Posted on 17 Apr, 2014 by Russell Poppe
You'll often hear Printed Circuit Boards (PCBs) being referred to as the ‘brains’ or the ‘heart’ of the machine.
And, with such importance placed on their function, you might well expect them to be treated with clinical care and attention.
But when it comes to panelising PCBs - the art of joining up the individual ‘circuits’ to make a ‘panel’ - you can often find that the levels of care and attention levels are less than desirable.
The problem is, choosing to ignore some of the more 'delicate' details associated with board panelisation can be costly. It can end up taking you longer to get your products built. And you could find yourself paying your assembly partner more than you'd expected in engineering charges.
Often the root of the problem is just simply 'lack of ownership' on the part of the Original Equipment Manufacturer's (OEM's) designer.
The designer's main task after all, is to ensure that the electronic device functions correctly and that a single circuit can be manufactured.
More often than not though, it's then left to your manufacturing partner to manipulate that data and to make the decision as to what panel size will be the most economical to produce.
The question is - is this really the best approach for your product? And if it's not, then perhaps you'd prefer to take control a little earlier on to prevent unnecessary costs and delays creeping into your business?
The good news is, with just a little bit of input from both the original designer and your Electronics Manufacturing Services (EMS) partner, a balance can easily be struck between achieving an economical panel size and maximising product build efficiency and quality - and all without impacting on the integrity of your data.
So, when it comes to PCB panelisation, here are 10 points to keep in mind:
- Size - There will be a maximum size for the panel. It needs to fit the surface mount production line equipment. This is typically around 400mm x 500mm, but your assembly partner will be able to provide you with the exact details.
- Efficiency - Circuit boards that only have Plated Through-Hole technology (PTH) and no Surface Mount Technology (SMT) won’t always need to be panelised – single circuits are often fine.
- Materials - Decide if a peel-able solder mask is required. If it is, then specify exactly where it should go and what material you want it to be made from.
- Waste - Typically there should be a ‘waste’ border of 12.5mm so they can be handled through surface mount production machines. There should also be a 2.4mm routed gap between the PCBs and the waste border.
- Alignment - There must be three 1.2mm round pads on the panel border, and on each individual circuit board (if possible) - two on one axis and one on the other. These are called fiducials and are invaluable for aligning surface mount machines, automated optical inspection and so on. The fiducial pads will need to have good contrast against their background colour so the cameras within the production equipment can accurately align to them.
- Tooling - Four 4mm tooling holes in the corners, 5mm in from border edge will be needed for test fixture alignment.
- Routing - For routed PCBs you'll need to ensure that appropriate pips are used, ideally internal ‘rat bites’ to provide a smooth finish once the boards are broken out of the panel. These should be equally spaced out to avoid the board flexing and warping. The PCBs should be partially routed using a 2.4 mm -0/+0.1mm diameter cutter and supported by 2.5mm pips, placed between 10mm and 15mm from each corner. Further support pips should not be less than 25mm, and no more than 100 mm apart, except where this may interfere with component assemblies that extend beyond the board profile. There must be a minimum gap of a 20mm on, at least, one side of every pip. This measurement is paramount and overrides the 10 to 15mm measurement from each corner.
- Cutting - If the PCBs are separated by scoring rather than routing, pads and traces must be at least 5mm away from the edges of the score line, to allow them to be cut from their panel.
- Scribing - If the board has scribing on, it should be equal from both sides, giving a 0.4mm web with a tolerance of ±20% about the centre line. Extended wastage on the short edge should be added to allow jump scribing.
- Bad circuits - Don’t allow ‘cross outs’, i.e. known bad circuits in a panel. These cause lots of headaches, so make sure your bare board supplier understands they're not acceptable.
Before asking any supplier to quote, there will be some additional information that you'll need to supply. This might appear in the Gerber file, or the Bill of Materials (BoM).
More frequently than you may think though, this information doesn’t always get written down - which can lead to small-print assumptions from your circuit board supplier which could come back to bite you and/or your assembly partner later.
Some points to consider could include: what is the required material, thickness, finish, solder resist colour, silk screen colour, copper weight etc?
Making sure all of this finer detail is collated, and clearly stated, before requesting a quote, will help save you precious time and money further down the (production) line.
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