First go at CNC'ing a brass stencil

[charliex]

I thought i'd have a go at CNC'ing a brass solder stencil. I used 3 - 5 mil shims of brass sheet from ace hardware. I had a couple of problems cutting the first pass, so its a bit wobbly in parts. Overall i think it came out ok for a first go. I used a V60 bit instead of a V90 so that meant my depth was a bit off. Some swarf left over, especially since i did a couple of passes.

[ktownsend]

My guess is you're going to end up with some hefty bridges on the QFP part with so much paste. Discretes are generally very forgiving, but you really need to either keep those pins seperate on the stencil for the QFP or REALLY adjust the size of the hole you're carving so that you're only laying a thin line of paste (covering maybe 1/3 the width of the QFP pad length). If you can't cut individual pads at 0.5mm pitch, I'd make the width 1/3 to 1/4 what you're current using. It's a trial and error process, though, so I'm sure you'll figure it out. Leaded solder has better wetting properties and is a bit more forgiving than lead-free if you're able to use it, but I'm still sure you'll need to work on those pad sizes. Looks good for a start, though. It definately saves time and headache being able to make your own stencils.

Kevin.

[charliex]

yep thats the way it came out of eagle, basically one large strip. i used a pre exisiting library part for it.

i did reflow a couple of boards with a stencil i cut, the paste i used reflowed into the leads pretty well. though i was using lead free and the problems i found where that some of the solder balls ended up under some of the smaller components and caused shorts, typically on 0805 parts or smaller. i did use some flux to help the process along, as i i couldn't get a melt with the hotplate i used, it only just got upto the temperatures in the reflow chart i used.. but that seemed to disperse the paste under the components. Perhaps it was struggling with the temps too.

i haven't tried leaded reflow paste yet.

[alex_dubinsky]

Why a V60 or V90 bit? You should use a regular diamond-cut PCB router bit, or at least some sort of endmill.

I agree about covering the QFP pads with a thin strip of solderpaste. That's what I do with a syringe and it works great.

[charliex]

Its what i had at the time, i ordered more bits from precisebits, and i was basically learning as i went.

[alex_dubinsky]

So how did the CNC'd stenciling work out? I've always used paste syringe under a microscope, which works pretty well for prototypes. Wondering if there's a benefit to stenciling?

[charliex]

hard to tell, i just think my hotplate didn't get upto the temperature required for lead free. i'm thinking about the QK but i'm away in los angeles for a month or so, so no access to my cnc at the moment,

it seemed to lay it ok, the brass stencils are way tougher so thats good.

ive got my new bits here so as soon as i head back i'll give it a bash, i've lso got some pcbs coming with masks so that might help, vs cnc'd.

[charliex]

I just realised i used the tStop/bStop for the mask.... I re exported them with them with tCream/bCream and shrunk some of the pads slightly in coppercam. My cnc is still at home though, and i don't think my GF is up for doing a remote first time machining so i'll give pololu a try

[charliex]

Just got my masks from pololu this morning, no drama there, and my QK870 arrived with a quick853 for good measure.

I decided to use leaded vs lead free, since its lower temp, i'm using the zephtronics paste. For a reflow profile for doing a QFP100 i'm using

100-150C 60-120 secs preheat

183C 60-150 seconds meltyman mode

6c/sec ramp down (well ok i'm setting it to cool and turning down the temperature)

183C didn't seem hot enough after about 120 seconds, so i went upto about 19OC

Any tips would be very welcome before i go off and destroy all life as we know it