On Jun 30, 10:12 am, "numeric" wrote:
Hi,

I would like to build a PCB board with fine pitch (LQFP32) and very small
surface mount parts (1206 and smaller) parts. The board is a simple 24 bit
A/D and D/A converter with a full speed (12 Mbs) USB2 interface; although I
would prefer the USB2 high speed interface (480 Mbs). The temptation is too
hard to resist using today's microprocessors such as SiLabs C8051F350
(basically an 8051 cpu). I know that a commercial 4 layer PCB with a solder
mask would probably work for homebrew construction; but the board cost is
high.

So what is the chance of a double sided PCB without solder mask working with
very small parts? The process I would use in either case would be:

1. Place solder paste on all pads. Typically, when heated the solder will
vacate between pins and will suck up, like a magnet, under the SMD pin.

2. Place glue under parts that will possible move when the board is handled.

3. Heat the PCB in a toaster oven until soldered. Manually follow the heat
up and cool down time curves for type of solder paste used.

Comments/suggestions are appreciated.

I agree with Leon and Tim that making a board and mounting parts on it
shouldn't be a problem, though I do greatly appreciate being able to
do my soldering under a stereo microscope using a Metcal iron.

I commonly use boards coated with positive resist (from Circuit
Specialists in the Phoenix area), etched by floating them on top of
ferric chloride. I find that (lacking a spray etcher) the float
process maintains fine line resolution--I have a theory that it has to
do with copper-laden etchant being heavier and falling away from the
etch site, but whatever, it works well for me. Surface tension will
hold fairly large boards up, but you can also just put support at the
corners and let the board submerge. Resolution for me is always
limited by the quality of the transparency. Thanks to Leon for
posting a reference to another film material to try. I've had the
best luck using an Epson inkjet printer: my HP printer doesn't do as
well, and laser printers typically don't print a dense enough black.
Pinholes WILL print on the board! I use a back-light film setting on
the Epson for highest density. An alternative is to get a
photoplotting shop to do the film for you: better quality but it
lacks the immediacy of being able go from layout to board in minutes.
Anyway, the best inkjet printed transparencies will allow me to do 5
mil lines with 5 mil spaces if I'm careful, and 8/8 pretty easily.
Most of my work is with 0805 as the standard passive, since that's
what I invested in an assortment of back a few years ago, but I also
do 0603 with no trouble, and lead pitches down to 0.5mm. We have
fancy equipment to do BGA rework here at work, and the trained techs
have marginal success with it at best (for large parts), so I've
avoided BGAs in my home projects.

By the way, if you get boards from Circuit Specialists, be aware that
what they call 1 ounce copper is really 0.5 ounce...seldom a problem
and allows faster etching and it's easier to make fine lines, but be
aware of it for high current traces.

Cheers,
Tom

On 1 Jul, 00:30, K7ITM wrote:
On Jun 30, 10:12 am, "numeric" wrote:



Hi,

I would like to build a PCB board with fine pitch (LQFP32) and very small
surface mount parts (1206 and smaller) parts. The board is a simple 24 bit
A/D and D/A converter with a full speed (12 Mbs) USB2 interface; although I
would prefer the USB2 high speed interface (480 Mbs). The temptation is too
hard to resist using today's microprocessors such as SiLabs C8051F350
(basically an 8051 cpu). I know that a commercial 4 layer PCB with a solder
mask would probably work for homebrew construction; but the board cost is
high.

So what is the chance of a double sided PCB without solder mask working with
very small parts? The process I would use in either case would be:

1. Place solder paste on all pads. Typically, when heated the solder will
vacate between pins and will suck up, like a magnet, under the SMD pin.

2. Place glue under parts that will possible move when the board is handled.

3. Heat the PCB in a toaster oven until soldered. Manually follow the heat
up and cool down time curves for type of solder paste used.

Comments/suggestions are appreciated.

I agree with Leon and Tim that making a board and mounting parts on it
shouldn't be a problem, though I do greatly appreciate being able to
do my soldering under a stereo microscope using a Metcal iron.

I commonly use boards coated with positive resist (from Circuit
Specialists in the Phoenix area), etched by floating them on top of
ferric chloride. *I find that (lacking a spray etcher) the float
process maintains fine line resolution--I have a theory that it has to
do with copper-laden etchant being heavier and falling away from the
etch site, but whatever, it works well for me. *Surface tension will
hold fairly large boards up, but you can also just put support at the
corners and let the board submerge. *Resolution for me is always
limited by the quality of the transparency. *Thanks to Leon for
posting a reference to another film material to try. *I've had the
best luck using an Epson inkjet printer: *my HP printer doesn't do as
well, and laser printers typically don't print a dense enough black.
Pinholes WILL print on the board! *I use a back-light film setting on
the Epson for highest density. *An alternative is to get a
photoplotting shop to do the film for you: *better quality but it
lacks the immediacy of being able go from layout to board in minutes.
Anyway, the best inkjet printed transparencies will allow me to do 5
mil lines with 5 mil spaces if I'm careful, and 8/8 pretty easily.
Most of my work is with 0805 as the standard passive, since that's
what I invested in an assortment of back a few years ago, but I also
do 0603 with no trouble, and lead pitches down to 0.5mm. *We have
fancy equipment to do BGA rework here at work, and the trained techs
have marginal success with it at best (for large parts), so I've
avoided BGAs in my home projects.

By the way, if you get boards from Circuit Specialists, be aware that
what they call 1 ounce copper is really 0.5 ounce...seldom a problem
and allows faster etching and it's easier to make fine lines, but be
aware of it for high current traces.

Cheers,
Tom

I use a Metcal and microscope, as well.

I use manual agitation in hot FeCl3. I don't have any problems getting
down to 8 mil tracks or even less.

Leon

On 1 Jul, 00:30, K7ITM wrote:
On Jun 30, 10:12 am, "numeric" wrote:
. *Thanks to Leon for
posting a reference to another film material to try. *I've had the
best luck using an Epson inkjet printer: *my HP printer doesn't do as
well,

The ordinary JetStar film is suitable for dye-based ink like that used
in Epson printers. Premium Jetstar is intended for use with pigment-
based inks used with HP printers. They are available from Mega
Electronics and Farnell.

Leon

On Jul 1, 3:02 am, Leon wrote:
On 1 Jul, 00:30, K7ITM wrote: On Jun 30, 10:12 am, "numeric" wrote:

. Thanks to Leon for

posting a reference to another film material to try. I've had the
best luck using an Epson inkjet printer: my HP printer doesn't do as
well,

The ordinary JetStar film is suitable for dye-based ink like that used
in Epson printers. Premium Jetstar is intended for use with pigment-
based inks used with HP printers. They are available from Mega
Electronics and Farnell.

Leon

These seem to be UK suppliers. So far, my Google searching hasn't
turned up a source in the US for the JetStar films, though I am
intrigued by the product lines that Valley Litho carry. There's been
something of a revolution in how printing is done: professional laser
printers are used for short runs, offset plates are made in various
ways much more directly from computer-generated copy, and laser and
inkjet printers are used to make positive and negative
transparencies. Valley carries several items that claim to produce
dense, clean copy for exposing plates or screen film, and that should
be fine for PCB work too. I'll probably call them to get
recommendations, since a hundred sheets of transparency film is a bit
pricey to buy to just try out.

Cheers,
Tom

On 1 Jul, 17:52, K7ITM wrote:
On Jul 1, 3:02 am, Leon wrote:

On 1 Jul, 00:30, K7ITM wrote: On Jun 30, 10:12 am, "numeric" wrote:

. *Thanks to Leon for

posting a reference to another film material to try. *I've had the
best luck using an Epson inkjet printer: *my HP printer doesn't do as
well,

The ordinary JetStar film is suitable for dye-based ink like that used
in Epson printers. Premium Jetstar is intended for use with pigment-
based inks used with HP printers. They are available from Mega
Electronics and Farnell.

Leon

These seem to be UK suppliers. *So far, my Google searching hasn't
turned up a source in the US for the JetStar films, though I am
intrigued by the product lines that Valley Litho carry. *There's been
something of a revolution in how printing is done: *professional laser
printers are used for short runs, offset plates are made in various
ways much more directly from computer-generated copy, and laser and
inkjet printers are used to make positive and negative
transparencies. *Valley carries several items that claim to produce
dense, clean copy for exposing plates or screen film, and that should
be fine for PCB work too. *I'll probably call them to get
recommendations, since a hundred sheets of transparency film is a bit
pricey to buy to just try out.

Cheers,
Tom

Silkjet film seems to be similar and is available in the US. They even
make two types for dye and pigment inks, like Mega do. It might be the
same stuff, renamed for a different market.

Leon

On Jul 1, 10:43 am, Leon wrote:
On 1 Jul, 17:52, K7ITM wrote:



On Jul 1, 3:02 am, Leon wrote:

On 1 Jul, 00:30, K7ITM wrote: On Jun 30, 10:12 am, "numeric" wrote:

. Thanks to Leon for

posting a reference to another film material to try. I've had the
best luck using an Epson inkjet printer: my HP printer doesn't do as
well,

The ordinary JetStar film is suitable for dye-based ink like that used
in Epson printers. Premium Jetstar is intended for use with pigment-
based inks used with HP printers. They are available from Mega
Electronics and Farnell.

Leon

These seem to be UK suppliers. So far, my Google searching hasn't
turned up a source in the US for the JetStar films, though I am
intrigued by the product lines that Valley Litho carry. There's been
something of a revolution in how printing is done: professional laser
printers are used for short runs, offset plates are made in various
ways much more directly from computer-generated copy, and laser and
inkjet printers are used to make positive and negative
transparencies. Valley carries several items that claim to produce
dense, clean copy for exposing plates or screen film, and that should
be fine for PCB work too. I'll probably call them to get
recommendations, since a hundred sheets of transparency film is a bit
pricey to buy to just try out.

Cheers,
Tom

Silkjet film seems to be similar and is available in the US. They even
make two types for dye and pigment inks, like Mega do. It might be the
same stuff, renamed for a different market.

Leon

Thanks, Leon! I really appreciate your inputs that have gotten me
pointed in some new directions. Maybe all this will get some other
folk experimenting and reporting back their results. First: Silkjet
is a brand of Kimoto, so if you search for Kimoto you may find more
suppliers and info. (They also make high quality laser printer
film.) Second: if you do searches like "screen printing inkjet film"
you'll find a bunch of stuff. My belief is that a film that's really
suitable for exposing screen printing emulsions is very likely going
to be suitable for transparencies for PC printing. Also, I found one
place at least that offers a ten-sheet sample pack of Silkjet
(recommended for Epson printers) for $10; I just ordered one--they
seem to not have a minimum order, even. (shop.poconoscreen.com) And
with the right supplies, you may be able to do as well (or better?)
with a laser printer; see for example the Valley Litho web site -- and
a search on "screen printing laser film" turns up lots of stuff too.
The key seems to be finding out where inkjet and laser printed
transparencies are being used commercially, and screen printing is
clearly one of those areas.

Cheers,
Tom