Pad size and via diameter dimensions consideration in PCB prototyping

When designing pcb prototype, pad size and via diameter dimensions should be appropriate. If the pad is too large, the solder spreading area is larger, the formation of the solder joint is not full, and if the pad is too small, copper foil surface tension is too small, can bring up cold solder joints.  

Through hole and component pin with too large gap, easy to cause pseudo soldering, when the via hole diameter is 0.05 ~ 0.2mm bigger than the component legs, is the more ideal conditions for welding.

According to the pad requirements to make pcb prototype is to achieve the minimum diameter, the diameter of at least 0.5mm larger than the maximum diameter of the solder terminal.  

Must be in accordance with ANSI/IPC 2221 requirements for all nodes to provide test pads. A node is an electrical connection point between two or more components. A test pad requires a signal name (node signal name), an x-y coordinate axis associated with the reference point of the printed circuit board, and the coordinate location of the test pad (indicating on which side of the printed circuit board the test pad is located).

The aspect ratio of the plated through-hole has a significant impact on the manufacturer's ability to effectively plating in the plated through-hole, and is also important in ensuring the reliability of the PTH/PTV structure. When the hole size is less than 1/4 of the basic board thickness, the tolerance should be increased by 0.05 mm. When the hole diameter is 0.35 mm or less and the aspect ratio is 4:1 or greater, the manufacturer should use a suitable way to cover or block the plated through-hole to prevent the entry of solder. In general, the ratio of printed circuit board thickness to plated via size should be more than 5 : 1.

Fixture information for SMT testing is required, as well as techniques for printed circuit board assembly layouts to facilitate testability in the circuit with the help of "in-circuit test fixtures" or "bed-of-nails fixtures" as they are often called. In order to achieve this, it is necessary to:

1) The diameter of the test pads dedicated to probing testing should be no less than 0.9mm.

2) The space around the test pads should be greater than 0.6mm and less than 5mm. If the height of the component is greater than 6mm, then the test pads should be placed outside the component 5mm.

3) Do not place any components or test pads within 3mm from the edge of the printed circuit board.

4) Test pads should be placed in the center of a 2.5mm hole in a grid. If possible, allow the use of standard probes and a more reliable fixture.

5) Do not rely on the edge of the connector pointer for pad testing. Test probes can easily damage gold-plated pointers.

6) Avoid plated through holes. Probing on both sides of the printed circuit board. Place the test tip through the hole onto the non-component/solder side of the printed circuit board. This approach allows the use of reliable and less expensive devices. The number of different hole sizes should be kept to a minimum.