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PCB pad design for machined interconnects

Introduction:
Specifying the correct printed circuit board pad, land and, hole sizes are critical to the device mechanical attachment of a machined pin or socket.

Through-hole PCB designs come in two flavors: plated and unplanted.

A plated hole provides a path for solder wicking due to capillary action.
The tolerance of the final plated hole is a function of the starting drill size and the plating thickness.

To soldering a pin lead:
A) Determine the maximum pin diameter including manufacturing tolerance. On some specifications, the pin diameter may be potentially 0,05mm larger than the nominal diameter.

B) Specify a minimum finished plated hole that is at least 0,05mm larger than the maximum pin diameter. The goal is to achieve at least a minimum of 0,05mm clearance between pin lead to the hole wall to promote solder wicking up the plated hole.

For press-fitting a pin into a plated hole:
A) Any press-fit geometry that is plunger-like (full diameter feature) or aggressive cutting (barb or knurl) is not intended to be pressed into plating. These features will either ream the hole or create connection voids, both essential ruin the PCB.

A press-fit feature must either be compliant to the plated PCB hole or if solid, it must provide contiguous paths of copper from the bottom to the top of the plated hole. An example of a solid polygon pin geometry is a square post. Follow the pin manufacturer’s guidelines for specifying the starting drill size and the plating thickness.

A drilled non-plated hole in FR-4 is very accurate as it is controlled by the drill tolerance, however, it can become rough and uneven due to excessive drill wear.

Typically, the tolerance of a drilled hole is +0 / -0.02mm (the tolerance of a new drill)

A non-plated hole is useful for non-electrical applications such as a plastic positioning peg. If a conductive pin is press-fit into a non-plated hole, it will be necessary to tack solder the pin to a top or bottom layer pad to tie it electrically to the PCB.

A PCB pad is a uniform round, square or rectangular shape.
Typically, the minimum PCB pad land is a minimum of 0,127mm annular ring around the SMT pin base feature. This allows a solder fillet to form and keeps the part accurately positioned.
When pin spacing is reduced by a nearby component or board feature, then the PCB pad can be of an irregular shape.

Summary:
When specifying a finished plated through-hole you must determine the largest possible pin diameter (considering maximum tolerance) and the minimum finished hole tolerance. You need to have at least a 0,05mm clearance between the two to allow for solder wicking.

Solder pads are typically symmetrical but when a component or object is interfering with your layout, you can design a non-symmetrical solder pad.

pcb pads for article.png


Here is a link to an article:
https://drive.google.com/file/d/16556QXjh4Vo_gFeu2Q84oYGigODArrb2/view?usp=sharing

If you have any questions on these concepts please feel free to ask.

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mhoulroyd
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