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Through-hole or Surface Mount crystal?

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Los Frijoles

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board issues surface mount through hole

I have two possible versions I have been designing for some time now. Since it is looking like I am going to be home long enough to finish it I guess I should get it fabricated. However, one thing needs to be decided before I order it and that is whether or not I should use a Through-Hole or Surface Mount crystal.

Originally I was going to just use a through hole crystal, but after having a friend of mine look over the board as another set of eyes he pointed out that my crystal traces were over 3in long and had several vias in them. So, we identified a spot where a surface mount crystal could go with no vias and 0.5" traces. The criteria for fitting in this space was that the crystal had to be no more than 7mm long, 3mm wide, and 1.5mm tall. The obvious problem here is that nothing really fit in that space except for a crystal made by Abracon that is 3.2x2.5x0.7mm. Now the problem is size: is the crystal too small?

My resources: 0.015" solder, 1/32" spade tip (on a variable temperature soldering iron), solder wick, and a relatively steady hand (successfully soldered a .4mm pitch 64tqfp by hand without overheating it).
Things I don't have: A magnifier, reflow oven
Things I don't have but could get: Heat gun, solder paste

I don't have enough experience with this sort of thing to answer it myself and I really can't afford to buy two versions of this board. Is using a crystal this small really worth it?

If this were you would you go with the through hole crystal at the sacrifice of clock clarity or go with the surface mount crystal at the sacrifice of ease of assembly?
 

flatulent

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using solder paste for through-hole -patent

The advantage of shorter traces are more than the disadvantage of a possibly lower Q crystal.
 

betwixt

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crystal, soldering

I would go with the shorter traces version. I'm not sure what frequency your crystal runs at but I do know that noise pick-up on the traces to it can be your worst enemy and very difficult to deal with after the board is built.

I find the problem with SMDs isn't the physics but the fear of using them. As you have already managed to handle tiny solder joints you should be OK.

There is anther alternative which is viable in situations where only one or two SMD components are present, use a dab of silver loaded paint instead of solder. If you get good quality (>10% silver) paint ( I get it from Farnell) it is not only very low resistance but has sufficient adhesion to hold small parts in place. You could also use a tiny amount of glue to hold it in place or coat over it after the paint has dried.
Its expensive but a 3g bottle will do several hundred, possibly thousands of joints and no heat is needed at all. A tiny drop is all that's needed on each 'pin' and it dries hard in about 12 hours. I would be careful if you use a solvent wash process to clean your boards unless the paint is protected, it may wash off!

Brian.
 

Los Frijoles

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why through hole clock crystal

That silver paint sounds cool, but the problem is that my board is completely SMD. After my first board with SMD about a year ago, I have become quite accustomed to using SMD in just about everything (I even use SMD resistors and capacitors on my through hole radio shack project boards). This board has 314 SMD pads, so I don't think that using that paint will work very well :D.

I just had a thought: If I were to use solder paste on every joint including the crystal and then use the heat gun for the entire board would that work? Or would I end up just blowing every component off the board?

Using solder paste like this has occurred to me several times before, but I always dismissed it because of the fact that it uses moving hot air rather than the ambient temperature of the air and it could disturb the part arrangement.

EDIT: Here is my annotated crystal layout:
 

betwixt

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through hole vs surface mount board cost

My apologies, I misunderstood your previous message and thought it was only the crystal that was surface mounted.

When PCBs are soldered commercially, a solder screen is used. If you haven't seen one, it's a thin steel stencil which is held against the board. It is perforated where the pads are so solder paste can be squeezed through the holes on to them. When lifted, the solder paste is exactly where you want it and nowhere else.

I have once made a board by smearing paste over the whole board then hand placing the parts. When heated, it soldered the components and tinned all the tracks in one go!

As the solder melts, it's surface tension will tend to pull components centrally in the pad footprint so you might get away with it as long as you 'blow' gently and not scatter them everywhere. The other option is to glue the parts down. If you are making a small number of boards it isn't too difficult to place a dab of glue from a syringe under the component. There are special glues for this, they have enough 'stiction' to hold the part during assembly and it cures in the heat of the soldering to a rock hard state.

Another alternative is to bake the board 'pizza oven' style so it can be kept horizontal. Possibly an IR heat lamp positioned over the board could be used.

Brian.
 

doraemon

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mount silver crystal layout

Hello!

Mounting the crystal on the opposite side is one solution, but depending
on the number of pieces you will produce, it can have a huge influence on
the cost (I mean, if you have no other components on that side).

And judging by the density, you can move a few tracks and have the
crystal soldered on the component side, and a lot closer to the processor.

By the way, you don't need specific soldering tools for that kind of crystal.
I do it with a plain iron (regulated).

Dora.


Los Frijoles said:
That silver paint sounds cool, but the problem is that my board is completely SMD. After my first board with SMD about a year ago, I have become quite accustomed to using SMD in just about everything (I even use SMD resistors and capacitors on my through hole radio shack project boards). This board has 314 SMD pads, so I don't think that using that paint will work very well :D.

I just had a thought: If I were to use solder paste on every joint including the crystal and then use the heat gun for the entire board would that work? Or would I end up just blowing every component off the board?

Using solder paste like this has occurred to me several times before, but I always dismissed it because of the fact that it uses moving hot air rather than the ambient temperature of the air and it could disturb the part arrangement.
 

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