Victor,
Here's all ya' need to know about double-sided SMT. First I'll address each
of yer' queries, and then throw my own two-cents in.
>> When there is a need to solder SMT components on both sides of PCB, I
>> understand that 2 approaches are taken generally.
Yeah, there's two approaches...
>> i) use low temp. solder paste on one side and high temp solder on the
>> other
Well, I guess ya' could do that, but be prepared to pay some "dinero"
for your solder paste. The two common metals I've heard of to alloy and
lower the melting point of solder is bismuth, and indium...but it's BIG
BUCKS! I thought about it one time quite a while back on a very, dense
7" X 10" board that we were building. I weighed a bare board, printed
one side with paste and weighed it again to get an idea of how much
paste I was laying down per board, checked into how much a paste with
indium was going to cost, and it worked-out to about $30.00 a board!
Whoa Nellie! Too rich for my blood! I've since learned it's not really
necessary to go and buy weird solder paste to do that...just do a
double-sided reflow
>> ii) use conductive adhesive on one side and solder on the other side
I don't think you mean conductive adhesive, unless there's something
out there I haven't heard of yet. I recall somebody working on "Z-axis"
conductive adhesives to replace solder paste a few years back, but you
also needed some pressure along with heat to cure it and get a good
electrical connection. I think what you mean is using some sort of
epoxy adhesive (thermal or Ultraviolet cure) to secure the bottomside
components in position, and then wave solder them along with the
through-hole.
>> My question are:
>> 1) Which one is more popular in practice?
Epoxy bottomside, then wave.
>> 2) What are the pros and cons for the two methods?
Double-sided reflow is the way to go if the board has very minimal
through-hole that needs to be waved, it's one less process step (no
glue-dispensing)...
>> 3) For i), what are the melting points for the 2 kinds of solder
paste?
Here, I'll give ya' a few melting points fer' different alloys:
1. 63% tin/37% lead (most commonly used)
Solidus = 183?C
Liquidous= 183?C
2. 60% tin/40% lead
Solidus = 183?C
Liquidous= 189?C
3. 62% tin/36% lead/2% silver(shiny joints soldering to gold)
Solidus = 179?C
Liquidous= 179?C
4. 43% tin/43% lead/14% bismuth
Solidus = 144?C
Liquidous= 163?C
5. 52% indium/48% tin
Solidus = 118?C
Liquidous= 118?C
6. 42% tin/58% bismuth
Solidus = 138?C
Liquidous= 138?C
>> 4) What are the precautions/possible defects one should bear in
mind?
Well, that's a pretty wide-open question...so I'll give some
pretty general comments. The design of the assembly is going to
be what dictates the method you use for attaching the bottomside
SMT components. Like I said earlier, normally a double-sided
reflow is used when through-hole is very, very minimal. BUT, I
have used a double-sided reflow EVEN if the board is waved
because there's been a crappy job done on the layout on the
bottom of the board that the components weren't oriented to
wave well. I've printed solder to make sure I can get some decent
fillets, and dispense the epoxy so I don't lose the parts in the
wave. Other than that, standard wave soldering rules apply.
Double sided reflow works great provided IPC design guidelines
have been followed somewhat as far as component footprints and
geometry's are comcerned. The solder's surface tension when it is
liquidous holds all but the heaviest components on the bottom
just as long as you don't "bow" the board during its trip down
thru the reflow oven, or have a oven conveyer belt that jerks and
jolts.
As far as all those strange alloys, in 9-years of assembly, I've
never used anything but tin, lead, and silver in my solder paste.
Not just because of the cost, but also because of the resulting
solder joint properties and appearance. Bismuth and Indium added
to tin or lead will reduce both the shear and tensil strength of
the joint, plus the solder joint looks like doo-doo compared to a
tin/lead/silver joint...it'll be real dull and gray looking.
>> 5) Can I use the same operating procedures for making 2 sided BGA assembly?
"Holy Solderbridge Batman! Double-sided BGA? MAN! I'm Sorry to say I don't
have any experience with that yet!" (Thank goodness!!)
>> Any literature that may describe the above issues?
Of course! The IPC manuals are a good start...(See that IPC? Now when do I
get my commisions? Ha!Ha!Ha!) But here's a few others:
"SMT Design Rules and Standards" by James Blankenhorn
"Mechanics of Solder Alloy Interconnects" by Darrel Frear/Harold Morgan/
and Steven Burchettt
"Guide to Reflow Soldering and Temperature Profiling" by James Blankenhorn
"Mastering SMT Manufacturing" by Michael Brisky (used to be my old boss)
These books and others can be obtained from SMT Plus Inc. (408) 438-6116
ask for Jim ([log in to unmask]). You can also get a wealth of info from any
one of the trade magazines that are out there...SMT, Printed Circuit
Assembly, etc...
__\/__
. / ^ _ \ .
|\| (o)(o) |/|
#------.OOOo----oo----oOOO.-----#
# Steve Gregory #
# [log in to unmask] #
# #
#________________Oooo.__________#
.oooO ( )
( ) ) /
\ ( (_/
\_)
P.S.
Before I forget, GOOD LUCK!
***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to: *
* [log in to unmask] with <subject: unsubscribe> and no text. *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] *
***************************************************************************
|
