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Introducing Quality into PTH Assembly

21st August 2014

Plated through hole or PTH assembly is a method of connecting between track layers on a printed circuit board (PCB). The holes are first drilled or punched and them metallised.

PTH assembly provides binds that are far stronger mechanically than those made with conventional surface mounted technology (SMT). However, the downside is that extra drilling is required and the circuit boards are more expensive to produce as a result. The circuit boards became too large and too heavy for the smaller and lighter electronic devices under development.

But the reliability of solder joints in SMT technology quickly becomes an issue as the hole sizes contract over time. It is also unreliable for connections that undergo any amount of mechanical tension, especially in large capacitors and other electronic installations.

The PTH assembly method has an advantage over SMT methods when used for heavy and bulky components, especially in conductors, connectors and relays used in power-generation technology.

A number of factors affect the reliability of such assembly methods. Drilling or punching is obviously a key factor. But the method of choice depends on the thickness of the circuit board, the diameter of the hole to be drilled and the resin material from which the board is made and the capability of the drilling machine.

Firstly, the operator must decide on the feed-in rate and speed of the drilling machine. Too great a feed-in rate may result in rough-edged holes or drilling bit breakage.

Secondly, the rate at which the drill spindle rotates also affects the quality of the hole wall. High speeds will generate heat that causes smear on the hole wall, as well as wearing out the drill bit.

Thirdly, minimising smear is always a priority. But if speeds are too low, the drill bit spends more time inside the material and so generates more heat. This is where a parameter called 'surface feet per minute' (SFM) comes into play.

SFM is defined as the distance within the board material that the tip of the drill bit moves over a period of time. The SFM calculation depends on the spindle speed in revolutions per minute and the diameter of the hole to be drilled.

Quality in PTH assembly methods depends first and foremost on the quality of the drilled hole itself. It is important to understand any defects in the drill being used as well as the characteristics of the resin material of the circuit board.

Please feel free to contact us for more information.