Bob Willis Guide to Solder Spots - A New Plague in Manufacture ?

So what are solder spots ? They appear to be the next big problem in modern
reflow assembly, in fact in any process that involves solder paste. The
spots are visible on the surface of pads which have not been pasted prior
to reflow. The spots are seen more commonly on copper, gold or any surface
which is not fusible during reflow. The spots are a tin lead alloy which
become visible due mainly to the different colour of the pad surface and the
wetting action which takes place at reflow. If the spots were on a tin/lead
or silver surface there would not be much of a problem. Out of sight out of
mind. Some companies do apply rigid standards or apply the IPC guidelines
very vigorously, failing the products at final test or during goods receipt
if products are contracted out.

If the surface of the pads are to be used for interconnection then any
contamination may not be acceptable. Contact strips, LCD interconnection or
key pads are examples where solder contamination of course, mechanical
connection failure or intermittence. This would, have cause be related to
the position and the size of the spots. It is, however much easier to just
reject the products than make a judgement call but that is a costly option.
Depending on the cost of the product, make repair viable the spots can be
removed without any obvious signs.

If the spot is high off the surface of the pad concern may be shown on the
effect on contact resistance on the switch pads. Any key contact or button
contacting the surface may be lifted off the pad by the solder spot which
will decrease the surface contact and increase the contact resistance. In
most cases the contact resistance tolerance on such actions is wide and less
likely to cause a problem.Surface analysis of the solder spots has confirmed that the spots are
tin/lead and are most probably caused by solder paste particles. More
detailed evaluation would be able to determine the alloy content and
separate the hand cored wire, wave soldering alloy or even plating from
component terminations. The particles present on the surface of the pads
during reflow will wet the surface and it only takes the smallest of
particles to have a visual impact on copper and gold features. The surface
spots can be seen as a random defect or consistently in the same position on
the circuit pattern. They are very commonly seen at the edge of tacks and
pads which is normally a fairly good indication of the root cause of the
problem.

Elimination of the spot plague needs close consideration of each of the
assembly stages in the modern factory. This is necessary as each could
contribute to the problem of surface contaminates.
So lets call in the spot inspector !

Screen Print

During solder paste printing thousands of small balls are printed on to the
surface of the board in an attempt to produce consistent solder joints.
During normal printing there are a number of things which make contact with
the top and bottom surface of the board. The most obvious is the metal
stencil which contacts the surface of the pads and depending on the pressure
applied during printing, the surface of the board.

Solder paste will squeeze out between the stencil and the pad if a poor
gasket is produced allowing paste to contaminate the base of the stencil.
Recent video recordings produced by the author showing paste squeeze out due
to uneven board surfaces show how much this can be a problem in manufacture.
In this situation the paste particles on the base of the stencil can
contaminate the adjacent areas of the next and future boards. When an under
stencil wipe is used the particles can be spread around on the base of the
stencil without being properly removed resulting in random spots after
reflow. Care needs to be taken on the maintenance of the wiper cloth and
the solvents used.

During printing supports are used to maintain the board flat and provide the
ideal surface for printing. The board support may be a solid machined plate,
groups of magnetic pins or specially moulded supports. In the case of the
machined or moulded support or nests vacuum is normally used to help nest
the board. Excessive suction has been shown to pull paste particles through
the board to the base or along the surface of the board between the stencil.
In each case this has resulted in surface contamination which can be random
or in a line from a pasted pad feature to a via or hole where the vacuum
pressure is seen. It is also most likely to be on the edge of pads.

There are very few examples of plating outgassing today but one example of a
surface mount component was analysed recently. The component terminations
outgassed during soldering caused voids in the joints and local spots. This
was originally blamed on the paste and reflow profile until the components
were tested.

PCB Wash Off

One of the most common reasons for random defects is washed off boards. The
board may look clean but a few solder balls is all it takes to cause solder
spotting. If the board is poorly printed or there is down time on a
production line boards may be washed off. When this is done paste particles
can still be found in via holes, tooling holes and in resist apertures
between the resist wall and the pad. This is why solder spotting is often
seen at the very edge of the pads after reflow.

There have been many occasions where poor washing was the cause but every
one said the boards had never been cleaned.
"We never ever do that" said one contractor at an workshop only to be shown
the particles in the solder resist undercut.

Placement

After components are placed into the surface of the paste the board should
be reflowed and that is the operation complete. Misplaced components can be
only just off the surface of the pad or completely misplaced. This can
result in manual pick up of a part and repositioning it in its correct
location. If the part has been in contact with the paste then the
terminations will have paste on them. If the part was on another pad which
is not pasted some paste will be left behind when the part is lifted. This
would be totally random and very few companies have records of the
occasional replacement exercise.

Printed Boards

There are times where the type of assembly or issues with the printed board
can cause solder spotting during reflow. There are occasions when
manufacturing engineers have printed solder paste into via holes to fill the
via during reflow. This may have been done to provide a test pad, prevent
vacuum loss during in circuit test or in an attempt to improve the
reliability of the via. Paste printed into a via will reflow but what if the
copper plating thickness is poorly controlled the via will gas causing the
particles of paste to be displaced.

The same can be seen during pin in hole/intrusive reflow assembly. A large
volume of paste in a confined space will prove difficult for the soak zone
of a reflow process to allow the evaporation of the solvent. Again small
particles can gas out causing random spotting.

Reflow

Some engineers have blamed the reflow oven for causing the paste to spit
resulting in random spotting. Solder paste should not spit during reflow if
the profile and the paste are compatible. To test your product simply take a
hot plate and a pasted board and reflow the board on the plate matching the
profile recommended by the solder paste supplier. This has been done in the
past by placing shims between the board and the plate to adjust the ramp
rate. The same test can be conducted in an oven during the normal reflow
cycle but it does not provide the opportunity to record or observe the
cycle, seeing is believing.

Take an hour glass plate normally used in a laboratory and place it over the
board. The smaller the glass and the closer it is to the paste the better.
This technique was originally used by the author to show when the different
solvents in a paste evaporated during reflow. Normally condensation is seen
on the surface of the glass. If any spitting were to occur with a paste this
would be visible on the hour glass. An alternative is to use a white card
when placed over a hot plate. The white paper or card trick is also used to
assess some cored solder wires for spitting during hand soldering and is
simple and cheap evaluation tool.

A number of engineers have considered the possibility of paste particles
being circulated in the convection reflow oven, either due to high
convection rates or re-circulating of gas at reflow or cooling phases. Poor
maintenance of the equipment could, it has been suggested allow paste
particles to fall onto the surface of passing boards prior to reflow. This
is less likely due to the weight of the paste particles but can easily be
determined. Production manager may not like it as throughput is king, but a
problem needs to be sorted. Take a few sheets of thin copper laminate and,
without any production running through the machine, pass the sheets
repeatedly through the process. Inspect the panels after each pass for
spotting.

There are still some companies who process double sided reflow products on
mesh belts with the board in contact with the mesh during its second pass. A
poorly maintained machine can easily have paste residues on the mesh which
could contaminate the base of the board.

A final area to investigate are the entry and zone curtains on some ovens.
The rubber curtains can, if incorrectly positioned or cut, will not only
contaminate the board but displace components.
Rework

If solder paste is used during rework there is an opportunity for paste
contamination. Normally paste used during rework is manually dispensed
although some companies use mini stencils to apply the paste. This is a more
obvious area for paste contamination and would normally be spotted quite
quickly during investigation.

Finally just handling the boards in production should be avoided as any
operator or technician knows paste gets everywhere. Generally when operators
are using bare hands to handle boards they tend to be more careful with
paste contamination. Some companies have the policy of wearing gloves which
reduces the need for care as the paste will not go on your hands.
Disposable gloves are used but they tend to cause sweating and not that well
liked. The alternative reusable gloves tend to get dirty and again cause
handling contamination.

If any one has any other examples of where paste residues appear on non
pasted areas of printed boards and then form the solder spot plague then
please Email us your causes and cures. This brief introduction can then be
updated and circulated again.


Bob Willis
Process Engineering Consultant
Electronic Presentation Services
2 Fourth Avenue Chelmsford Essex CM1 4HA
England
Tel: (44) 01245 351502
Fax: (44) 01245 496123
Email: eps@bobwillis.co.uk
Web Page: www.bobwillis.co.uk