# BGA soldering

A

#### Active8

Jan 1, 1970
0
hi:

i think win asked about using an oven. here's a snip from something that
turned up in a search:

******************
I recently came across and found the idea and the performances
interesting
as I'm balancing about using BGAs for homebrewing.
They have "selected" 3 or 4 toaster ovens and as far as I understand the
important thing is that they have the power resistors on the bottom side
of
the oven. This way the IR heat the PCB and then the solder past. This
ensure
a good temperature uniformity across all the PCB's surface (even under
the
component package) and a good temperature control.

To ensure an optimum soldering quality, i.e. follow as closely as
possible
the recommended temperature profile, you could one or several
thermocouple
on a dummy board and make some measures to optimize the process.

There's still the problem of solder past dispensing which is normally
done
by printing through a laser cut steal stencil. That's of course out of
question for personnal use (cost).
******************

note: in case anyone hasn't read the page on reflowing solder paste in
an oven with SMD parts, the paste was just laid down as a bead across
all the pads and the surface tension lined up the part and cleaned
everything up when the paste reflowed.

FWIW,

brs,
mike

T

#### Terry

Jan 1, 1970
0
Active8 threw some tea leaves on the floor
and this is what they wrote:

There's still the problem of solder past dispensing which is normally
done
by printing through a laser cut steal stencil. That's of course out of
question for personnal use (cost).

I can say from experience a that silk screened screen works almost as
well and have produced around 2000 smt boards a day using this method.

I've also used the laser cut stainless stencil method, both have their
pros and cons.

Don't be afraid to use a silk screen, it works. All you need do is save
up for the solder paste ;-)

A

#### Active8

Jan 1, 1970
0
Active8 threw some tea leaves on the floor
and this is what they wrote:

I can say from experience a that silk screened screen works almost as
well and have produced around 2000 smt boards a day using this method.

I've also used the laser cut stainless stencil method, both have their
pros and cons.

Don't be afraid to use a silk screen, it works. All you need do is save
up for the solder paste ;-)
i *know* you realize that stencil thing was a quote from someone else...

none the less, thanks for confirming my suspicions. IIRC in "Printed
Circuits Handbook" - Coombs goes to some length discussing the
fabrication of those steel stencils and how undercutting, etc. can foul
up the paste. who wants to screw around with steel and failure analysis
when a silk screen will do?

now since we have a flame war going elsewhere and this is a topic i've
been researching, perhaps you could share some of your experiences with
me. crap! i was hoping to find the latest file i found on making a
screen. it mentioned three methods and parts were unclear.

i'm really interested in knowing what works best for you and how to get
started. why don't we start with the solder paste screen while i try to
dig up that article on liguid imageable solder mask and etch resist
screening.

TIA,
mike

G

#### Garrett Mace

Jan 1, 1970
0
I can say from experience a that silk screened screen works almost as
well and have produced around 2000 smt boards a day using this method.

I've also used the laser cut stainless stencil method, both have their
pros and cons.

Don't be afraid to use a silk screen, it works. All you need do is save
up for the solder paste ;-)

I wonder if a silk screen for solder paste could be constructed with the
toner-transfer method. Print a positive and iron it into the material, then
use that to fill a negative on *another* piece of material.

Nah, stuff would probably move around too much. Maybe you could use a
positive to mask etch-resist onto a piece of thin metal shim stock, and then

A

#### Active8

Jan 1, 1970
0
On Sun, 12 Oct 2003 14:33:19 GMT, Active8, said...
note: in case anyone hasn't read the page on reflowing solder paste in
an oven with SMD parts, the paste was just laid down as a bead across
all the pads and the surface tension lined up the part and cleaned
everything up when the paste reflowed.
of course, with BGAs you don't need paste.

mike

A

#### Active8

Jan 1, 1970
0
On Sun, 12 Oct 2003 14:33:19 GMT, Active8, said...
note: in case anyone hasn't read the page on reflowing solder paste in
an oven with SMD parts, the paste was just laid down as a bead across
all the pads and the surface tension lined up the part and cleaned
everything up when the paste reflowed.
of course, with BGAs you don't need paste. te point is that the surface
tension of the solder *may* pull the chip in as it does for solder
paste. think of how you can pick up leads with an iron or get a chip cap
to follow the iron around if you don't hold it down.

mike

A

#### Active8

Jan 1, 1970
0
I wonder if a silk screen for solder paste could be constructed with the
toner-transfer method. Print a positive and iron it into the material, then
use that to fill a negative on *another* piece of material.

i really think the normal sesitized emulsion method would easier
Nah, stuff would probably move around too much. Maybe you could use a
positive to mask etch-resist onto a piece of thin metal shim stock, and then

check the links john gave in my toner xfer thread. they claim you can
etch metal (at least brass - don't ask me why brass is speacial) with
the film in the first link. i don't see why you couldn't do it with the
PnP blue or wet, though. the dynaart/pulsar site doesn't mention
anything special about their product that prevents undercutting of the
metal. IIRC Coombs doesn't mention using a trapezoidally (is that even a
word? ) undercut stencil, mirror imaged and flipped, so that it
doesn't lift the paste off the board. he may have mentioned that it
would be hard to squegee the paste in that way, though. i might play
with it one day, i think i know the trick. i also found another link
while checking out photoimagable solder mask. a company that etches
steel and aluminum stiffeners. Coates or someone. it's all doable.

but from what i've read, a silk screen will do just fine. i've seen
elargements of screened images and for paste and soldermask, it's fine.
i see resolutions that might work for etch-resist, also, but i'm
skeptical as to whether this would be acceptable. the enlarged images
i've seen were for traces and one of 'em looked, let's say "not too

let's see what Terry has to say about the pros and cons if he'll be so

crap! i can't find the link.

now here's the real ball-buster if 150 ain't too much... Ed (GPS) supplied this in this group. see: Re: Succes - first finepitch device soldered www.stencilsunlimited.com brs, mike H #### Hal Murray Jan 1, 1970 0 of course, with BGAs you don't need paste. I think they normally use paste to get the flux in there. A #### Active8 Jan 1, 1970 0 I think they normally use paste to get the flux in there. depends on what you mean by paste. the "balls" in "Ball Grid Array" are made of solder and you add flux. brs, mike G #### Georg Acher Jan 1, 1970 0 |> depends on what you mean by paste. |> |> the "balls" in "Ball Grid Array" are made of solder and you add flux. This is not true for all packages. There are a few BGA-packages made of ceramic with so much weight, that they would squeeze out the solder when warmed up. Thus, their balls don't melt (non-eutectic) and you need solder paste for proper contact. Normal packages (ie. the cheap epoxy-based ones) have solder balls and only need some flux. K #### Klaus Vestergaard Kragelund Jan 1, 1970 0 Terry said: Active8 threw some tea leaves on the floor and this is what they wrote: I can say from experience a that silk screened screen works almost as well and have produced around 2000 smt boards a day using this method. I've also used the laser cut stainless stencil method, both have their pros and cons. Don't be afraid to use a silk screen, it works. All you need do is save up for the solder paste ;-) -- I think I have missed that thread. Do you just use the silkscreen as the stencil for applying the paste? Could you direct me to the original thread, a link perhaps? Thanks Klaus D #### Da Man Jan 1, 1970 0 I think it was future active or digikey that sells little disposeable stick on stencils for BGA rework. A #### Active8 Jan 1, 1970 0 I think I have missed that thread. Do you just use the silkscreen as the stencil for applying the paste? Could you direct me to the original thread, a link perhaps? it was archived at google IIRC. could have been an archive or search hit of a web forum. but there was no mention of silk screen AFAIK. it was about pcb fab and the poster just mentioned the stainless stencil. you'll find more stuff on this than you can shake a tree at via google. don't worry, i think i got all the relevant stuff as far as toasters are concerned so you didn't miss much. i just found that snip in my note. IIRC, win hill or someone was asking about BGAs in this group and the question came up about reflowing them in toasters. brs, mike T #### Terry Jan 1, 1970 0 Active8 threw some tea leaves on the floor and said: i *know* you realize that stencil thing was a quote from someone else... Opps, your marks for clairvoyance aren't too high ;-) none the less, thanks for confirming my suspicions. IIRC in "Printed Circuits Handbook" - Coombs goes to some length discussing the fabrication of those steel stencils and how undercutting, etc. can foul up the paste. who wants to screw around with steel and failure analysis when a silk screen will do? I found the stainless stencil to be nice and accurate, but it had problems of paste leaking underneath. now since we have a flame war going elsewhere Eww, I'm not trying to start any flame wars and this is a topic i've been researching, perhaps you could share some of your experiences with me. crap! i was hoping to find the latest file i found on making a screen. it mentioned three methods and parts were unclear. I had my silk screens made at a local screen printer. I'd supply the artwork and they would make the screen. It all went together in a simple wooden jig, the pcb aligned by hand into two pcb thickness stops which were then fixed to the base of the jig. Apart from careful usage and cleaning the screen clean after each run they were fine for the task. Lets face it, IR reflow allows for a lot of errors anyway including pick and place machine alignment errors. i'm really interested in knowing what works best for you and how to get started. why don't we start with the solder paste screen while i try to dig up that article on liguid imageable solder mask and etch resist screening. My experience is getting a bit old now, and I imagine that there are many new techniques around. I used the standard solder paste that comes in a300 tube, it was
squeeged onto the silk screen carefully by a experienced operator
and then the parts were placed by an Amistar SMT P&P machine with a

From there it was into the IR reflow oven and out the other end to
inspection and testing.

I guess like many things, when you work with them everyday, they soon
seem very simple.

I haven't made any SMT assemblies (professionally) for years now, but
I'm thinking of setting up a small home system at the moment to compliment
my little home development workshop.

Pics of my setup and pcb efforts are at :-
http://milkstone.d2.net.au/software/pcb.html

T

#### Terry

Jan 1, 1970
0
Active8 threw some tea leaves on the floor
and said:
i really think the normal sesitized emulsion method would easier

I do as well, tho I've never made a silk screen myself.

Some kind of light sensitive emulsion would do, perhaps even PVC (the
whitish clear liquid) ?

A friend uses that as a photosensitive coating for etching aluminium
labels, and it works nicely judging from the results.

but from what i've read, a silk screen will do just fine. i've seen
elargements of screened images and for paste and soldermask, it's fine.
i see resolutions that might work for etch-resist, also, but i'm
skeptical as to whether this would be acceptable. the enlarged images
i've seen were for traces and one of 'em looked, let's say "not too

let's see what Terry has to say about the pros and cons if he'll be so

From my pov, and I'm not the expert here by any means:

Stencil:
Pros: Nice and accurate in terms of area to be covered and *depth* of
solderpaste.
Cons: Can get solder paste running underneath and smearing on the pcb.

Plus hard to make at home without a 50W NC laser ;-)

Silk screen:
Pros: Accurate enough, the solder paste flows around the silk strands.
easy to make compared to stencils, low in cost.
Cons: depth of solder paste is hard to control, needs an experienced op
to get repeatable results, must have good tension etc.

T

#### Terry

Jan 1, 1970
0
Klaus Vestergaard Kragelund threw some tea leaves on the floor
and said:
I think I have missed that thread.

Sorry Klaus I missed the whole thread as well, and just jumped in as is
my way
Do you just use the silkscreen as the
stencil for applying the paste?

I used to be the technical manager of a small SMT assembly plant. We
had one Amistar SMT P&P machine, a couple of Dynapert leaded p&p
machine, a die bonder and a wire bonder, plus re-flow oven and solder
bath etc.

We used silk screens to apply the solder paste at that time. Later in
another job, I came across stainless stencils and so I have had
experience with both methods.

The silk screen method is really simple. The screen has been coated
with a photosensitive paint and exposed thru the CAD generated artwork
which contains the images of all the SMT footprints (or the inverse).

So where a footprint exists on the pcb, the that area of the silk is
clear and will allow solderpaste to be squeegeed thru onto the pcb.

The operator makes a thin line of solderpaste at the top of the screen
after the pcb has been placed below into its alignment holders and one
pass of the squeegee is done. The screen is then lifted off (its hinged
at the top or side) and the pcb removed. The SMT parts are then placed,
naturally they stick to the solderpaste and even a decent knock against
a wall fails to move them!

If too little paste is placed, a second squeegee can be done, but this
isn't a great idea.

Oh, one more thing, the silk screen sits *above* the pcb by a few
millimeters and in theory only the area below the squeegee comes into
contact with the pcb, unlike a stencil which lays flat on the pcb.

T

#### Terry

Jan 1, 1970
0
Active8 threw some tea leaves on the floor
and said:
On Sun, 12 Oct 2003 14:33:19 GMT, Active8, said...

of course, with BGAs you don't need paste. te point is that the surface
tension of the solder *may* pull the chip in as it does for solder
paste. think of how you can pick up leads with an iron or get a chip cap
to follow the iron around if you don't hold it down.

mike

I found that this is true, however one needs a little more paste for
the reflow to move the SMT parts into place, and this makes the final
job a little less neat due to the extra solder and resin present.

With perfect alignment of the smt part, less solder paste can be used
and the board is ready to go without any cleanup, however too little
solder paste and some parts may "toombstone" or stand up slightly with
one end not touching the pcb.

This naturally is a "bad thing" (tm)

T

#### Terry

Jan 1, 1970
0
Active8 threw some tea leaves on the floor
and this is what they wrote:

<deletia>
I agree that the temperature of the "oven" should be well regulated
and follow the accepted profile of reflowing solderpaste.

Our IR commercial reflow oven was a bit like a pizza makers oven and
had about 5 heat plates above and below a metal mesh conveyor belt.

Each bank had its own temperature controller and the settings could be
altered, as could the speed of the conveyor belt.

The unit was about 10 feet long and the boards went thru in about 3
minutes (from memory).
note: in case anyone hasn't read the page on reflowing solder paste in
an oven with SMD parts, the paste was just laid down as a bead across
all the pads and the surface tension lined up the part and cleaned
everything up when the paste reflowed.

This makes me cringe because too much solder paste can be a real
problem and wicking *can* occur.

If that technique works, it's got to be more luck than anything else
imho.

I know that working with SMT is a hassle in many ways (especially at my
age and eyesight) but I don't think that trying to make it easy by just
placing solder paste across all the pins of a part is a process that
will be very reliable.

I imagine most of us here just make a few prototypes at a time?

In that case we need something that's as simple and cost effective as
possible, but what's the answer ?

I think that a syringe and a good eyeglass may allow the dispensing of
solder onto each pad of small prototype pcb with reasonable results ?

Or some easy way to produce a silk screen, but I haven't looked into
this yet.

Can some laser printer paper manufacturer please develop a semi stiff
silk A4 sheet please ? ;-)

Hmm, whats to stop someone using a a4 sticky backed paper sheet and
sticking some silk screen to it, then running thru a laser ?

If the silk melts, its *your problem*, but then again what about a fine

If the brass or the cut ends of the brass screen damage the laser toner
roller, once again its *your problem* not mine!

I think there are a few workable ideas here ?

I've got an expendable laser and nearly empty cart, so if I can find the
materials, I'll give it a go as soon as I can

A

#### Active8

Jan 1, 1970
0
Active8 threw some tea leaves on the floor

Opps, your marks for clairvoyance aren't too high ;-)

oops. it was clearly mentioned and delineated within asterisks ;-)
Eww, I'm not trying to start any flame wars

and i'd rather have a discussion with you than read that bs. it looks
like normal posting is back up. weekend's over and all. and my friggin'
(hmm... how many days has it been) or the damn think takes naps.
I had my silk screens made at a local screen printer. I'd supply the
artwork and they would make the screen.

ok, i'm planning on making my own for now. done a little research.

i noticed you mentioned PVC as a photosensitive emulsion. i didn't know
that it was used like that. anything else you can add to that. i know
nothing of PVC other than pipe and it's various uses.<snip>

brs,
mike

A

#### Active8

Jan 1, 1970
0
Active8 threw some tea leaves on the floor
and this is what they wrote:

<deletia>
I agree that the temperature of the "oven" should be well regulated
and follow the accepted profile of reflowing solderpaste.

profiles have been run on 4 toasters, but the pics can't be read and i'm
not convinced it's right because you have to tailor the profile for
certain situations.
Our IR commercial reflow oven was a bit like a pizza makers oven and
had about 5 heat plates above and below a metal mesh conveyor belt.

and those toasters tested had only top heaters which was claimed to be a
good thing. that's something else with which i don't neccessarily agree.
Each bank had its own temperature controller and the settings could be
altered, as could the speed of the conveyor belt.

The unit was about 10 feet long and the boards went thru in about 3
minutes (from memory).

This makes me cringe because too much solder paste can be a real
problem and wicking *can* occur.

If that technique works, it's got to be more luck than anything else
imho.
the pic of the beads of paste looked like it was a sloppy way to do
things but the result looked real good. i think the soldermask helped.
I know that working with SMT is a hassle in many ways (especially at my
age and eyesight) but I don't think that trying to make it easy by just
placing solder paste across all the pins of a part is a process that
will be very reliable.

it was the pads. still, i'd just as soon hand solder than do that.
I imagine most of us here just make a few prototypes at a time?

yeah, but my time is valuable and i'd like to have little robotic
foreigners doing my light work for me.
In that case we need something that's as simple and cost effective as
possible, but what's the answer ?

I think that a syringe and a good eyeglass may allow the dispensing of
solder onto each pad of small prototype pcb with reasonable results ?

ouch!! silk screen it.
Or some easy way to produce a silk screen, but I haven't looked into
this yet.

Can some laser printer paper manufacturer please develop a semi stiff
silk A4 sheet please ? ;-)

the best screen turns out to not be silk. it's polyester and that's the
one most commonly used.
Hmm, whats to stop someone using a a4 sticky backed paper sheet and
sticking some silk screen to it, then running thru a laser ?

If the silk melts, its *your problem*, but then again what about a fine

what about getting some screen from a graphic arts supply biz?
If the brass or the cut ends of the brass screen damage the laser toner
roller, once again its *your problem* not mine!

I think there are a few workable ideas here ?

I've got an expendable laser and nearly empty cart, so if I can find the
materials, I'll give it a go as soon as I can
great! if you don't **** it up and it's expendable. send it to me. i
need one.

has been a lot better than that flame war.

talk to ya soon.

brs,
mike

R
Replies
4
Views
2K
Martin Riddle
M
Replies
17
Views
6K
Replies
11
Views
3K
M
Replies
11
Views
3K
josephkk
J
Replies
1
Views
1K