Profiling Printed Circuit Boards
It seems an obvious statement that once a board is finished, it needs to be shaped to suit enclosures but as usual in the manufacturing of any product, there are options which can impact heavily on the cost.
The cheapest and most cost effective shape is a rectangle and this is because it fits on a production panel which is rectangular and generally yields the best utilisation. Any change or addition to a rectangle, so cropped corners, slots and cut outs, radii or bits that protrude will all add to the cost and limit the process options.
Profiling is about mechanical forming. When circuits first appeared, profiling on a table saw with a tungsten blade was common. Once cut, the boards were usually linished to give a smooth edge. You can appreciate that the accuracy is very operator dependent and the repeatability a bit hit and miss.
As enclosures became more designed for the PCBs, the accuracy was required to allow a snug fit. Most boards were then profiled using NC and later, CNC routing machines. Solid carbide drill blanks were profiled to give a cutting edge in the lateral plane hence giving a fast and accurate edge which was smooth. Generally, in volume production, the PCBs are left in the panel attached in a few places with ‘nibs.’ This allows the product to be assembled in a panel then cut out afterwards. The downside is the nib needs careful cutting and finishing normally with a file. There are influencing factors that need to be compensated for, height of stack, speed of cut, direction of cut, wear on the cutter, location to bed, all in the set up and programming.
The next option on a rectangular board is scoring.
Scoring is very popular and is a very quick process. Basically, it is 2 cutting wheels, usually about 100mm diameter and 2mm thick, edge to edge with a gap in between. The board is driven through that gap using CNC controlled guides removing material from the top and bottom of the board. The web left is strong enough to go through the assembly process after which a bending motion will snap the boards out. This is a very fast profiling method and allows the PCBs to be laid edge to edge so gives the best panel utilisation. It is limited to cutting straight lines and recently, it is being heavily used in the aluminium backed PCB market.
Widely used in the Far East, punch profiling is very common. The companies in the Far East prefer this method but in the West, the cost of a press tool prohibits the process for all except very high volumes. A typical tool may be £6 to £9k. A piece of tool steel is profiled exactly to the board with allowances for the shearing of the material, a very skilled job requiring a press tool designer, then placed onto a tool bolster. It then goes into a power press, a mammoth machine with a flywheel up to 4 ft diameter and weighing many tons.
The boards are placed into the tool mouth using a pin location and the press fired. The energy in the fly wheel is transferred into a vertical ram which hits the board like a hammer with the correct profile shearing the board out of the panel. Very quick and especially useful for sophisticated profiles like car dash boards and the like. They can also be designed to pierce out slots and non standard shapes at the same time.
Although very fast, this is a very dangerous process and I personally have been with people who have lost fingers on these machines despite numerous safety guards and procedures.
This again is just an appreciation of mainstream profiling. There are many variants including laser stitch cutting, and even within the press process there are options to return the PCB to strip even punch holes if they are non pth.
If you have any queries I will be happy to answer them if you want to email me.
Until next month, enjoy the season and I will be putting bare board testing under the eyeglass… unless anyone has a better topic.