Wave soldering of SOT23 is often a problem due to wave not being the correct
height or flux gassing during wave contact with the board. Use the following
procedure the correctly set up your machine.

Wave soldering of SOT 23 devices is best performed with a double wave a single
wave will not provide the 100% yield. I would tell you about the workshop at
Nepcon West but that may get me in to trouble again.

Procedure for Setting  Wave Soldering Parameters

The following procedure may be used when setting up a wave soldering system for
a new board design or the test board. To fully prove the soldering process only
fully populated boards should be used. The component lead length should be
checked along with the correct position of all surface mount components. The
samples should be soldered with all the necessary jigs or support clips to
prevent any sagging of the boards during the soldering operation.

Engineering should specify the flux to be used for the new design and it should
be used in the machine's fluxing system during final trials  The speed of the
conveyor should initially be set at approximately 3.0/3.5ft/min for double sided
and multilayer boards and 4.0ft/min for single sided boards. The solder
temperature should be 235-240oC when using 63% tin /37% lead alloys.

Fluxing Operation

The spray fluxer should be set-up and checked for correct operation. A sample
unsoldered board should be passed through the machine. The board should be
removed just after passing through the fluxer and the air knife. There should be
evidence of a thin coating of flux over the complete board with evidence that
the flux has just entered the holes, (plated through hole boards only).

In the case of foam fluxers the flux head should be stable and have the smallest
bubbles possible when adjustments have been made to the air pressure. Using an
unsoldered board or a Lev Check, confirm that good contact is being made with
the foam head. When using the Lev Check, the foam head is depressed, again check
that the foam bubbles making contact are relatively small.

In the case of both fluxers the air knife should be set to remove excess flux
from the base of the board.  In the case of spray fluxing with plated through
hole boards the air knife should be adjusted vertically to aid flux rising into
the plated through holes.

To monitor flux penetration into through hole with a spray unit a piece of
thermal fax paper is placed on the top side of the un-populated printed board
and passed through the spray fluxing system. There should be evidence of the
flux wetting the paper due to the flux penetration. Using the air knife at 90o
to the board will aid flux rise and allow the amount of flux applied with spray
fluxing to be reduced.

It is common to have poor solder rise on any board regardless of the surface
finish on copper or nickel/gold. Its easy to over coat the board with flux and
get good results the trick is to get good results with a minimum deposit. 

Pre Heat Operation

A fully populated assembly should be passed through the machine to establish the
temperature profile. This will be done with the fluxer and solder wave switched
off. The assembly may have been previously soldered to locate the components in
position. The temperature on the topside of the board should reach a minimum of
100oC prior to passing over the solder bath.

Either temperature labels or the Datpaq profiler can be used for this operation.
Ideally the profiler should be used as this provides a full record of the
profile for future reference. If the profiler is used thermocouples should be
positioned on the bottom of the board and on the topside. The probes should be
soldered in position using high temperature solder and only the leads held in
place with tape. The additional probes should be located in areas of the board
where there is the greatest heat sinking effect.

In the case of surface mount boards the termination points of selected parts on
the underside of the boards should be checked. There should be no more than an
80oC temperature difference between termination temperature and solder wave
temperature prior to hitting the first wave. 

If topside surface mount components are included in the design the peak
temperature over the wave is also important. If fine pitch terminations reflow
again during wave contact it can cause them to be lifted leading to open joints.
This can occur due to the increased movement of the board during wave contact,
although not desirable all boards do flex during wave contact.

A record of the profile file name saved on the computer should be recorded for
future reference along with a copy of the profile attached to the prototype
result report. The file reference should be the board name or board reference
number for ease of location.

A test board should be passed through the machine with the fluxer and pre heater
section operating. The board should be removed prior to passing over the solder
wave. There should be no evidence of solvent left on the underside of the board
which may cause gassing and spitting during wave contact.

Solder Wave Operation

As the board goes through the solder wave the board should not sag. If it does
the support is inadequate and should be reviewed. If surface mount parts are
positioned on the bottom of the board then the double wave should be used. The
solder contact should be checked by using the Lev check before processing the
prototype boards. It can be valuable to check for flux gassing which can affect
satisfactory soldering of chip components. 

The contact width on the solder wave should be approximately 15mm for the chip
wave and 30mm for the second wave. This should provide a minimum of 2 seconds in
the chip wave and 4 seconds in the second wave. During wave contact the back
section of the second wave should be moving at the same speed as the board
assembly. After soldering the first board check that on any through plated holes
full solder penetration has been achieved. If this is not visible check flux
application again in the through holes and the pre-heat settings to confirm
topside temperature is correct in the area of poor solder penetration.

The solder wave height setting should achieve a board depth of 50% of the board
thickness with or without jigs or fixtures. In the case of tooling holes or
breakout routs the solder meniscus should be clearly visible. In the case of
finger transport the fingers should fully depress the wave with the solder
lapping into the finger "V" groove.

After soldering the minimum of flux residues should be left on the board
surface. Provided the soldering performance is satisfactory with no shorts or
missing joints the quantity of flux applied by spray may be reduced. The
inspection department should be asked to inspect the board samples after
soldering with reference to the current soldering standard. A photocopy of the
bottom board layout should be made and any defects marked with colour pens.
Shorts should be marked with red, unsoldered joints should be marked in blue.

A full report on the soldering trials should be produced highlighting any
soldering problems encountered. This should be attached to any new design
assessment form which should accompany all new prototype production designs. If
specific soldering faults are encountered or the correct jigs or support clips
are not provided the soldering section of any design review should not be
approved.


 

Bob Willis
Process Engineering Consultant
Electronic Presentation Services
2 Fourth Avenue, Chelmsford, Essex CM1 4HA. England.
Tel: (44) 01245 351502
Fax: (44) 01245 496123
Home Page: http://ourworld.compuserve.com/homepages/Bwillis
Email: [log in to unmask]


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