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PCB Via-in-Pad

As electronic products continue to trend toward being lighter, thinner and smaller, PCBs are also evolving toward high-density and high-complexity designs.These constraints are forcing chip manufacturers to produce chips with smaller pitches, such as QFNs, BGAs, flip chips, etc. In order to meet the requirements for routability and signal integrity, PCB designers are adopting VIPPO structures and combining them with traditional design methods such as dog-bone routing, micro-vias, skip vias, and solder pads with trace routing, thus achieving high-quality PCB designs.

What is Via-in-Pad?

Traditional vias are located in non-component areas of the PCB, whereas the Via-in-pad (VIP) technique involves placing the via directly on the SMD pad and then plating it. The following diagram illustrates the difference between via-in-pad and traditional vias.

via in pad.png

Via-in-pad involves the deposition of conductive material, typically copper, into a PTH which is then covered with a layer of solder mask. This allows for direct connection between the pad and the via, eliminating the need for separate traces or vias to connect the pad to other areas of the board. Via-in-pad can help reduce the overall size of the board, thereby decreasing signal path lengths and improving signal performance. 

In comparison, traditional PTHs require passing through non-component areas of the PCB and connecting to traces on the other side of the board. However, PTHs have been the standard in through-hole technology for many years and are still widely used due to their reliability and ease of manufacture.

The manufacturing process for via-in-pad is highly challenging as it requires precise drilling and plating of holes and may be more expensive than traditional PTHs. The choice between via-in-pad and traditional PTHs depends on the specific requirements of the PCB design and manufacturing process.

The Design about Via-in-Pad

No Need for Via-in Pad

Prior to PCB routing, it is necessary to perform fanout work to facilitate inner layer wiring. For the fanout of BGA-type components with fanout located at the center position between solder pads, there is no need to design a via-in-pad structure. As for the setting parameters of BGA fanout, the via hole diameter should be 0.15-0.2mm, the trace width should be 3-4mil, and the annular ring width should be 0.3-0.4mm. Therefore, the spacing between BGA pins needs to be greater than 0.35mm in order to achieve normal fan-out.

Need Via-in-Pad

Before the fanout of the BGA, it is necessary to set the diameter of the vias. Otherwise, if the via diameter is not appropriate, effective fanout cannot be achieved. When the pitch between BGA pins is too small to fan out, it is necessary to design via-in-pad structures for inner layer routing or for routing on the bottom layer.

Via-in-Pad on BGA

For components with fewer pins, it is not necessary to design via-in-pad. However, for BGA components with a high pin count, the large number of fan-out vias can consume a significant amount of routing space. By designing these vias as via-in-pad, additional routing space can be preserved. When the pitch between pins is too small to route on the same layer, via-in-pad should be implemented and routing should be done on other layers.

vippo_2_update.png

Via-in-Pad on Filter Capacitor

When routing within a BGA component requires many vias, it becomes difficult to place filter capacitors on the backside of the BGA component without interfering with the vias. Therefore, vias are placed above the pad as via-in-pad to allow for this.

VIPPO_3_update.png


Advantages and Disadvantages of Via-in-Pad

To decide whether to use the via-in-pad, the advantages and disadvantages of this method must be considered. Designers should evaluate the project reasonably and correctly according to the actual situation to determine whether to use this technology.

Advantages

• Increase PCB space utilization: By placing vias directly beneath SMD pads, designers can better utilize the surface space of the PCB, which will better meet design requirements.

• Improve thermal performance of the circuit board: In high-power circuit boards, using via-in-pad technology can improve heat dissipation and maintain component temperatures below specified limits by reducing thermal resistance.

• Improve the voltage capacity and performance of PCB:All vias have inductance and resistance characteristics that affect the flow of current. These complex variables are detrimental to the functionality of the PCB. Via-in-pad design can shorten the power supply path and significantly improve the performance of the PCB.

Disadvantages

• Increased complexity: Covering the via-in-pad may result in an uneven surface, which requires additional steps to fix.

• Longer production time: After drilling the vias on the pad, it must be filled with epoxy resin. With these additional steps, manufacturing also takes longer.

• Higher production cost: Via-in-pad technology is more complex and requires more time and effort to complete, resulting in higher production costs. Additional costs are one of the main issues facing this technology in the PCB industry.

Note

• The via on the BGA is generally defined as via-in-pad, and requires resin filling, with an electroplated cap for convenient customer soldering. The exception is when customers request non-resin-filled holes for the BGA.

• In addition to the BGA, when customers require all through-holes to be filled with resin, the vias on the surface mount components are also defined as via-in-pad.

Production Process of Via-in-Pad

vippo_4_update.png

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