Back when I was in high school, I played the clarinet and saxophone. On these instruments, the pitch is changed by how many holes in the instrument are open or closed. The more holes that are closed, the lower the pitch. The holes are usually closed by covering them with a round disk made of a soft material called a “pad.” If these pads become worn, cracked, or too hard, the hole will not get completely closed and sealed. The result in a fuzzy-sounding note instead of the crisp tone you are expecting.
I never would have thought back in those days that I would eventually end up in a career where problems with a different sort of “pad” were just as important. The difference now, however, is, instead of getting a lower grade from my high school band director, I may end up with a circuit board that may not be manufacturable due to a bad pad.
How we define the pads used in the footprints of our printed circuit board designs can make or break the board when it comes to assembling it. Here are some PCB pad design guidelines that can help you to create pad shapes that will contribute to success when it comes time to manufacture your PCB.
The Importance of PCB Pad Design Guidelines
First of all, let’s define exactly what a “pad” is. A pad, also known as a “land,” is the exposed area of metal on a circuit board that the lead of a part will be soldered to. Multiple pads are used to create the component footprint, or land pattern, on the printed circuit board.
For through-hole parts, the pad is usually round in shape and will have a plated hole going through it for a component lead to be inserted into and soldered. For surface mount technology (SMT) parts, the pad will vary in size and shape depending on the component to be soldered. Sometimes, there are unusual sized and shaped pads for thermal or mechanical reasons, and small round pads are usually used for registration and alignment by automated optical equipment.
For many years after PCB design tools were introduced, the lack of automation in the tools required layout engineers to manually define their own pads. This was done by drawing the pad shapes in the design tools using information from datasheets and generic pad size and shape formulas.
This process could be very error-prone, however, as manufacturers’ specifications didn’t always adhere to the same formulas. This could result in layout designers using the wrong sizes and shapes in their pads. These incorrect pad sizes and shapes, unfortunately, could have disastrous consequences during manufacturing, including:
- Through-hole breakout: Through-hole pads must have a solid annular ring for solderability, which is the metal between the hole wall and the outer perimeter of the pad. The annular ring specification is designed to be large enough to allow for the expected amount of drill wandering from the center of the hole. If the pad is too small, however, there could be some breakout of the annular ring, and too much breakout can result in improper soldering or broken and incomplete circuits.
- Insufficient solder joints: SMT parts that have too small of pads run the risk of not getting a proper solder fillet during manufacturing. Without a good fillet, the solder joint will be weak and can break.
- Floating parts: SMT parts that are soldered to pads that are too large could end up floating out of position during solder reflow. This can cause conflicts with other parts or even shorts between circuits.
- Tombstoning parts: Smaller two pinned SMT parts, like resistors and capacitors, can have problems soldering if their pads are not the same size. One pad will heat faster than the other, and the melting solder will pull the part up and away from the other pad, sticking up like a tombstone.
- Shorts to other metal: Pads that are too small will allow surface traces too close to the parts that are soldered onto them creating potential areas of metal shorting together. On the other hand, pads that are too large could restrict trace routing between them, making it more difficult to route the board.
All of these problems force PCB manufacturers to resort to manual assembly techniques to resolve them, as well as considerably more inspection and rework time. It is essential to create PCB pads at the right size and shape, and thankfully, there is a lot of help for that today.
Industry Standards and Calculators for PCB Pad Sizes
PCB designers have many options available to them today for creating proper pad sizes. There are different specifications for pad sizes, such as IPC-7351, that detail the pad specifications that PCB designers should be using. Additionally, designers have these resources at their disposal:
- Pad and land pattern generators: Most PCB design tools today contain library wizards or generators. These software features are usually paired with the IPC standards and will automatically create the size and shape pad you need for your components.
- PCB design CAD vendor libraries: At the same time, CAD tool vendors usually have pre-made libraries of pads and land patterns you can download and use. These are usually freely available to those users on maintenance contracts for the tools.
- Third-party CAD library vendors: Designers can download pads and land patterns from third-party PCB library part vendors for a cost. In some cases, you may find the pad and land patterns you need directly available from the component vendor, which means they have been created specifically for that part.
- Pad and land pattern calculators: There are also different pad and land pattern calculators available online.
By using industry-standard parts that are either automatically generated by your tools or available to you online, you can save yourself a lot of time that used to be devoted to library creation. You will also have the assurance that your CAD parts are built according to industry or vendor specifications, and will, therefore, pass your manufacturer’s DFM requirements.
The Greatest Resource of All: Working with Your Contract Manufacturer
Another great resource for pad information or to get your questions about PCB CAD tool pad creation answered is to go to your contract manufacturer. Your CM will be able to tell you what they need in your PCB design for manufacturing success, and in many cases, can help you with layout questions or problems.
In some cases, you may have a unique pad size that requires a different shape. Your CM can help you to optimize these pads so they will be the most effective with the manufacturing process being used.