Using Zeph paste and a 25 gauge plastic syringe + hot air gun , I can solder
a 128 pin .5mm pitch QFP in under 10 seconds (including the pasting time)
with no bridging.
I cannot usually solder .4mm pitch devices without some bridging. However,
I just use kester flux in a syringe and a wide soldering iron and drag it
across the pins and this moves 99% of bridges as long as I made my pcb pads
big enough.
Syringes have no cleanup.
I have a laser cutter and it's pretty effortless for me to make a stencil
but I don't use it unless I need to make more than 2 boards because the
cleanup time is more than my syringe time.
On Mon, May 9, 2011 at 5:27 PM, Terry <twgray2007@...> wrote:
>
>
> For prototyping you can make your own solder stencils using a 3-4 mil
> copper gasket material from a good hardware store. I use toner transfer to
> apply the solder mask and then etch as you normally would a board. Very
> fast, clean, and simple.
>
> For fine pitch parts in prototyping or rework I always tin the pads with a
> solder, wick up any bridges and then use a hot air rework tool to solder the
> part in place. Much easier than hand soldering the fine pitch leads and then
> trying to wick up the bridges.
>
> Using a 3-4 to 1 solder paste/flux mix ratio usually thins the solder paste
> enough to apply on very small pads.
>
>
> --- In Homebrew_PCBs@yahoogroups.com, "Boman33" <boman33@...> wrote:
> >
> > For regular SMT assembly I use commercial stencils to screen print the
> > solder paste but I am having problems dispensing solder paste by hand in
> > small amounts for rework or prototypes.
> >
> > I have seen somewhere that individual stencils are sold but that adds the
> > requirement to accurately align the stencil which will be very difficult
> on
> > a rework board.
> >
> > Using a syringe and placing a single long bead across all the IC pin pads
> > work fine on wider pitch ICs but as the pitch is getting smaller I am
> having
> > trouble getting the line thin enough. In other words, there is too much
> > solder so when I reflow there are solder bridging.
> >
> >
> >
> > If I use a finer tip on the syringe I cannot squeeze it hard enough for
> the
> > paste to come out. If I add some flux it gets thinner but too runny. I
> > like the normal consistency since it helps to keep the ICs in place with
> its
> > stickiness. My current smallest bead diameter is 0.5mm (20mils). I like
> to
> > be able to get about half of that size.
> >
> >
> >
> > I am basically looking for a solder paste with less solder and more thick
> > flux material so I can squeeze out a reasonably sized bead but after
> reflow
> > there is less solder than a normal paste.
> >
> > Any suggestions on a paste like that?
> >
> >
> >
> > Any other suggestions on how to place a very small amount of solder on
> the
> > IC pads?
> >
> > Thanks in advance for suggestions.
> >
> > Bertho
> >
> >
> >
> >
> >
> >
> >
> > [Non-text portions of this message have been removed]
> >
>
>
>
[Non-text portions of this message have been removed]