Todays world of high speed, high pin count high power density electronics means designing multilayer PCB’s is almost essential for any design. Multilayer boards enable a designer to be able to integrate controlled impedances for high speed and RF circuits with power circuits easily and efficiently without taking up large amounts of space and eliminating the need to use jumper links to make connections.
But as engineer’s knowing how our multilayer boards are built is key to ensuring design success. Knowing the materials that will be used at the fabricator, so the right cores and pre-pregs are used is are particularly important for impedance control. Allowing enough time and effort to work out the number of layers required and their stackup will be time well spent. While this can be changed during the design, it may mean that other design elements will be affected and time wasted in re- design to account for this. So it needs to be planned well.
So, how does a PCB Designer work all of this out?
Layers – Deciding on grounding and power strategies will give us the best guide on the layers we need. Ask these questions:
1)How many ground planes needed? Are there different ground on different planes or are they split?
2)How many power rails are there? Is a layer being allowed for each power rail or are we splitting them? Will they be split into area above each other to stop ground bounce?
3)What and how many signals are there to route? Will there need to be signals routed on inner layers and will these need to be protected from the power layers?
4)Just a plain through hole board or are blind / buried vias being employed?
The plane arrangement is typically number of planes one on top of the other on different layers. They will be a mix of power and ground nets. This is an acceptable way to arrange power and ground planes.
However in order to minimise ground bounce in a circuit, the power and ground planes could follow the same shaped profile on each layer, this reduces the coupling between power and ground. It could be any shape, but would follow the same profile on each layer for power and ground.
For ground planes it’s generally accepted that splitting planes creates signal integrity ‘gaps’. While sometimes it’s unavoidable to split a plane, if possible it’s best avoided.
Once the basis for the multilayer board is decided, talked over with the fabricator, the placement and routing can begin in earnest. Placement is one of the key elements in achieving design success – get it right and you’re a big step towards making routing much easier. The big question here is – will there be components on both sides of the board?
Routing is next and is one of the elements of board design that can go wrong if not carried through methodically:
1) Ground comes first – always. If the ground connection is being made to an inner layer on a via, make the track thick – around 0.5mm and keep the via close to the component, particularly on decoupling components. For larger components make multiple ground connections, to give a low impedance connection.
2) Power – don’t underestimate this, get it right to avoid issues later, think about the power path. For example make sure the power connection flows from the plane to decoupling to component in that order. Not the other way around.
3) Controlled Impedances / Sensitive Signals – These signals are next in priority. The signals’ sensitivity will dictate it’s priority.
4) Everything else in it’s own natural order.
Following these steps should ensure the right circuit elements are routed with the right priority to achieve design success!
Finally, once all the connections have been made check the copper balance. Area’s of different copper density could cause the PCB to warp during fabrication. Filling in the blank area’s with ground pour where this won’t affect function or safety will ensure a uniform copper density and flat boards.
This of course is a list of the basic elements of multilayer design, but if the designer thinks along these lines and how the circuit will work before routing a single track – the path to success is less troublesome.
© Circuit Mechanix 2016