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Vias and PADs
Vias and PADs
What is the difference between a via and a pad? what are the Pad sizes, shapes and dimensions will depend on ?
Vias connect the tracks from one side of a double-sided board to another, by way of a hole in the board.
On all but cheap and low-end commercial prototypes, vias are made with electrically plated holes, called "Plated Through Holes" (PTH). Plated through holes allow electrical connection between different layers on your board.
What is the difference between a via and a pad? Practically speaking there is no real difference - both are electrically plated in the "electroless" process but vias are subsequently hidden by the solder mask. So there are differences when it comes to PC board design packages. Pads and vias are, and should be, treated differently. You can globally edit them separately and do some more advanced things to be discussed later. So don't use a pad in place of a via or vice-versa.
Holes in vias are usually a fair bit smaller than component pads, with 0.5-0.7mm being typical (although they should be larger when they need to carry substantial current).
Using a via to connect two layers is commonly called "stitching", as you are effectively electrically stitching both layers together, like threading a needle back and forth through material. Throw the term "stitching" a few times into a conversation and you'll really sound like a PC board professional!
Pad sizes, shapes and dimensions will depend not only on the component you are using but also the manufacturing process used to assemble the board, among other things. There are lots of standards and theories behind pad sizes and layouts and these will be explained later. Suffice it to say at this stage that your PC board package should come with a set of basic component libraries that will get you started. For all but the simplest boards though, you'll have to modify these basic components to suit your purpose. Over time you will build up your own library of components suitable for your own requirements.
There is an important parameter known as the pad/hole ratio. This is the ratio of the pad size to the component lead hole size in that pad. Each manufacturer will have a minimum specification for this. As a simple rule of thumb, the pad should be at least 1.8 times the diameter of the hole or at least 0.5mm larger. This is to allow for alignment tolerances on the drill and the artwork on the top and bottom layers. This ratio gets more important the smaller the pad and hole become, and is particularly relevant to vias .
There are some common practices used when it comes to generic component pads. Pads for leaded components like resistors, capacitors and diodes should be round, with around 70 thou diameter being common. Dual In Line (DIL) components like ICs are better suited with oval shaped pads (60 thou high by 90-100 thou wide is common). Pin 1 of the chip is commonly a different pad shape, usually rectangular, with the same dimensions as the other pins.
Most surface mount components use rectangular pads (with circular ends) and the pads should not be any wider than the component itself. Surface tension of the molten solder is an issue and if the wrong pads are used, surface tension can pull the component off line or even upright.
Other components that rely on pin numbering, like connectors and SIP resistor packs, should also follow the "rectangular pin 1" rule.
Octagonal pads are seldom used and should generally be avoided. As a general rule, use circular or oval pads unless you need to use rectangular.