Just returned from a holiday catching some sun before the winter sets in. One thing with running your own business is that even on holiday things need to stackup, you want to be contactable just in case there are some questions. I took with me a laptop and my mobile, mobile for the email monitoring and calls and the laptop to access anything that required investigation. Thankfully, with good planning and communication, we did not need to breakout the laptop! But this got me thinking about the ease with which I could stay in touch with everything using a portable hand-held device, making calls from another country, keeping in touch with Microsoft® Teams/Slack/GotoMeeting (other messaging systems are available) Note: There is always a comparison at this stage about the computing power of a smart phone and the Apollo lunar missions; you just need to have seen the Apollo 13 movie to have an appreciation of how far we have come.
The Speed of the electronics industry has progressed
So, as I was pondering the speed at which the electronics industry has progressed, I considered how as a PCB designer, the major focus of any step forward is the latest chipset while the humble PCB is hardly mentioned at all.
The PCB substrate is, at times, an almost after thought and this should definitely not be the case! For a long time, the industry has recognized that at certain frequencies the PCB is a major electronic component affecting signal quality; but another aspect that is not always considered is the manufacturability of the substrate itself.
We see a number of designs using different technologies and each one has different requirements from the build of the Printed Circuit Board. The overall technology of the design is the main driver; whether the board contains RF, Digital, Analogue, Power, Microwave or a mixture of some or all of them. The second is then the type of components being used, especially the pitches of the devices.
PCB Strategy for Stackup
Here is a short list of some the points that we consider with our PCB Consultancy, when we look at the PCB and start to build a strategy for the stackup, and therefore the layout. (This is not an extensive list but just an illustration of the types of data to be considered)
- Impedance requirements
- Digital – USB, CAN BUS, Ethernet etc.
- RF– CPW or Non-CPW
- Microwave – fine feature sizes
- Power requirements
- Are there specific requirements for current carrying capability
- Via sizes required
- PCB/or &/ Electronic components used
- If uBGAs are used what are their pitches and how many pins does it have
If we consider a simple case of a digital signal based design which is using a fine pitched component, e.g. 169 pin 0.5mm pitch BGA, we have a couple of requirements to accommodate. Firstly, we have to have a PCB stackup with a series of plane and signal layers that will enable us to track in the impedance traces, lets assume we need 100 Ohm differentials and 95 Ohm differentials. For a standard through hole technology this would be quite straight forward, but now if we consider the design requirements imposed by the 0.5mm pitch BGA, we will have to move to a stackup that includes the use of uVias. Two effects of this are firstly cost; there will have to be multiple press-cycles and additional drilling and plating operations and secondly, the thickness of the pre-pregs will be much thinner which impacts our impedance calculations for the outer layers. So, faced with this the options are to have the tracks thinner for the differential pairs, increasing losses, or to use another reference plane further away.
As a PCB Design consultancy, we do not see too many standard ‘PCB Stackups’, mainly owing to the diversity of the requirements within the broad electronics industry. This means that for each design we spend more time planning and talking to the fabricator (see our PCB Board Fabrication Guide) to ensure that we satisfy all the requirements than we have done previously.
So, as I lay on my sun bed, my conclusion is that the latest silicon enables products to be faster with more features and functions, while at the same time creates more challenges for the PCB designer and PCB fabricator when defining the stackup.