Electronic – How to deal with crossing wires when designing a PCB

pcbpcb-fabrication

I'm finally dipping my toes into PCB design as I got around to finishing up my schematic for an upcoming project that I'm working on.

If it helps in finding a solution to my problem, this PCB will take a mixed signal approach. If not, please ignore it.

The issue at hand is essentially: how can I deal with rat lines that are crossed over each other? The only solutions I see is jump the trace from the top layer to the bottom layer, or redo the footprint from the IC.

It doesn't matter how many times I rotate or organize other components. The ratlines are always crossed over on each other.

I'm currently using EasyEDA as the schematic and PCB editor using the community's footprint design, so I am not sure if that's where I went wrong.

Be warned. This is my first time attempting a PCB design, so what can I do to improve?

Here's a picture with what I'm dealing with.

This is not the final product as I'm stuck on the left side of the board where the microcontroller is.

Best Answer

You can use vias to move a trace from top to bottom and back again. That's like your "jump the trace from top to bottom solution." That's the usual way of handling crossings - make one trace go below the other.

What you really need to do is to move your parts around and rotate them to minimize the number of crossings before you start routing.

Use the entire bottom side as ground. Try to keep it in one large piece. Anything you run on the bottom to get around a crossing should be as short as possible.
You have audio and fast digital stuff on there. Try to keep the traces for the two types of signals away from one another.
Group your parts so as to minimize crossings, even if it offends your sense of organization/aesthetics.
Group your parts so as to keep digital stuff and analog stuff separate. When in doubt, keeping analog and digital seperate is more important than minimizing crossings.
Use vias to connect all ground connections straight down to the ground plane on the back side.
You can pretty much ignore the ground connections until the very end. Route all of your signal lines and power traces, then pour the ground plane on the back. Drop the ground connections to the ground plane with vias.

Related Solutions

Electronic – PCB prototyping with Presensitized copper boards

Not sure about exposure time (for my lamp).

Not sure about UV light position and height required.

With these you just have to experiment, I'd suggest putting the board fairly close to the lamp (20-50mm or so).

I suggest you take one longish strip of photoresist PCB and put something on top of it that blocks UV light, then expose the strip while moving the UV blocker away at predefined intervals (say, every 10 seconds). What you will end up with is a PCB exposed in steps for different exposure times (10s, 20s, 30s, 40s, 50s etc.), then just pick the exposure time that gives off the best result.

Not sure if my mask pattern is opaque enough (to UV light).

If it's a normal transparency sheet it should be fine. Printing to these with a laser printer works but this depends on your printer. Large areas of black might get overexposed but this depends on your printer. If your quality of black is bad you can print two sheets and align them on top of eachother.

Not sure about correct proportions of caustic soda to water

I have been using a 3% solution of NaOH (KOH works too) which seems to work well.

Not sure about time to leave the board in the caustic soda - whether I'm going too far developing.

You just basically move it around in the solution until it seems developed (black stuff comes off the board), usually it develops quite quickly (under 30s) but this depends on your solution. If your solution is too strong or you develop it too long it will strip off all the photoresist if you keep it in the solution too long. A solution not concentrated enough won't successfully develop the board (again, 3% solution seems to work well). Washing the board under a faucet afterwards works well. You can continue developing after taking it away from the solution. You can actually even continue developing after etching the board a bit, just remember to wash the board. Putting the board in the etching solution shows you pretty well where the board has photoresist and where it doesn't, the copper exposed to the etching solution goes to this "dull" color/texture in under a minute in the etching solution.

I also dont know if a developed pre-sensitized board stays so, and wont fade as its left in daylight over time. How long before normal daylight affects a pre-sensitized developed board?

A board with the plastic shield SHOULDNT be affected by daylight but to be sure I'd keep them in a place shielded from light (like a drawer).

Btw. I'd suggest you try to expose your boards with a 11W fluorescent table lamp (the ones with a "U"-shaped lamp, they should be pretty common and cost like 10e from Ikea). Put the lamp quite close to the board (like 50mm close, and have a thin plate of glass over the board to keep the mask close to the board). You can get suitable plate of glass from picture frames (again, Ikea is a good place to get these), just make sure its real glass and not plastic. Expose for 12-15min (I've used 13,5min for my boards and setup). I have been using this method succesfully for a long time. Won't work for large boards due to the lamp being so narrow but for small boards it works well.

Electronic – PCB (Auto-)Routability

The Eagle autorouter is a decent tool, and I use it a lot. However, like any tool, you have to know how to use it well and understand its limitations. If you are just expecting to throw everything at the autorouter, you will be dissappointed. No current auto router, and probably for a number of years to come, can do that for anything beyond contrived or toy problems.

You say there are settings in the Eagle autorouter you don't understand and never mess with. This is a bad attitude, and probably a good part of your problem. There is no set of control parameters that works on all boards. Even within 2 layer boards there are various tradeoffs. You absolutely have to read the manual and adjust the parameters for your particular situation.

For two layers boards, I often try to keep most of the bottom layer a ground plane. I therefore use the top layer for interconnects as much as possible, and the bottom layer for short "jumpers" to make the routing topology work out. In this case, I set a high cost for routing in the bottom layer.

Before autorouting, you have to look at the board and think about the critical areas that you can't explain to a autorouter. For example, you want to keep the loop currents of a switching power supply local and off the main ground plane. The same holds true for high frequency currents local to a digital chip, like bypass caps and crystal with its caps. If you are using the pseudo ground plane layer as I described above, then you want to manually connect every ground connection immediately to the ground plane with its own via. That leaves maximum room on the top layer for routing everything else.

The process of routing a board even when letting the auto router do most of the grunt work looks like this:

Manually route the critical paths, as I mentioned above.
Do basic housekeeping pre-auto routing. This includes connecting all the ground pins directly to the ground plane for example.
Look for problem areas where you can see the autorouter might get itself into trouble. If there are short connections in dense areas you might want to make some of them. This takes some experience and intuition, so if you're new to the particular autorouter, skip this step for now.
Save a copy of the board, then run the auto router. If this is the first thru here, just have it do the minimum to find a solution. The purpose of the first few times is to see where the problem areas are so you can adjust the layout and your manual pre-route accordingly.
Look carefully at the resulting route. See where the problem areas are. Revert back to the saved copy from step 4 and adjust your layout and manual pre-route according to what the auto router did. Repeat back to step 4 until the result looks reasonable. As you do more iterations thru here, you crank up the autorouter optimizations and other parameters to make a more final route. In the beginning you are just trying to see if it can find a solution and what the large problems are. In later passes you converge on a real route. I start out with no optimization passes, and use 8 for final routes. I also configure early passes to find a solution, then later passes to optimize it.
Do manual cleanup on the route. In the case of a two layer board with mostly ground on the bottom, you want to minimize the maximum dimensions of islands in the ground plane. It is better to have a large number of small islands than fewer large islands. Sometimes you can see ways of rearranging signals locally to minimize the jumpers on the bottom layer. In this stage, the big picture has already been taken care of and you are focusing on manually optimizing small areas. This is similar to a peephole optimizer of compilers.

Here is a Eagle autorouter control file I used on a two layer project with the bottom layer a ground plane to the extent possible:

; EAGLE Autorouter Control File

[Default]

  RoutingGrid     = 4mil

  ; Trace Parameters:

  tpViaShape      = Round

  ; Preferred Directions:

  PrefDir.1       = *
  PrefDir.2       = 0
  PrefDir.3       = 0
  PrefDir.4       = 0
  PrefDir.5       = 0
  PrefDir.6       = 0
  PrefDir.7       = 0
  PrefDir.8       = 0
  PrefDir.9       = 0
  PrefDir.10      = 0
  PrefDir.11      = 0
  PrefDir.12      = 0
  PrefDir.13      = 0
  PrefDir.14      = 0
  PrefDir.15      = 0
  PrefDir.16      = *

  Active          =    1
  ; Cost Factors:

  cfVia           =   50
  cfNonPref       =    5
  cfChangeDir     =    2
  cfOrthStep      =    2
  cfDiagStep      =    3
  cfExtdStep      =    0
  cfBonusStep     =    1
  cfMalusStep     =    1
  cfPadImpact     =    4
  cfSmdImpact     =    4
  cfBusImpact     =    0
  cfHugging       =    3
  cfAvoid         =    4
  cfPolygon       =   10

  cfBase.1        =    0
  cfBase.2        =    1
  cfBase.3        =    1
  cfBase.4        =    1
  cfBase.5        =    1
  cfBase.6        =    1
  cfBase.7        =    1
  cfBase.8        =    1
  cfBase.9        =    1
  cfBase.10       =    1
  cfBase.11       =    1
  cfBase.12       =    1
  cfBase.13       =    1
  cfBase.14       =    1
  cfBase.15       =    1
  cfBase.16       =    5

  ; Maximum Number of...:

  mnVias          =   20
  mnSegments      = 9999
  mnExtdSteps     = 9999
  mnRipupLevel    =   50
  mnRipupSteps    =  300
  mnRipupTotal    =  500

[Busses]

  @Route

  Active          =    1
  cfVia           =   10
  cfChangeDir     =    5
  cfBusImpact     =    4
  cfPolygon       =   25
  cfBase.16       =   10
  mnVias          =    0
  mnRipupLevel    =   10
  mnRipupSteps    =  100
  mnRipupTotal    =  100

[Route]

  @Default

  Active          =    1

[Optimize1]

  @Route

  Active          =    1
  cfVia           =   99
  cfNonPref       =    4
  cfChangeDir     =    4
  cfExtdStep      =    1
  cfHugging       =    1
  cfPolygon       =   30
  cfBase.16       =   10
  mnExtdSteps     =   20
  mnRipupLevel    =    0
  mnRipupSteps    =  100
  mnRipupTotal    =  100

[Optimize2]

  @Optimize1

  Active          =    1
  cfNonPref       =    3
  cfChangeDir     =    3
  cfBonusStep     =    2
  cfMalusStep     =    2
  cfPadImpact     =    2
  cfSmdImpact     =    2
  cfHugging       =    0
  cfPolygon       =   40
  mnExtdSteps     =   15

[Optimize3]

  @Optimize2

  Active          =    1
  cfVia           =   80
  cfNonPref       =    2
  cfChangeDir     =    2
  cfPadImpact     =    0
  cfSmdImpact     =    0
  cfPolygon       =   50
  mnExtdSteps     =   10

[Optimize4]

  @Optimize3

  Active          =    1
  cfVia           =   60
  cfNonPref       =    1
  cfPolygon       =   60
  cfBase.16       =   12

[Optimize5]

  @Optimize4

  Active          =    1
  cfVia           =   40
  cfNonPref       =    0
  cfPolygon       =   70
  cfBase.16       =   14
  mnExtdSteps     =    5

[Optimize6]

  @Optimize5

  Active          =    1
  cfVia           =   20
  cfBase.16       =   16

[Optimize7]

  @Optimize6

  Active          =    1
  cfBase.16       =   18

[Optimize8]

  @Optimize7

  Active          =    1
  cfBase.16       =   20

Best Answer

Related Solutions

Electronic – PCB prototyping with Presensitized copper boards

Electronic – PCB (Auto-)Routability

Related Topic