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1.
Tools for inspecting assemblies:
§
Magnifying glasses 5 to 10X with halo lamps
§
Printed workmanship standards to eliminate confusion
§
10 to 30X microscope may be required for inspecting difficult parts such as fine
pitch components and solder paste quality.
§
For high volume manufacturing automated visual
inspection such as 3 dimensional laser scan or X-ray scan may be
justified.
2.
Techniques for Inspecting Assemblies:
§
Boards need to be tilted to examine under components when inspecting for trapped
contaminants or J type leads. Also, if rework and touch-up are to be done along
with inspection, a board positioning table may be useful.
§
Dental picks or pointed wood sticks are useful to verify the soldering joints of
leaded components when in doubt. Use pick or stick to gently push the top edge
of lead to check for joint attachment. This prevents damage to the board and
lead. An unsoldered lead or one with cold joint will move when pushed and this
needs to be touched-up.
§
Write down or log all defect types using a common vocabulary. This information
should be useful to find common defects and trends so their source can be found
and eliminated.
Good
solder joint:
o Smooth and shiny. No voids, or blowholes.
o Good wetting of soldered joints
o A concave fillet shape and free from excessive
solder deposition.
3.
Touchup, Rework and repair:
o
Rework or repair is required to be done on components/ boards that are not
meeting the workmanship or performance standards.
o
Rework tools that are commonly used are soldering irons, desoldering
stations using suction, hot air jets, solder wick etc.
o
Rework on SOs and other high lead count packages needs prior training and some
practice on dummies will be helpful.
o
Since leads and terminations of SMT packages are small, the thermal
requirement is less compared to through-hole component pads. The components are
to be removed only after ensuring that all leads are desoldered or solder has
reflowed. If not done properly, the chances of damage to the pad area is
significant.
o
Before removing the component gently push it to check for complete solder melt.
o
When the connecting solder melts on each lead or termination of the component,
it’s readily removed and replaced with a new one.
o
SMT rework requires operator
training as it requires new techniques and new tools.
o
Alcohol in dispensing bottle
o
Cotton swabs
o
‘R’ , ‘RMA’ flux in a small dispensing bottle
o
Dental picks or pointed wooden sticks
o
Long, thin tweezers
o
Solder wicks in sizes as required
o
Fine tip temperature controlled soldering iron and spares as required
o
Desoldering station with appropriate bits
o
Manual hot air jet and nozzles and hot air rework station
o
Solder paste and dispenser
o
24 gauge ( 0.015”) flux cored solder wire
o
Static free work station with wrist strap and proper grounding
o
Workmanship guidelines
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