As is always discussed, how a PCB fabricator makes the PCB’s that you design can make or break a project. The same is true with reliability, knowing what to ask and what’s needed is most of the battle. If we’re hoping to create a circuit that is going to operate reliably in harsh conditions, what is it that we need to be looking for in our fabricator and asking them to do?
Finding the higher grade PCB materials for use in yours boards is all very well – but can your PCB fabricator get them and use them? Some of the cutting edge materials can be very hard to get hold of, or can have a long lead time attached to them. Similar products are often available from a fabricator if your needs are discussed, who knows, perhaps even something better.
With the materials decided, the circuit has to be manufactured in a way that will enhance it’s reliability. As always, it depends on the application, but if we’re going to consider harsh environments and temperature changes the considering the type of copper will make a difference.
Typically two types are available on PCB laminates, Electro Deposited (ED) and Rolled Annealed copper (RA). Both processes are self explanatory, but rolled annealed will typically be more robust as it’s a sheet of thinly rolled copper presses onto the laminate surface. These sheets will also be used when making inner layers on multilayer assemblies and bonded between layers of prepreg and core materials. It’s not generally known, but a tiny amount of lead is added to increase it’s durability, but not enough to break RoHS directives.
Electro deposited copper will be more fragile and fractures are more likely to be made when subjected to mechanical stresses such as thermal expansion and contraction.
The most likely part of a PCB to fail when subjected to thermal stress are the via’s. The z-axis will expand and could fracture via’s. The via wall thickness will thin in the centre, creating a weakness that can break in thermal cycling.
A fabricator will typically specify the thickness of via wall they’re able to manufacture. Specifying a minimum via wall thickness of 25um will reduce the risk of via fracture considerably. There is no way to eliminate this completely of course especially in cases of extreme temperature change.
The quality and accuracy of via drilling also has a distinct affect on via reliability. Drill bits need to be replaced a regular intervals to ensure the drill is sharp when every via is drilled. A blunt drill will create a rough via wall that could fail, especially when stressed.
Drill accuracy is how on target every via has been drilled. On larger via’s an pads it’s easy to see this, but on smaller.
The condition that needs to be avoided is called the keyhole effect (as shown above) where the drill hole is not on target and can create a potential weakness and failure effect where the track leaves the bad or via.
It needs noting that no via hole will ever be spot on – but it needs to be within the stated manufacturing tolerances of the fabricator. The IPC-A-600 standard will give guidance on the acceptability criteria of this an many other manufacturing defects that may arise from a fabricator. It will also give an idea of the acceptability criteria to specify when asking them to supply boards.
Circuit Mechanix © 2016