Electronics help

[Rajinder]

Hi,
Please could someone explain the difference between solder paste, solder mask, solder resist and paste mask.
It confuses me.

My second question is what is the connection of pcb track length with wavelength? How can a track become an antenna at certain frequencies.

I look forward to any help.

 

[AnalogKid]

Solder paste is a paste made of solder. It is a bit wetter than toothpaste. It is applied to the pcb pads of surface mount components with either a paste mask and a squeege, like screening ink on a t-shirt, or a robot arm with a nozzle that deposits a very precise little dot. A paste mask is a thin sheet of steel (usually), photo-ethed with a hole pattern than matches all of the SMT device pads in shape and location.

Solder mask and solder resist are essentially the same thing, a coating applied to a pc board so than when it is hosed down with liquid solder, there are some areas where the solder does not stick. For example, a ling trace between two resistor pads does not need to have solder on it the entire length, only at the two pads.

Any conductor (pcb trace, hookup wire, bread knife, whatever) radiates both electric and magnetic fields. Most conductors are such a small fraction of the wavelength of the signal they are conducting that they make very inefficient radiators, but they do radiate something, always. You can optimize the shape of a conductor to make it a more efficient radiator at specific frequencies, but all you are doing is improving its efficiency; it was gonna radiate something anyway.

ak

 

[davenn]

How can a track become an antenna at certain frequencies.

Any conductor (pcb trace, hookup wire, bread knife, whatever) radiates both electric and magnetic fields. Most conductors are such a small fraction of the wavelength of the signal they are conducting that they make very inefficient radiators, but they do radiate something, always. You can optimize the shape of a conductor to make it a more efficient radiator at specific frequencies, but all you are doing is improving its efficiency; it was gonna radiate something anyway.

AK covered that pretty well except that he didn't tell you ...
As frequency increases into the high VHF, UHF and microwave radio bands, the physical lengths of tracks start becoming 1/4, 1/2 or full wavelength in their length
it is then that the real problems occur.
Large amounts of EM radiation from the tracks or other interconnect wires, even long component legs
Large interactions between adjacent tracks and inter connect wires

 

[FuZZ1L0G1C]

(3*(10^11))/f = Full wave length mm.
3E08m = c = velocity of light (EM radiation)
f in Hz.
So half wave is FWL/2
and quarter wave is FWL/4
The symbol for wavelength is the Greek Lamda which looks like an upside down lowercase y.
Unicode character.

 

[kellys_eye]

There can also be issues with timimg at higher frequencies caused by trace length mismatching. This is very obvious if you look carefully at PCBs made for high speed processors - note the 'wavy' trace lengths between CPU and (usually) memory devices.

The PCB designers try to keep each trace length equal to minimise propagation (time) losses and the consequent data loss if it should occur.

 

[AnalogKid]

The PCB designers try to keep each trace length equal to minimize propagation (time) losses and the consequent data loss if it should occur.

Nope. The "wavy" tracks actually *increase* propagation time - intentionally. They are there to make all tracks in a group (such as 8 or 16 data bits coming off of a backplane, or a high speed differential signalling pair such as LVDS and Ethernet) equal length, not to minimize the propagation time but to minimize *differences* in the propagation times among the group by minimizing length differences among the group. Differences in length cause differences in timing among the bits. With modern ultra-high-speed chips such as FPGA's and 10 gigabit Ethernet parts, differences of a couple of millimeters can have serious effects on signal integrity.

ak

 

[Wiginometry]

Huh... you learn something new everyday...

 

[davenn]

equal length, not to minimize the propagation time but to minimize *differences* in the propagation times among the group by minimizing length differences among the group. Differences in length cause differences in timing among the bits.

that's what kellys_eye said .... you misread

 

[Rajinder]

Thankyiu all for your help and answers. Much appreciated.
Could someone please give me an example (simple one to start with) on what track would radiate at what frequency?
Thanks in advance.

 

[Harald Kapp]

"Electronics help" is not a very useful title. Electronics help is what this forum is all about.
I recommend you use a more concise title for your posts so the members of this forum can see from the title what the thread is about and can decide whethe they can contribute.

 

[AnalogKid]

that's what kellys_eye said .... you misread

No, I didn't. And I quoted him so my response would be clear. The purpose of the wavy tracks is *not* to minimize time "losses". The only way to decrease propagation time is to change the substrate material, but that still would affect all traces equally. Maybe what he meant is that it reduces a form of propagation time *distortion* called skew.

The purpose of a wavy track is the opposite, to *increase* time delay. This can be needed for a single-ended signal, like delaying a clock edge until a data signal (or all of the members of a group of signals, like 8-bit data into a latch) has arrived and settled. But it is more common to use it on one part of a differential pair to correct for unequal routing path delays. The minimum pulse width of a 10Gbps signal, such as 10 Gb Ethernet on a full mesh backplane, is only 50 ps. A 1% difference between the propagation times of the two parts of a differential signal has a measurable effect on signal integrity.

https://electronics.stackexchange.com/questions/34959/why-wiggle-nearby-tracks-on-a-pcb
https://electronics.stackexchange.com/questions/39846/sawtooth-traces-on-a-pcb/39850
http://www.ti.com/lit/ml/slyp173/slyp173.pdf (page 12)

ak

 

[kellys_eye]

If you take my first paragraph in explanation you should get the principle inferred. The first SENTENCE even....

There can also be issues with timing at higher frequencies caused by trace length mismatching.

Then again, I should have used 'equalize' in the second paragraph instead of 'minimise'.

But pedantry isn't part of my skill set.

 

[Rajinder]

Thanks all for the help.
I have another question.
How can I calculate at what frequency a track length will radiate?
For example if my track length was 4cm, what frequency will that radiate at?
Thanks in advance

 

[davenn]

Thanks all for the help.
I have another question.
How can I calculate at what frequency a track length will radiate?
For example if my track length was 4cm, what frequency will that radiate at?
Thanks in advance

300 / freq(in MHz) = wavelength ( in metres)

eg 300 / 150 MHz = 2 metres

rework the formula to give you frequency

 

[Rajinder]

What is meant by 1/4 wavelength in relation to pcb tracks and how they radiate. Can someone gives simple answer with some working outbid the equations involved. Also capacitor selection to minimise effects of radiation through pcb tracks. Thanks

 

[davenn]

What is meant by 1/4 wavelength in relation to pcb tracks and how they radiate.

try doing what I suggested
you haven't even shown an attempt

 

[Rajinder]

ok
so your example was this:
300 / freq(in MHz) = wavelength ( in metres)

eg 300 / 150 MHz = 2 metres

rework the formula to give you frequency

frequency will then be ;
wavelength / 300 i.e. 2 metres / 300 = 0.0066 MHz
Is that correct?

So if I have a track length of 10cm, using a frequency of 16MHz (not sure about this value but I made it up for an example)

10 / 160MHz = 0.0625

frequency would be 0.0625 / 10 = 0.00625 MHz

Is that the frequency at which the track will radiate?

 

[BobK]

No.

300 / f = w

multiply both sides by f

300 = w * f

now divide both sides by w

300 / w = f

Where w is in meters and the result is in MHz.

For a 4 cm track to be 1/4 wavelength, the wavelength would be 4 times 4 cm.

w = 16cm = 0.16m

300 / 0.16 = f = 1875 MHz or 1.875 GHz

That would make 4cm a full wave antenna. But we were looking for what is a quarter wave antenna.

Note that that is roughly the frequency region of a cell phone, which have antennas that are roughly in the range of 4 cm!

Edited to add: Unless you are working in the Gigahertz range, you do not need to worry about this.

Bob

 

[Rajinder]

Thanks Bob for the great explanation. I understand it now

 

[Rajinder]

So please could you explain why the 1/4 wavelength is so important?
Thanks in advance