PCB Pad

Choosing the Right PCB Pad Size for your Printed Circuit Board Design

The quality of assembly of a printed circuit board is highly dependent on some factors. These include the interface in between the components of the board and the board itself.

This is why the solder pad is a very important aspect of the design and development of PCBs as it functions as the surface areas that ensure electrical contact between the board and the component.

What Role does PCB Pad play in Printed Circuit Board Design?

PCB pads are a metal’s exposed region on circuit boards through which the lead component is soldered into. Furthermore, multiple solder pads that are in conjunction are useful for the generation of land pattern or component footprint on printed circuit boards.

There are two major types of PCB pads. These are surface mount and through-hole pads.

Surface Mount Pads

Surface mount pads are those pads useful in the mounting of surface mount components and surface mount devices. They have these features

  • Solder pads showing the copper area. The contact pad could be oblong, square, round, or rectangular in shape.
  • Solder paste and solder alloy
  • Solder mask layer
  • Pad number (these are the number of pads that are available for the component)
  • Pad and land patterns

Through-hole Pads

Thru hole pads are PCB pads that help in mounting the through-hole components. They come in two major types.

Non-plated through-hole

This refers to solder pads without any plating present in the hole. Most times, the solder pad is useful for one-sided boards. A thru hole can also be useful for PCB mounting in enclosures with screws being mounted through the holes.

Right from time, unplated holes usually have a specific area surrounding the thru hole, which does not have any copper. This is just like the board edge clearance. Note that this is done to help in preventing shorts between the copper layers as well as the parts to be placed.

A thru hole pad has different pads, which are referred to as a pad stack. This solder pad stack is made up of inner pad, drill, top pad, bottom pad, pin number, and annular ring.

Plated through hole

Plated thru hole has to do with a pad having a thru hole. The plating of this thru hole wall will be done with copper. At times, it is done with a solder mask, or you can use different protective plating. Using electrolysis, you can achieve this hole plating. With this plating you can be sure of electrical connections formed between the board’s layers.

Is it Possible to Place a Via on Pads: Via-in-pad?

For HDI designs, where space becomes a constraint, placing vias on pads becomes very necessary. Conventional vias feature signals that carry traces, which are routed from the pad to the via.

Furthermore, via-in-pad is useful in minimizing the form factor of a printed circuit board. It achieves this by the reduction of the space which has been taken by trace routing. Also, via-in-pads are useful for BGA components having pitches of at most 0.5 mm.

What are Bonding Pads?

 Bonding pads help in connecting circuits present on a die to a packaged chip’s pin. One of the sides of the gold wire helps in connecting to the bond pads. The other side connects to the package. These bonding pads are manufactured from all the metal layers that are stacked together and then connected via the vias. With this, there is a connection between the pad and chip core.

Also, this die requires a passivation thin layer or insulator on the chip. This ensures that environmental contamination doesn’t come near the core. Also, the bond pads should be accessible to be connected to the chip. This is why you cannot wrap it by the insulating layer yet. Also, the glass inner layer indicates to the printed circuit board manufacturer where those openings should be when bonding.  

Problems with Wrong PCB Pad Sizes

Note that the solder pad size, solder pad position, and solder pad shape in the footprint of a printed circuit board has a direct link to the extent to which the manufacturing of the printed circuit boards can take place. Working with wrong solder pad sizes, or one in the wrong position could lead to different issues while soldering during printed circuit board assembly. Below are some of these problems you may encounter.

Floating Parts

When the surface mount sits on solder pads which are spaced incorrectly or too large, it may leave its position when solder reflow is ongoing. This could cause solder bridging to other metals, as well as insufficient spacing of the component for PCB test, rework, and thermal cooling.

Solder bridging

Too large solder pads can cause one of the parts to float. This could cause solder bridging. Solder crosses to a metal feature or pad on a different net and then creates a specific direct short. Also, solder bridging can happen if the right solder paste and solder mask features aren’t designed in the pad shape present in the CAD tools.

Solder wicking

Thru hole pads may also have some difficulty when not constructed the right way. If the drill size used in the lead is too large, the solder mask may end up wicking down via the hole before it makes a good connection. Same way, a too small drill size will make the insertion of the component lead very difficult. This will also slow down the assembly.

Tombstoning

Anytime a discrete, small surface mount part is unbalanced thermally when solder reflow is on, the solder paste present on one solder pad could melt much faster compared to the other, pulling this part up in a specific tombstone or vertical position.

Drill breakout

When the solder pad size for that drill hole used is too small, this drill may end up wandering slightly during the drilling operation. It will also break out of that pad shape.

Incomplete solder joints

Too small pads or solder pads spaced very close together may prevent enough room to allow the forming of enough solder filets and solder alloy. This can cause that part to have no solder joint or a wrong solder connection.

Conclusion

Here comes to the end of our article on PCB pads. We have also discussed possible problems you may encounter when you choose the wrong size for your PCB pad. We hope that you avoid these errors to ensure you get a perfect circuit design.

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