EMRFD Message Archive 2421

Message Date From Subject
2421 2008-11-18 14:43:05 bob_5b4agn Re: Soldering Fine Pitch SMD - Sorry but this is long.
I've done a lot of this kind of work at home over the past few years
and at the outset tried a number of different approaches. The key
issues are getting good visual feedback, holding components in place
for soldering, applying the solder and then confirming integrity of
your joints.

Visual Feedback

Many people give up on the idea of soldering fine pitch SMD just
because their unaided vision renders the component a blur and if
that's not enough to put them off they almost certainly run scared
when they see how much their hand shakes whilst trying to get a clear

Of course there are medical exceptions but for most of us our shakes
are caused by poor visual acuity. When we fix the visual feedback
problem our shakes go away. Thank goodness.

I am one of those people who sees nothing but a blur when I look at
even quite large SMD with unaided vision. Even with powerful reading
glasses I have trouble. I borrowed a good quality stereo microscope
for my early experiments with a plan to invest in one if it solved my
problem. With this, even the smallest fine pitch device became
crystal clear and all signs of the shakes disappeared. Results were
good but I couldn't help but feel disconnected from the work-piece
when viewing it this way.

I decided to invest $15 in a 5 inch illuminated desk magnifier at a
Maplin's sale. Much less expensive than a good microscope. This when
paired with my normal reading glasses proved to be perfect. I can see
even the finest pitch device very clearly and have plenty of space
between the magnifier and the work piece to get my soldering iron in.
The daylight fluorescent tube ensures the work is well lit. My $15
investment in the desk magnifier has been money very well spent.

Holding the component in place.

Most of us learnt to solder long ago and well before fine pitch SMD.
We were taught that to avoid dry joints both solder and iron must meet
the component to be soldered at the same time. With resin cored
solder in one hand and soldering iron in the other we would approach
the component to be soldered. Now is the time to reconcile ourselves
to no longer doing this. The simple truth is we don't have enough
hands! One hand and a small pair of tweezers will typically hold the
component in place while soldering takes place. The other hand holds
the iron bearing a very small bead of solder on its fine tip.

A problem with these devices is that registration against the PCB
tracking has to be very good but even the smallest movement with the
tweezers can have the darned thing skating around the board. Your
choice of flux can play an important part in keeping the component
where you want it. I use flux supplied in a syringe applicator from
Chip Quick. The advantage I find, is this flux has a high viscosity
at room temperature and this prevents the skating tendency making it
easier to hold the component in place.

Applying the solder

I have tried the flood and wick process and I've tried soldering
individual pins. Experience has given me a strong preference for
the latter. This because the latter is progressive and provides for
on-going adjustment. Flood and wick really requires your first effort
at registering the component to the tracks to be spot on. I use a
standard fine point T/C Weller type iron with narrow gauge silver
baring multi-core solder.

I apply flux to the PCB on all tracks to which the fine pitch device
will be soldered. I then drop the component in place, thereafter
gently manoeuvring it with my tweezers. I want it to be registered as
accurately as possible overall but more importantly I want one leg on
one corner of it to be registered perfectly. This is the leg I will
solder first. With the device held in place by the tweezers in my left
hand gently pressing down on it, I bring the soldering iron in my
right hand to the work. The tip has on it a very small bead of
solder. In the old days this approach would almost guarantee a dry
joint but today we have flux applied to the track. I place the tip of
the iron on the track immediately in front of the leg to be soldered
and watch through the magnifier as the flux draws the solder towards
the leg of the device making a perfect joint. Next I re-check
component registration after rotating the work to make the leg on the
diagonally opposite corner accessible for soldering. Having soldered
the second leg I very diligently check for accurate registration of
all remaining legs, perhaps adjusting the odd stray leg with the point
of a pin. When everything is confirmed aligned I move to solder the
remaining legs one side at a time. With diagonally opposing legs
soldered the tweezers are no longer required to maintain registration.
I am free to hold solder in my left and the iron in the other. I
still do not bring solder and iron together to meet at the joint but
do instead what I did for legs 1 & 2. Having solder in one hand and
iron in the other merely speeds the process as re-charging the iron
tip is quickly accomplished. Occasionally I manage to bridge solder
between legs despite the help the flux gives in avoiding this.
Bridges are quickly dealt with using good quality small gauge solder wick.

Confirming Integrity of your Joints

A visual check is good only in so far as it gives feedback on the
neatness of your work. It is otherwise cold comfort. A good joint
has both mechanical and electrical integrity too. I check for
mechanical integrity with the point of a pin or needle. I gently push
each leg to see if I can make it move. If I can, I resolder that leg
continuing until all legs appear mechanically sound. Next up is the
electrical check. This is best achieved with a low voltage, low
current continuity tester with an audible sounder. My good friend the
homebrew legend W4ZCB devised a circuit based upon an LM339
comparator. This has an open circuit voltage of <100mV and a short
circuit current of <1mA. It beeps at resistances less than 60-70 ohms
avoiding false scares due to normal circuit resistances. This type of
circuit should be considered close to mandatory, as the open circuit
voltage and short circuit current of many a DVM can be enough to
destroy your carefully soldered fine pitch SMD.

Harold's beeper uses needles pushed through small corks as probes.
The audible warning allows eyes to concentrate on the work
without the need to look up to check a visual indicator. Each chip
leg should be checked for continuity to the track to which it
attaches. One needle is gently applied to where the leg meets the
plastic body of the IC and the other to the track. A check for
bridging between tracks must also be carried out. Leg to track should
get you a beep if all is well. Leg to adjacent leg should not unless
they are intended to be strapped together.

Pheeww.... that's it as I see it. I could have soldered at least 20
fine pitch SMD devices in the time it has taken me to write this. I
hope someone finds it useful.


Bob, 5B4AGN
2425 2008-11-19 07:01:10 rebruhnke Re: Soldering Fine Pitch SMD - Sorry but this is long.