin plugs the hole to increase the soldering area and wettability. If the plating cap is uneven or not secure, it can lead to gaps or separations between the solder paste and the copper layer, which can also form voids.
3.Solder paste itself contains oxygen or other impurities. Solder paste is a metallic material used to connect components and PCBs, and it usually contains a percentage of flux and other additives. These fluxes and additives react chemically or volatilize at high temperatures, producing oxygen or other gases.
4.The welding temperature is not suitable. Soldering temperature is an important factor affecting the fluidity and wettability of solder paste. If the temperature is too high or the heating time is too long, it will cause excessive oxidation or volatilization of the solder paste, producing a large amount of gas; If the temperature is too low or the heating time is too short, it will cause the solder paste to flow poorly or solidify too quickly, hindering the gas discharge.
How to avoid the problem of hollowing holes in the plate
1.Optimize the resin hole plugging process. The resin hole plugging process needs to control the type, viscosity, temperature, pressure, time and other parameters of the resin to ensure that the resin can fully and evenly fill the vias, and there is no shrinkage or cracking after curing.
2.Optimize the plating cap process. The electroplating cap process needs to control the composition, temperature, concentration, current, time and other parameters of the electroplating solution to ensure that the electroplating layer can be evenly and firmly covered on the pad, and has good adhesion to the resin and substrate. At the same time, it is necessary to choose a suitable copper reduction method to remove excess copper and maintain a flat surface.
3.Select and use the right solder paste. The selection and use of solder paste needs to consider its compatibility with components and PCBs, wettability, oxidation, volatility and other characteristics to reduce gas generation and carryover.
4.Optimize welding temperature and time. Soldering select the appropriate reflow temperature and time to ensure that the solder paste is fully wetted and flowing.
Fitech solder paste
Fuyingda is able to produce high-quality, high-performance, and high-reliability solder pastes, which are suitable for a variety of PCB design and manufacturing processes, including pan-in-tray vias, microvias, etc.