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SMT question

O

o

Jan 1, 1970
0
hi, a smt assembly question ..

assuming we are talking about a double sided boards , each side active
chips with .5mm spacing and 0805 resistors etc.

a local assembly house told me that they would make a solder stencil
for each side of the board ( $250 each is that ok? ) ... so they print
solder on the first side , p and p the components and run it on the IR
reflow belt , then repeat for the other side

my question #1 ... in the older days with bigger components i know
they used glue dots on the first side to hold the components so they
didn't fall off when they reflowed the second side ? do they still do
this today or won't the back side of the board get hot enough for the
solder to flow and the components to fall off ?

the reason i wonder .. is i thought the fine pitch parts need to self
center during reflow on side one and that wouldn't happen if they were
glued down ..

my question #2 ... i guess you can't batch oven relow 2 sided boards ?
( unless i guess everything is glued down .. is would seem tough to
have glue dots small enough for 0805 )

thanks for any insight ... o
 
M

Mac

Jan 1, 1970
0
hi, a smt assembly question ..

assuming we are talking about a double sided boards , each side active
chips with .5mm spacing and 0805 resistors etc.

a local assembly house told me that they would make a solder stencil
for each side of the board ( $250 each is that ok? ) ... so they print
solder on the first side , p and p the components and run it on the IR
reflow belt , then repeat for the other side

my question #1 ... in the older days with bigger components i know
they used glue dots on the first side to hold the components so they
didn't fall off when they reflowed the second side ? do they still do
this today or won't the back side of the board get hot enough for the
solder to flow and the components to fall off ?

I'm pretty sure they don't use glue. I think most small parts will stay on
due to surface tension, even if the solder does reflow.
the reason i wonder .. is i thought the fine pitch parts need to self
center during reflow on side one and that wouldn't happen if they were
glued down ..

That's my understanding, too.
my question #2 ... i guess you can't batch oven relow 2 sided boards ?
( unless i guess everything is glued down .. is would seem tough to
have glue dots small enough for 0805 )

thanks for any insight ... o

Don't know. If you can keep one side, at least, to small parts, I don't
think they will fall off when they are upside down.

I'm not that involved in fabrication, so I could be wrong.

Mac
 
L

Leon Heller

Jan 1, 1970
0
o said:
hi, a smt assembly question ..

assuming we are talking about a double sided boards , each side active
chips with .5mm spacing and 0805 resistors etc.

a local assembly house told me that they would make a solder stencil
for each side of the board ( $250 each is that ok? ) ... so they print
solder on the first side , p and p the components and run it on the IR
reflow belt , then repeat for the other side

Perhaps they cool the underside of the PCB with cold air. Fibreglass
isn't a very good conductor, anyway.

Leon
 
G

Glenn Ashmore

Jan 1, 1970
0
It is probably not a problem with IR reflow because there is a major
difference in temperature between the top and bottom sides. With a
toaster oven it takes more planning. In designing the board put all the
large components (ICs, inductors etc) on one side and small descreets on
the other. Small discreets will stay on by surface tension so you bake
that side first.
hi, a smt assembly question ..

assuming we are talking about a double sided boards , each side active
chips with .5mm spacing and 0805 resistors etc.

a local assembly house told me that they would make a solder stencil
for each side of the board ( $250 each is that ok? ) ... so they print
solder on the first side , p and p the components and run it on the IR
reflow belt , then repeat for the other side

my question #1 ... in the older days with bigger components i know
they used glue dots on the first side to hold the components so they
didn't fall off when they reflowed the second side ? do they still do
this today or won't the back side of the board get hot enough for the
solder to flow and the components to fall off ?

the reason i wonder .. is i thought the fine pitch parts need to self
center during reflow on side one and that wouldn't happen if they were
glued down ..

my question #2 ... i guess you can't batch oven relow 2 sided boards ?
( unless i guess everything is glued down .. is would seem tough to
have glue dots small enough for 0805 )

thanks for any insight ... o

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com
 
W

Winfield Hill

Jan 1, 1970
0
Glenn Ashmore wrote...
[ snip ]
I'm building a 45' cutter in strip/composite. Watch my
progress (or lack thereof) at: http://www.rutuonline.com

"pictures on this site are current as of February 2000."

Glen, it's February 2004, how's your boat project coming?
Also, what's Papa's hangout? :>)

Thanks,
- Win

whill_at_picovolt-dot-com
 
G

Glenn Ashmore

Jan 1, 1970
0
Hopefully we set the engine next month and MAY be in the water in September.

"Poppa's Hangout" is Key West

Winfield said:
Glenn Ashmore wrote...
[ snip ]
I'm building a 45' cutter in strip/composite. Watch my
progress (or lack thereof) at: http://www.rutuonline.com


"pictures on this site are current as of February 2000."

Glen, it's February 2004, how's your boat project coming?
Also, what's Papa's hangout? :>)

Thanks,
- Win

whill_at_picovolt-dot-com

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com
 
R

R.Legg

Jan 1, 1970
0
hi, a smt assembly question ..

assuming we are talking about a double sided boards , each side active
chips with .5mm spacing and 0805 resistors etc.

a local assembly house told me that they would make a solder stencil
for each side of the board ( $250 each is that ok? ) ... so they print
solder on the first side , p and p the components and run it on the IR
reflow belt , then repeat for the other side

my question #1 ... in the older days with bigger components i know
they used glue dots on the first side to hold the components so they
didn't fall off when they reflowed the second side ? do they still do
this today or won't the back side of the board get hot enough for the
solder to flow and the components to fall off ?

You still have to ensure that any heavy components presented inverted
in a reflow process are secured. Normally this means the heavy parts
all have to be on the last side reflowed, to avoid the requirement.

Special considerations are required if the finished product is
intended to be installed using subsequent reflow processes.

SMD parts are also used on the wave-solder side of an assy. These will
require adhesive regardless of weight. Obviously this restricts the
parts that can be used, as not all parts can reliably survive the
wave, can wave without bridging or can support an adhesive spot within
their pattern.

One of the aims of a board designer is to minimize the steps required
for completion, as each process carries a fixed cost. The intended
assy process and methods are usually fleshed out before a
manufacturable board design begins.
the reason i wonder .. is i thought the fine pitch parts need to self
center during reflow on side one and that wouldn't happen if they were
glued down ..

If it doesn't work, you can't do it. The trick is to get everything
located so that the process steps intended are physically practical.
Fine pitch devices, or devices with leads on more than two package
edges cannot be wave-soldered reliably, for a start. Whether thay can
be reflowed after an adhesive application will depend on your
materials, equipment and methods.

Some adhesives can be applied to a part after reflow, if they are
absolutely required. Final assemblies can be protected from end-use
re-application of reflow environments, by coating, or by using higher
temperature processes and materials in their original construction
($).
my question #2 ... i guess you can't batch oven relow 2 sided boards ?
( unless i guess everything is glued down .. is would seem tough to
have glue dots small enough for 0805 )

Reflowed parts can be inverted in a succeeding reflow process if their
mass does not cause the parts to separate from the surface tension
holding them in place, and if they are individually specified for
repeated reflow cycles.

You can get answers to some fab questions in the IPC technet forums;

http://www.ipc.org/html/forum.htm

RL
 
R

Roger Gt

Jan 1, 1970
0
X-No-Archive: yes

It is probably not a problem with IR reflow because there is a major
difference in temperature between the top and bottom sides. With a
toaster oven it takes more planning. In
designing the board put all the
large components (ICs, inductors etc) on one side and small descreets on
the other. Small discreets will stay on by surface tension so you bake
that side first.
<snip>

Excuse me, but in a "toaster Oven" don't you still
solder one side at a time, by placing a plate
(tray) under the work? So far I have only
soldered top side SMT parts with the toaster oven,
and have always placed the board on rails mounted
on a tray to keep it from laying against the
grill. (shelf) Wouldn't this also keep the lower
side from heating as much, perhaps just warming it
instead?
 
G

Glenn Ashmore

Jan 1, 1970
0
Roger said:
Excuse me, but in a "toaster Oven" don't you still
solder one side at a time, by placing a plate
(tray) under the work? So far I have only
soldered top side SMT parts with the toaster oven,
and have always placed the board on rails mounted
on a tray to keep it from laying against the
grill. (shelf) Wouldn't this also keep the lower
side from heating as much, perhaps just warming it
instead?


That is what I thought I said. I bake the back side with the small
stuff first and then place and bake the top side. I use a piece of
single side PCB board as a tray thinking it might slow the heat from the
lower element. About the biggest package I have gotten to stick
reliably upside down is a 1206 but I think I may be getting some over
run on the temperature just watching that wax pellet melt.

I am thinking about rigging up the Fuji PRX3 process controller from the
epoxy post cure oven to do double duty as a toaster oven controler.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com
 
W

Winfield Hill

Jan 1, 1970
0
Glenn Ashmore wrote...
Hopefully we set the engine next month and MAY be in the
water in September.

Wow, that's good news! But you've been holding out on us
with progress photos on your build-a-boat-project website.
"Poppa's Hangout" is Key West

I wanna go too!

Thanks,
- Win

whill_at_picovolt-dot-com
 
J

John Larkin

Jan 1, 1970
0
Don't know. If you can keep one side, at least, to small parts, I don't
think they will fall off when they are upside down.

Correct. Small parts (0805's, SOT-23's, stuff like that) won't fall
off if the board is reflowed with them on the bottom.

John
 
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