On 10 December, G-L-O-R-I-A (I'm gonna' shout all night...)
GLOOOOR-EEYA (I'm gonna' shout all day...)
GLOOOOR-EEEE Yeah-yeah-yeah....asked:
>> I am looking for some information regarding the application of adhesive for
>> bottom side soldering. I can't seem to find a spec anywhere. IPC has very
>> little coverage of this.
>> thanks
>> gloria
Gloria,
I read your email and when I saw your name, that ol' classic song from a
while back jus' popped into my head...(I better be careful here, I'm dating
myself...heh,heh,heh)
Anyways...
adhesives...bottomside...the application of. Ya' know, you are absolutely right,
there ain't much in the IPC 'tall...'Bout all you can find in the -610 is on
page-106, and all they say there is don't get the junk all over the pads.
There is something recent I read in the May 1996 "SMT Magazine" about
adhesives/epoxies and dispensing by a Dr. Ken Gilleo of Alpha Metals
([log in to unmask]), and there's been other articles dealing with the SMD epoxy
process in the magazine in the past. In fact, you can talk to a Jane Harrop at
area code (847) 362-8711 extension-503, she's in charge of reprints at SMT
magazine and she can probably turn you to some good articles.
I'll tell ya' what I know 'bout this stuff...
1. First thing, all yer' tryin to do is somehow secure components on the bottom
of a PCB so you can wave them along with the thru-hole, nothing more. The
adhesive isn't adding any mechanical strength, or anything else to the
soldered connection.
2. Ya' gotta' be precise when dispensing this stuff, too little and the
components will fall off in the wave, too much and ya' make a mess all over
the pads (this stuff works as a good soldermask too), or when the epoxy cures
it'll expand and lift the component clean off the surface of the PCB...
3. Most everybody uses a dispenser made just for this kinda' work, but you can
stencil it, or even some pick-and-place machines use the placement heads and
have a pin instead of a nozzle in the head which gets dipped in the adhesive
and then goes to a X/Y location on the PCB and transfers some of the adhesive
onto the surface of the PCB...thus the "Pin Transfer" method. Myself, I like
dedicated dispensers.
4. Each one of the above methods needs adhesives with certain unique properties
to perform best with the method used...eg: viscosities, slump charateristics,
etc.. The best way to find out which adhesive to use in your process is get
some different kinds and play around with it...go ahead, get it all over you,
put some dots down on some PCB's, get it all over the machine,...go ahead,
don't be scared! You might think I'm being silly now, but seriously, you do
want to find out how easy it cleans off of the equipment, or the PCB when you
mis-dispense one, if it irritates your skin if you get it on you, how well it
does dispense, and what kinda' dots it makes.
5. The reason I like dedicated dispensers, is because of the dots they make. The
best adhesive dots for components look like "Hershey Kisses"...no joke. You
want the adhesive to be able to hold that shape until the component gets
placed. It's gotta' be tall enough to contact the bottom of the body of the
component. You also want the epoxy to be able to be dispensed in individual
dots without a "string" or "tail" pulling off one dot while the dispense head
is moving to the next one to be dispensed...kinda' like what honey does when
yer' trying to put some on yer' toast in the mornin'. Those "tails" or
"strings" can get all over the solderable surfaces and cause problems during
wave...
6. And lastly, you want adhesive that can be reworked easily somehow. You don't
wanna' be rippin' up PCB laminate replacing a chipped capacitor do ya'?
One other tidbit:
* If yer' using multiple dots for one component such as IC's, or even small
passives using a double-dot needle, all the dots used for that one component
MUST be of the same shape, size, and volume. Otherwise during the curing
process the components will mis-align themselves. Thermal cure adhesives
expand while they cure, and if you have different sized dots, the amount of
distance they expand is going to be different. That was driving me stark
raving loony until I finally figured it out...placement accuracy was looking
perfect going into the oven, but coming out the components were off the pads!
I hope this helps a little bit...
P.S. My favorite brands are:
Ciba Giegy Epibond
Loctite Chipbonder
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