6 Horribly Common PCB Design Mistakes

[exxos]

[UnderTheRain]

Electronics is my day job and I see new customer PCB's I would say multiple times a week as one of my tasks is vetting the customer data and programming the PnP and AOI machines and here are some of my observations.

People aren't taught PCB design they are taught electronics and learn PCB design by doing and most of the time not very well.

A lot of professional boards are very badly designed.

If you really MUST stick SMD components right along the edge make sure your boards are in a frame or they will not go through the machines.

People do not design for production (Fiducials are your friends use and abuse them, put them on the PCB put them on the frame if they are in a panel put them on large BGA's and QFP's/QFN's they don't cost anything and they are super helpful)

People don't think about repairs or points of failure and stick 0201's between two 10mm coils shielding the 0201 from the reflow oven this one I'm sure they do on purpose. Stack high things as close as you can on your design will make them harder to solder.

There is an increasing trend of lets use the smallest components we can find (0402 bga's anyone? yep about a month ago we built a product with 0402 BGA's WHY!!!!! same goes for 0201 components when half the board is empty and lets put them on a massive ground plain just for the hell of it.....) and leave 90% of the board empty.

Lets not properly test the design lets just sim and then when we design it we can leave essential bits off.

Let's design our board using parts that went obsolete 20 years ago (no seriously again we see this a lot).

Let's not talk to the people who build, test and repair boards to see if there are any design issues lets just make them and work it out later.

Let's use all black components with no legend because its fun. (This isn't really a problem building just if something doesn't work fml we audit the reels of components when they are on the machines and do first off's to ensure there aren't mistakes).

Lets design a circuit that relies on accurate capacitor values to function (some caps are super tight on tolerances and stability and some are horrendous 20% some of them).

Lets send the manufacturer a BOM and a overlay drawing but no Gerber's/design files/pick and place data they can sort that out.

Let's change BOM's 5 times and don't tell anyone :)

There are many many more issues but I'm going to sit in a dark room with a towel over my head for a but to calm down.

When laying out a board please don't rotate a component 360 degrees or a passive 180 or 270 because when the pick and place picks that part it spins the head one complete revolution to get back to where it started before placing and this takes time (ok not much for one component but when you have a 1000 and 10,000 PCB's it adds up and yes we change them back.

Put you 0,0 reference point actually on or in the corner of your actual PCB not 15 feet away all PCB design software software lets you reposition the 0,0.

DO NOT USER A HOLE AS YOUR 0,0!!!! Put a fiducial if you can

I have to agree with the chap in the video with points but most of the time we see people with antenna's these days they are using off the shelf modules so all of the work to make those function is already done. An I think trace length matching isn't really a problem below 100MHz which for PC ram is nothing but very very very very very few PCB's run those speeds they just don't need that power I would think 50% of the boards we make have processors of some description on but you rarely see anything over 25MHz unless its radio which we do a little of. But then again that could just be down to the design environment this chap works in. Were a small fab that specialise in small to medium productions runs, prototypes that being said we do have customers that buy thousands of boards monthly but we can flex to do anything which is why we see so many designs larger fab's wouldn't touch a run of 10 dev PCB's whereas will do 1 offs to as many as you can pay for :D and we work for other fab's quite often.

Thanks

Brian :D

[exxos]

Yeah all good valid points.

People think as long as point A connects to point B then all good :roll: Autoroute it all and make a huge mess.. zero pride or thought as well. Use larger as possible PCB area as possible and squash parts into small area which hard to get to.

I don't get people's obsession with small parts. Once you get below 0805 in capacitors you might as well not bother fitting them as they lose value. Don't get me started on how designers don't understand inductor saturation either. The list is endless.

I think it's in part why most electronics items don't work properly anymore. Nevermind cheap china construction. I've ranted on about it all already over the years.

[UnderTheRain]

The chap in the video also suggests having both supply plains internal you don't see this very often usually its from top to bottom data, positive, data ground on the back is more common and linking ground traces for 99% of applications is fine you do not need to route them all back separately yes 'floating grounds' is a thing but I haven't heard anyone talk about it since I was at college and that was a lifetime ago (1993) it was more of a problem if you were grounding chassis in multiple places for panels and the like you don't really have any issues on modern average sized PCB's with it (and just for reference we build a PCB for a distribution system that about 2ft x 3ft+ in fact I'm sure it's even bigger than that I've never actually measured it). I think some of the issue this chap see's are probably very specific to his environment maybe he works of a PC parts maker or radio maker with a lot of high frequency but for anything else it's a non issue. We have one customer that must put VIA's on his ground every mm I've no idea why I have asked why he does it but we've never had a reply.

Just looking online there seems to be every combination of layer configs it seems a bit like the old wild west everyone seems to have a different opinion I guess it's down to what type of board you want to make and how you want to make it. There are recommendations for different effects (decoupling optimizations etc) I guess most people will just go with what they feel like or what their software defaults too for most boards it makes little difference.

This has a good article allaboutcircuits . com/technical-articles/common-layer-stack-ups-for-a-four-layer-board/ (not sure if we are allowed to post links)

I think the improved two stack implementations will be better for most things.

Brian

[exxos]

The H5 uses internal power planes and others do the same. Though mostly it's to allow access to the tracks on top and bottom layers. I've had to junk a batch of ST536 boards because there was a short on a internal track which wasn't fixable :( But also you get distributed capacitance across the power planes if they are internal.

Data lines on internal layers can be screened with power planes if they are on the top and bottom layers. Obvious advantages. But it has side effects of loading capacitance on both power rails. It's can help snub noise or slow your signals and increase current consumption. Once you get into the MHz ranges things quickly add up. Most designers don't understand high frequency currents. They seems to design based on DC type circuits and you can't create a high frequency PCB based on a DC based set of design rules. It all gets very complicated.

It's all swings and roundabouts unfortunately. There are always multitudes of design compromise to be made in one respect or another. PCB designers need to be aware of all of this but information is pretty bad.

[UnderTheRain]

You realise you've started me watching eevblog videos on youtube I have 6 queued atm I'm going be here for hours lol

One of our guys had to buy some 1mm drills a month ago to remove a short on an internal layer on a first batch of boards. I'm just glad it wasn't me :)

Brian

[exxos]

Have fun ;)