Etching for SMT

[MoltenVoltage]

Any suggestions for etching SMT PCBs?

The toner transfer/muriatic acid and hydrogen peroxide technique I use can't handle the fine pitch parts.

[davent]

Any suggestions for etching SMT PCBs?

The toner transfer/muriatic acid and hydrogen peroxide technique I use can't handle the fine pitch parts.

Photoresist pcb and muriatic/peroxide, how fine are the traces that are causing the problems?

[StereoKills]

I think switching from toner transfer to using photosensitive PCB would give the accuracy you need. The enchant, as long as it is good and strong, shouldn't matter too much I would think.

[MoltenVoltage]

Thanks for the replies.

Any particular brand you use?

I am looking to etch traces for some .05" pitch chips, do you think that the photoresist technique can get that kind of resolution or should I just use batchPCB?

[Gurner]

0.05" is 1.27mm pitch (SOIC packages)   ...that pitch is easily do-able at home.

I can comfortably work down to SOT23-6 pitch (which is about 0.95mm pitch =0.037401" ....ie about 0.75mm/0.029527" gaps between the pads) ...in fact I can go down even lower than that, but it then becomes a bit more 'hit & miss '....and soldering such pitch starts becoming a problem (even with a compressed air solder paste  dispenser, I get too many bridges, which then take too long to braid off!).

I don't use anything particularly special ...an old LaserJet4  laser printer (needs to be at least 600dpi), I print onto overhead/laser transparencies ......I then use a UV box made from an old EEPROM eraser my work were throwing out  (that said, I replaced its old analogue dial timer with a PIC digital timer I knocked together -getting down to accuracy plus/minus 10 seconds is important wrt exposure).

I live in the UK....& used presensitized copper board from either Maplin (Brits - it's suprisingly well priced there) & or Mega UK (when Iwant a lot of boards) ....I use either the paper based economy board or the FR4.

I use bog standard caustic soda (drain cleaner) to develop...and ferric chloride to etch - I bought a bubble tank off Ebay for about $40 to help ( I can heartily recommend buying such a device - no having to worry about temperature or agitating, also the ferric chloride just stays in there until it's showing signs of exhaustion ...therefore using one of these, you're always 'ready to 'roll' so to speak)
.

I pushed the boat out & bought a pneumatic solder paste dispenser from china (about $130) & I bought a s/h fan assisted pizza type oven and then converted it for SMD ...but truthfully it's just as quick for one-offs to use a small hand held hot air gun to flow the paste.

In fact I'm making a PCB as I type using all the above kit (it has 0.95mm/0.037401" pitch devices on it)

EDIT: Just finished it (ok, not completely uniform traces, but more than good enough for quick prototyping - it took me about 20 minutes from pressing the 'Print' button in Eagle, to have the board at this stage - total cost is only 50cents  - so, a fair bit quicker & cheaper than BatchPCB!)...



that's a british 10p...about the same size of one of your Quarters. The pitch that you're wanting to to achieve 0.05" 1.27mm (SOIC) is that IC in the middle...you can see there's sufficient space to hold a Disco in between the pads)

[davent]

I usually use boards and developer from MG Chemicals which being local, i can pick up locally. Have also used the other brands available at Digikey and they all work fine. I've always used just regular flourescent bulbs for exposure, placing the board about an inch from the light. I've got a plexiglass "shelf" attached to the ceiling joists above my workbench to hold the exposure package for exposure. I'm not as efficient as Gurner but it's easy to go from printer to etched board in half an hour with my primitive setup.



dave

[MoltenVoltage]

Very impressive!  I had no idea that type of quality was possible.

Please tell me more about the pneumatic solder paste dispenser.

Do you use a plastic stencil with it?

I've seen videos of using solder paste with a stencil and a squeegee, but it looks pretty sloppy.

[Gurner]

The dispenser is just a way of precisely dispensing the correct amount of solder paste manually (by hand) on each smt pad (else it's a right old faff using needles) .....you feed compressed air into it (so you'll need a small air compressor) - all it does is open a valve for a split second allowing the compressed air to push a little bit of paste out a syrine needle (the syringe holds your solder paste).

Here's a similar one to the one i bought...

http://cgi.ebay.co.uk/Solder-paste-glue-liquid-auto-dispenser-SMD-PCB-Pro-/270669380388?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item3f0526fb24

here's a video of an automated one (attached to a machine) , but the end result is the same (albeit not as uniform!)

http://www.youtube.com/watch?v=lVUU9hFeQzY

This method is only good for prototypes (ie each time your messing about with a particular circuit's pcb layout - which can take a few versions) ...once you've proved the board, then a stencil is the way to go.