Well as I have received a lot of good responses for the last FAQ's on soldering
here is another  lot of questions asked in the Bob Willis assembly surgery, or
manufacturing defect clinic. Next time it will be BGA FAQ's if I continue to get
good feed back from the WORLD.
Dear Bob

We have recently introduced surface mount components into our products. We are
suffering a high level of rework on 0.025" devices due to solder shorts. The
assembly is stencil printed with paste using 0.010" stencil and reflow
soldering.


The stencil mask being used is too thick causing too much paste to be applied.
The stencil thickness should be reduced to 0.008" with a possible aperture
reduction of  in each direction 10%. You should also check your profile as in
some cases too short a time at reflow temperature and cause shorts and not allow
them to break.

Dear Bob

Our production department has problems wave soldering 0.050" connector pins. The
connector is positioned in the Centre of the board.


When changing from 0.1" pitch connectors to 0.050" there are often an increase
in shorts. The use of this type of connector should be avoided unless absolutely
necessary. Factors which effect yield are pin length, pad and resist aperture
size.

A simple change in the pad shape on alternative pins can aid drainage when the
connector is mounted across the board. Elongating alternative pads increases the
separation points during wave soldering. Its like a piece of gum it stretches
then breaks. The pad trick is featured on a SMART Group DFM video available in
the UK.

Dear Bob 

Currently we are building prototypes of our new telecoms product. During
soldering some suppliers transistors in the SOT23 packages melt. What is the
cause of this fault ?


There are many suppliers of this package you should select a supplier which
meets the European process requirements standard. You should also reduce the
number of suppliers you use in your designs. At peak temperatures used in
production for rework certain plastics do melt. Change your supplier.

Dear Bob

We currently suffer solder shorts on SOIC devices during wave soldering. Our pad
width is 0.028" is this OK? If we reduce the pad width will the joint strength
drop.


Reducing the pad width to 0.020" this will significantly reduce solder shorts.
There will not be any significant drop in joint strength.

Dear Bob

Our current designs do not allow the traditional screen print legend to be used
on our boards to identify components. What options can be used?


Component identification can often be etched into the copper foil. Simple pin
one identification can also be etched into the foil eliminating the legend stage
in PCB manufacture. It can save time and the cost of you boards. Alternatively
you could use map referencing, this is where 1,2,3,4 etc. is printed and the top
of the board with A,B,C etc. down one side. Components are identified by grid
reference. I would say legend is becoming a thing of the past, boy can it srew
up printing solder paste!!!!!!!

Dear Bob

We are not sure if surface mount is right for our company our products do not
need miniaturisation. Should we convert our products?


If you don't need SMT then don't use it. But be aware that over 45% of products
world wide use SMT. Many of the new components are only available in the new
format. If you are not sure then arrange for a training course for your design,
production and management team. Understand the benefits and disadvantages and
review designs and costs, obtain an independent opinion, give me a call.

Dear Bob

My production department needs a different PCB aperture size for the production
of stencils for solder paste. Is this normal, it means having another set or
artwork.


This is standard practice in the industry, but I would suggest that you talk to
your companies stencil supplier. Most stencil manufacturers take, and prefer
design data not film work, so they can modify the data for you.

Dear Bob

What advice can you give on hole size for through hole components. We manually
assemble and hand solder our boards.


The same design rules should be used for any board, always design for automation
regardless if it is to be used or not. Design for manufacture is good practice
and maintains good design discipline. 

As a guide the hole size is determined from the pin size. The hole size is the
pin size plus 0.010" plus any board tolerance. Remember use this as a guide but
also try to minimise the number of different hole sizes as it may affect PCB
cost. This is also featured on the DFM video from the SMART Group office. I
think its also now available from the SMTA main office in the USA. Give my
regards to Diana S in the US office.

Bob Willis  Electronic Presentation Services  England 
Tel:  01245 351502 
Fax: 01245 496123
Email: [log in to unmask]