The electronic industry uses printed circuit boards (PCBs) with multiple layers to reduce the physical size the board occupies. This increases the component density on the top layer of the board, and it also splits the space available for routing traces onto different inner layers, with vias being the only way to electrically connect between different layers. Although this makes vias an essential part of board design, they also affect solderability and introduce weaknesses, leading to a lower reliability of the board. However, by following good design practices offered by Rush PCB, you can make your PCBs more reliable even with vias.
Types of Via
A via is a hole in the PCB, spanning a single layer or multiple layers, with the fabricator plating the hole with copper forming a barrel. The ends of this barrel connect to specific traces on the respective layers, forming electrical contact between two layers. If the via spans multiple layers, the inner layers may also connect to it, provided the circuit requires it to do so. Primarily, there are three types of vias, through-hole, buried, and blind.
A through-hole via traverses the entire thickness of the PCB, beginning at the top layer and ending at the bottom-most layer. The fabricator may plate the through hole via or leave it without plating, depending on the requirement. A plated through hole via has the advantage that other inner layers may connect to it, if the circuit demands.
A buried via spans two or more inner layers and is not visible either at the top or the bottom layer. The layers a buried via spans may connect to it, if the circuit requires. A blind via begins either at the top layer or the bottom layer, and finishes at an inner layer. On the way, a bind via may span several inner layers, and they may connect to it if necessary.
High Density Interconnect (HDI) PCBs and flex PCBs use another type of vias known as micro-via. These are very small, and fabricators drill them using laser beams. Each micro-via spans only one layer, and the fabricator can stack them on other layers to function as through-hole vias, blind, and buried vias.
The bigger the diameter of the via, the stronger it is. This is because a large via has greater mechanical strength, and higher electrical and thermal conductivity. However, a large via subtracts from the already scarce PCB real estate that could be useful for routing traces. Rush PCB recommends vias with a minimum drill width of 20 mils, an annular ring of 7 mils, and a minimum aspect ratio of 6:1.
When a PCB heats up, whether during processing or in the work environment, the difference of the coefficient of thermal expansion (CTE) between the copper and laminate can lead to issues. The structural latticework of the PCB laminate limits expansion in the horizontal direction (along the plane), but can freely contract or expand significantly in the vertical direction (perpendicular to the plane).
For instance, FR-4 laminates can contract or expand at four times the rate at which copper can. Therefore, the copper barrel in a via hole is under tremendous strain every time the board heats up. If the thickness of the copper barrel is not adequate, and the PCB too thick, the board will expand to the point where the copper may break. For a drill width of 20 mils, a pad diameter of 34 mils will work adequately with a maximum board thickness of 120 mils.
Positioning a via is as important as its size. Locating a via very close to a solder pad may cause several problems to arise, foremost among them being the effect of solder wicking.
As the via and the solder paste on the pad heat up, capillary action in the via draws solder from the solder pad into the via. The solder travels through the via and collects on its bottom side, leaving the pad deficient or entirely free of solder. A larger via will wick more solder more quickly, and the joint will likely be mechanically weak and electrically resistive. Rush PCB recommends three ways for preventing this, and any one of them is effective:
Solder Mask Barrier: a barrier of solder mask placed between the via and the solder pad will prevent solder from moving into the via hole. However, to place an adequately wide solder mask barrier, the designer may have to move the via farther from the pad. This may not be possible if the board is already crowded, or the board handles high-frequency signals.
Tented Via: If it is not possible to move the via away from the pad and put a solder mask barrier in between, masking the via pad totally may help. Completely sealing the via does prevent solder from flowing into it, but it also prevents using the via as a test point. It is possible that some contaminant may enter the via and corrode the copper barrel.
Filled Via: It is also possible to fill up a via entirely with a conducting or non-conducting material before tenting it. Totally sealing a via with filler material has the advantage of providing a better barrier against contamination.
Rush PCB recommends undertaking Design for Reliability studies as a valuable process for improving reliability, lowering costs, reducing time to market, and improving customer satisfaction. Implementation of Design for Reliability requires a proper combination of personnel, tools, and time limitations.
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