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.

On Jun 30, 1:12*pm, "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.

LQFP32 is not ridiculously fine pitch (I do 0.65mm pitch parts by hand
all the time), and 1206 is just enormous in the SMT world.

I do both by hand all the time - just a Weller WTCPT with 0.015"
solder. No solder mask for me (I use ExpressPCB cheapo boards) but
pretinning does help. Sometimes flux comes in handy but is not really
necessary. Just lay the solder down across the leads, melt on with the
soldering iron, go back and clean up any blobs with solder-wick.

Toaster oven is great for BGA's but if there's only a few leaded SMT
parts it seems like way overkill.

Tim N3QE

On 30 Jun, 18:12, "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 don't have any problems making PCBs at home for 0.5 mm lead spacing
QFPs, or soldering them. by hand (drag soldering).

Leon

"Tim Shoppa" wrote in message
...

LQFP32 is not ridiculously fine pitch (I do 0.65mm pitch parts by hand
all the time), and 1206 is just enormous in the SMT world.

I do both by hand all the time - just a Weller WTCPT with 0.015"
solder. No solder mask for me (I use ExpressPCB cheapo boards) but
pretinning does help. Sometimes flux comes in handy but is not really
necessary. Just lay the solder down across the leads, melt on with the
soldering iron, go back and clean up any blobs with solder-wick.

Toaster oven is great for BGA's but if there's only a few leaded SMT
parts it seems like way overkill.

Tim N3QE

Thanks for the reply.

You are correct, a 1206 is a large SMD component and I will try to use them
wherever possible. However, smaller (0402 and larger) may also be used. It
is encouraging to hear about successful fine pitch results with hand solder
techniques. In addition, it will certainly be useful for parts placed on
both sides of the PCB.

"Leon" wrote in message
...
I don't have any problems making PCBs at home for 0.5 mm lead spacing
QFPs, or soldering them. by hand (drag soldering).

Leon

Thanks for the reply. I have though about using a laser printer to print a
negative image on clear Mylar. Then expose a photo sensitive PCB to the
negative image. The laser printer does print very sharp lines as visible
using a 10X loop. The quality should be good enough for fine pitch. The
etching process however may end up in disaster with fine pitch lines.
How do you make the PCB?

On Jun 30, 4:33*pm, "numeric" wrote:
You are correct, a 1206 is a large SMD component and I will try to use them
wherever possible. However, smaller (0402 and larger) may also be used. It
is encouraging to hear about successful fine pitch results with hand solder
techniques. In addition, it will certainly be useful for parts placed on
both sides of the PCB.

For my purposes, 0603 is the "standard" size SMD part to use when I
have a choice. Of course power parts are gonna be bigger.

I mean, just so you don't scare yourself, look at the bottom of a
TO-92 transistor that you've been soldering for 30 years or more. The
pitch there 0.05 inches or 0.127 mm. And you do those all the time.
Your LQFP is finer pitch, but not ridiculously so. So don't set
yourself up with the mental attitude that it's gonna be hard, because
it isn't going to be hard!

I have occasionally used 0402 and even 0201 parts when they weren't
available in 0603... I struggled with 0201, and 0402 required a lot of
concentration. By comparison the 0.65mm pitch parts were a piece of
cake.

And finally, don't ignore the option of doing "dead bug" with SMT
parts. Very fun for low-lead-count analog parts... and possible though
questionable for digital stuff with lotsa leads.

Tim.

On 30 Jun, 21:45, "numeric" wrote:
"Leon" wrote in message

...

I don't have any problems making PCBs at home for 0.5 mm lead spacing
QFPs, or soldering them. by hand (drag soldering).

Leon

Thanks for the reply. I have though about using a laser printer to print a
negative image on clear Mylar. Then expose a photo sensitive PCB to the
negative image. The laser printer does print very sharp lines as visible
using a 10X loop. The quality should be good enough for fine pitch. The
etching process however may end up in disaster with fine pitch lines.
How do you make the PCB?

I use an inkjet printer and JetStar Premium film for the positive
artwork. UV exposure on Fotoboard 2 with pre-coated resist and etching
in ferric chloride.

Leon

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