[SOLVED] PCB- mixing SMD & through-hole components.

[David_]

Hey.

I'm building a lab power supply, I've finished the schematic but when I begun to route the PCB found my self in a inferno of wires crossing over each other. I started out using all SMD but in the end I had to use some through-hole just to get the layout done, the power supply has a transformer 230V/28V-0-28V 0-12V, the 0-12V winding is being used to supply a micro-controller and some other digital stuff and the 28V-0-28V feeds a regulator circuit. Its is controlled from the uC but the analog control circuit calls for multiple regulated voltages, these voltages will have to reach pretty much all over the board and there are both positive and negative voltages. I like the idea of only using SMD but I ended up the SMD bypass capacitor for various ICs with a small through-hole cap just to use the legs to pull up the relevant voltage from the bottom layer, the top is occupied everywhere and does not allow for the voltage rails to be routed and i ended up slathering the bottom ground-plane with voltage rails going like a maze to get them to all the places they need to be. I would feel ashamed to show anyone the finished layout, looks like it has been done by a drunk squirrel but I'm really rushing to get a working prototype going, all experience I have comes from my own DIY circuits and if nothing ells I've conclusively proved that this type of sloppy routing is from start to end a bad idea. But I wonder what the performance impact could be, I'm sure I've created a ground-loop de loop paradise and I thinking of dumping this design and make a new relying on cables to jumper voltages over the board. But i would like to ask a few questions that i have been reasoning about for some time.

There are a I2C bus that stretches over the hole board but aside from that is there any down side to have cables running over the board with steady voltages?

How about a I2C bus going from one IC to the next, to the next, to the next, in the and jumping pretty much all over the board?
Is there any point in using one of those mini coax cables? I have no idea what there name might be but a coax that has a outer diameter of 3mm or something.

I'm prone to use 1206 resistors and capacitors instead of 0805, they are i little easier to solder but I like having the gap between pads to route small traces, are these "under 1206 packages routes" a bad idea or is its just fine?

What about routing between two holes of a through-hole capacitor?

I don't like the idea of using lots of vias to for exampel go from top to bottom, straight line 1cm and then back up to the top again. There would be a lot of that down and under strips, but would it have any impact on the circuit function? Enough to care, its a linear power supply with a isolated digital uC board. I don't know but i imaging that in such a low frequency situation that kind of board layout would have marginally noticeable consequences.

If routing happens to produce cuts in a ground fill as to produce a kind of maze with many narrow passages, is it a good idea to use vias in remote areas of the maze to areas of ground fill on the other layer with wider openings in its ground fill?

Is it a odd practices to occasionally use a through-hole resistor instead of a SMD motivated only by opening up and sneak traces under it?

Regards

 

[crutschow]

Usually when you have a problem routing trances you add a layer to the PCB. If you are stuck with two layers, then you may have to do some of the tricks you mentioned. If you can, use a rats-nest wire layout to initially determine how best to arrange the parts to minimize the crossovers and vias required.

It's perfectly fine to route underneath surface-mount or through-hole parts as long as the minimum trace width and spacing rules are met.

If you cut up your ground plane too much then it can loose its effectiveness as a ground. Can you flood the top of the board and use that as a ground plane also?

 

[FvM]

I think your questions can be answered by studying the layout of existing electronic devices.

 

[David_]

I'm stuck with two layers though, the best arrangement that I could find was not good enough but I have changed the design to use cable-jumpers to distribute the supply voltage rails and that solved many inconvenient routing solutions.

I use to have ground fills on both sides and then if needed I could put a via or something to connect the two planes, aside from the "main" connection near the power supply. But I realize that i should have some ground scheme planed from the beginning and not just see how it goes and make the best of the situation.

I have looked at lots and lots of produced PCBs, it started out as scavenging but turned into a great learning opportunity. But for some reason I have not observed the kind of routing I asked about, I suppose that if one could use more than two layers.

Thanks for the advice.

 

[FvM]

Some time ago you saw consumer elecronic products, e.g. PC graphic cards that had been hand routed on two layers to cut productions costs. There you could find all kinds of routing tricks mentioned in your post. I'm not sure if these boards easily passed EMC tests (or had been seriously tested at all).

As IC package pin and circuit desnsity increases, two layer boards become rarer. Also higher circuit speed and EMC requirements often demand for continuous ground planes which aren't feasible on two layers.

 

[Jiadong Yao]

Hi, I am going to design a equalizer this semester. Can I use SMD components and through-hole components on the same PCB?

 

[crutschow]

Hi, I am going to design a equalizer this semester. Can I use SMD components and through-hole components on the same PCB?

You can use any combination of types that you want. Will the board have plated through holes?

 

[Jiadong Yao]

You can use any combination of types that you want. Will the board have plated through holes?

We will use eagle to draw the schematics and make the layout there. then we hand in all the material and the coach will contact certain company to make PCB for us.

If my PCB is not good the company will return it thus i will get a low grade at last. So it is better ask it here.

what's more, we have to find the correct components. I also have questions about selecting components. We will buy the components on Farnell. But sometimes the components in the eagle library do not correspond to the components on the website. And the library has little description about the components. I feel it is really hard work.

Could you give some suggestions regarding selecting components and finding the corresponding one?

 

[crutschow]

...........................
what's more, we have to find the correct components. I also have questions about selecting components. We will buy the components on Farnell. But sometimes the components in the eagle library do not correspond to the components on the website. And the library has little description about the components. I feel it is really hard work. .............

Welcome to engineering. If it were easy they wouldn't need engineers. ;-)

Getting the correct layout pattern for the components your are using is a big source of errors on PCBs so double and triple check that the layout is actually correct for the part you are buying. Few things are more embarrassing on a board then to find the part you ordered doesn't fit the layout you used. Often semiconductor parts come in different packages and you need to be sure the ordered part number is for the package you laid out.
Also make sure the package and layout dimensions are correct as sometimes packages may look the same but they actually have different pin spacing or package size.
And, for through-hole devices, make sure the hole is the proper size for the lead diameter. Another egg-on-your-face is finding that the component lead won't fit through the layout hole. For example, some rectifier diodes have leads that are significantly larger in diameter then typical resistors or diodes, to better dissipate heat through the circuit board connections.