PCB with surface mount components

[useer]

Hi,

Looking for advice on what surface finish to use on contact pads for fine pitched SMD type of PCB.
The minimum pitch of any of the components pins is 0.5 mm and I am planing to use a small reflow machine with IR heater + fan + stencil. Some components are DFN type ...

The ordering options mention HASL for contact pads and its benefits but I have read somewhere that it is not advisable for fine pitch components. If not HASL than what?

How about stencil, what is good, thick or thin?

Any recommendations in general will be appreciated. I've never had to consider these things before working for employers, always ordered complete assemblies from subcontractors. This time it's my money and project, have to  take things slow and be diligent...

Cheers.

[T3sl4co1l]

I'm comfortable with HASL (leaded or lead-free) for larger, leaded components (like SOICs and SOTs), but I wouldn't recommend it for flat blocky components, like DFNs, QFNs and LGAs.  The tinned surface is bumpy, and parts won't sit flat.  Maybe the paste will pull everything together still, but maybe not.  The amount of solder on pads is also highly variable.  An excess of solder will tend to reduce the amount of paste applied, but not proportionally (paste has lower density than solid, so you still end up with an excess).

The absolute go-to is ENIG.  And it's so standard these days, it's little added cost, even at cheap fabs.  Unlimited shelf life.

I've seen tin and silver plate, but these tarnish and become unsolderable in storage, so that's bad.  They're fine if you don't need shelf life (PCBs get assembled immediately).

There's also OSP (organic surface preservative), which prevents bare copper from tarnishing, while being solderable.  Shelf life is reasonable (a year I think?).  I haven't seen it much in design and manufacture, but I've seen products made with it (e.g. http://seventransistorlabs.com/Images/8510W_1.jpg the orange blocks are tant caps; the orange rings are now-aged, OSP'd copper pads!).

Tim

[mrpackethead]

OSP shelf life is only abotu 3 months if you want it to be reliable. Its fine for reflow but a PITA for any hand work..   I'd only ever use it, if you are in produciton manufacture and are using your boards as they arrive.  For small run/diy stick with ENIG.. Though if you are hand soldering, HASL is easy.

[useer]

Thanks mrpackethead,

So when you are saying that HASL is easy for hand soldering, how about the surface bumpiness and fine pitch components that T3sl4co1l mentioned ?

Or do you mean HASL is good for anything that is "big enough" for hand soldering ?

Cheers

[poorchava]

I guess he meant that if HASL produces a bumo that becomes a problem, you can always wick it away. I find it that HASL creates dome-like shapes on small pads and a "dune" like rise on one end of bigger pads.

Obviously ENIG is the best, but I can't aggree about fabs not charging much for it. Certainly more popular chinese places like Elecrow or iTead charge quite a lot. Places that are already not cheap (like Technoservice in Poland which i use quite often when time matters or when i need good communication because of complicated design) have much lower premium for ENIG.

[ajb]

For a one-off, HASL can be okay for a couple of small leadless or fine pitch parts.  If you're assembling by hand, then you generally expect/tolerate a higher level of touchup and rework anyway, and the HASL won't make a massive difference.  ENIG is certainly preferable, though, and becomes mandatory with larger leadless packages. 

Equally important for doing non-production boards with fine pitch and leadless parts is optimizing your pad size and shape.  You need to pay close attention to the fab's minimum mask sliver and expansion to make sure that you'll get mask between your pads.  For QFN/DFN parts, you likely want to extend the pads out a little further from the part so that you can rework them more easily. 

[IconicPCB]

On the question of stencil I make my own from 0.2mm double sided FR4 laminate.
I reduce the stencil opening dimensions to 70% of copper pad dimensions. This results in a VOLUME of paste equivalent to a full size pad opening in 0.1mm stencil.

Problem with this approach it is not good for cubic components smaller than 0603 and other features where the minimum opening dimension becomes smaller than 0.25mm. This last requirement ( 0.25mm ) is a limitation imposed by the cutting tool.

Otherwise 0.1mm is a better choice as it can deliver the correct and uniform volume of paste with the view to minimising tomb-stoning.

[T3sl4co1l]

Yeah, hand soldering doesn't much mind HASL because you're likely adding excess solder anyway, and adjusting things as you go.

For a one-shot process like reflow, you want as much consistency as possible.

Tim