Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

Hi, folks,

New member here (glad to find the group!) but long time electronics
designer and fabricator. (Have my own metalworking shop, various chem lab
equipment, etc.)

Due to other scheduling problems, I've backed myself into a corner on
producing some small PCBs (1x2", 2-layer, could even make due with 1-layer
if I have to :-) Would love to have the assembled wisdom here advise me on
how I can get out of this jam with DIY PCBs that I make over the weekend.

Would normally send these to a service bureau (hey, someone else is picking
up the bill :-) but it'll take me another half-day or so to get my Eagle
files together, thus missing today's submission window. Next "business
day" is Monday, which means that even with expensive 2-day turnaround I
won't have boards until late Wednesday for a critical Thursday demo.
Sigh.. Drag out the chemicals....

The critical requirement is probably minimum trace width and separation.
I'm using some QFN40 parts, with pads on 0.5mm pitch, 0.3mm wide, 0.2mm
separation. Boards will be small, i.e., about 1x2", so that's not an
issue. Standard thickness, 0.062", is fine.

∗Positive emulsion:∗
I have used the following ∗chemical ∗materials and process before, with
moderate success:

- MG Chemicals pre-sensitized single-sided boards, positive emulsion
- Transparency material printed from PDF files on a "good", e.g.,
1200dpi, laser printer, with careful attention to print density (hand
painting in any pinholes, faint areas)
- Contact printed with a FML-27EX-D 27-watt "four finger" daylight lamp
(about 6x4") at 6" for around 10 min (test exposure at 2:4:8:16 showed
somewhere between 8 and 16 was best).
- Developed with MG Chem 418 Developer (i.e., sodium hydroxide,
lye) at ∗one-half
∗the recommended strength, i.e., 1:20, instead of 1:10, at room
temperature. Still developed very quickly (~15 sec.) Note: tried sodium
carbonate at the recommended strength (20g/l?) at room temp. Didn't do
anything in several minutes.
- Didn't do anything to "fix" the image. I've attached a picture of my
exposure test strip (which might be stripped by the Yahoo mail handler
before you see this message).

The test pattern was pretty simple. Not a lot of fine detail like I need
in the new board. But a couple features (sides of eyelets, details in the
lettering) are close to the resolution I need, so I'm hopeful. Last time I
tried this same pattern, my results weren't as good. Had problems with not
enough contrast in the resist (i.e., hard to get clear open areas without
overexposing the holdback areas and getting holes. Suspect the pre-sens
coating was stale.)

Any comments on my process or ways to insure the highest definition?

∗Negative emulsion:∗

Do people have better success with hand-applied negative emulsion and
process? Source of supplies?

∗Toner transfer:∗

I've even tried toner transfer process in the past. Had a great 1200 dpi
laser printer (sadly, not anymore :-) Found that printing on inkjet
photopaper (i.e., with a glossy surface and a clay coating) led to the best
toner transfer on "ironing". Even made an aluminum plate with thermocouple
temp sensing (and a silicone rubber pressure platen to get full contact :-)
to do a couple experiments, but didn't use for producton. Wonder where I
put it? :-)

∗Solder resist:∗

On commercial boards, I gotten used to (lazy? :-) having solder resist over
the copper, especially to help prevent bridging between the 0.5mm spacing
pins. Any solution here (other than hand painting something between the
copper lands by hand using my stereo microscope :-) Squeegy something
between the lands?

Resist also helps a lot when using a stencil, solder paste and hot air.
Won't have a stencil in this case, but could hand paint the resist. (Does
the resist pen MG Chem sells work?)

Sorry for being such a newbie, but since I'm so short on time, I leaning on
all of you for some quick advice rather than reading the message archives
:-)

Mike

Palo Alto, California (if that matters in terms of finding local supplies
:-)

Attached pics:

Exposure test

Fine details in lettering (taken with $5 microscope attachment from Hong
Kong with my iPhone :-)

Some dropout (or damage?) on some fine details


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