Posted on 17 Apr, 2014 by Russell Poppe
I’ve often heard Printed Circuit Boards (PCBs) referred to as the ‘brains’ of the product or the ‘heart’ of a machine. With such importance placed on their function, you would expect them to be treated with clinical care and attention at all times. Yet when it comes to panelising PCBs - the art of joining up the individual ‘circuits’ to make a ‘panel’ - care and attention levels often can be deficient.
Ignoring some of the delicate details associated with board panelisation could end up costing you. For example it could take you longer to get products built or you may end up paying more in engineering charges from your assembly partner.
Perhaps the issue comes down to a lack of ownership? It’s rarely seen as the responsibility of the Original Equipment Manufacture (OEM) designer. Their main task is to ensure the electronic ‘brain’ or ‘heart’ functions correctly and that a single circuit can be manufactured. Frequently then it is left to the manufacturing partner themselves to manipulate your data and make decisions on what panel size will be the most economical for them to produce.
But are you really happy with this? Should you take control early on and help reduce unnecessary delays and costs creeping into your business? With just a bit of input from both the original designer and your manufacturer, a balance can be struck between economical panel size and maximising product build efficiency and quality, without impacting the integrity of your data.
When it comes to PCB panelisation, here are 10 points you should consider:
- 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 details.
- Circuit boards that only have Plated Through-Hole technology (PTH) and no Surface Mount Technology (SMT) don’t always need to be panelised – single circuits can be fine.
- Decide if a peel-able solder mask is required. If it is needed then specify exactly where it should go and what material it should be made from.
- Typically there should be a ‘waste’ border of 12.5mm so they can be handled through surface mount production machines and there should also be a 2.4mm routed gap between the PCBs and waste border.
- 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 must have good contrast against their background colour so the cameras within the production equipment can accurately align to them.
- Four 4mm tooling holes in the corners, 5mm in from border edge are needed for test fixture alignment.
- For routed PCBs you 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.
- 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.
- 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.
- 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 these are not acceptable.
Finally, before asking a supplier to quote, there’s some additional information that you will need to supply. This might appear in the Gerber file, or the Bill of Materials (BoM). More frequently than you may think, this information doesn’t always get written down. This can lead to small-print assumptions from your circuit board supplier which could come back to bite you and/or your assembly partner later. For example, what is the required material, thickness, finish, solder resist colour, silk screen colour, copper weight etc?It is worth making sure all of this information is collected and clearly stated before requesting a quote, and again when you come to order your boards, to help save you time and money further down the (production) line.
Image by Laura C Hewitt