I dont' know if this sort of question and answer session will be of interest to
you all. Please let me know. Its a question page from a magazine I produce. 

Dear Bob 

I want to profile my reflow oven to try and reduce reflow temperatures where
should I place my thermocouples?

Positioning of thermocouples comes down to experience. They should always be
soldered in place using high temperature solder. First position a probe at the
centre of the board and one at the corner edge of the board. This should
theoretical give you the lowest and highest temperatures. Next consider where
the largest mass of components are located on the board and place a probe at one
termination point. In a convection oven you should see a maximum temperature
differential of 15-20oC. Experiment again with the same board placing the probes
in different positions noting the temperature changes.

Dear Bob 

If I change my extraction system or my settings do I need to perform further
profiles?

Yes depending on the type and position of the extraction points it can have an
effect. If the ducting is poorly designed other equipment in the factory can
effect the extraction rates and your board temperature during reflow.

Dear Bob 

How can I justify the use of nitrogen gas during reflow?

You can justify it on quality grounds, some people have justified it through
moving to no clean due to the saving on CFC materials. No clean materials can be
run in air so this is not a good justification. Its better to justify it on cost
savings if for example you use copper based board finishes. This will improve
other manufacturing yields and save enough money per board to pay for gas usage.
You may find the new SMART Group video on inert gas soldering useful.


Dear Bob 

If I have flexible circuit to assemble how can I hold them flat during reflow?

Yes this is a nice one, as we increase convection rates during reflow the
flexibles tend to flap, moving components out of position. Depending on volumes
the best way is to hold the circuits flat  on jigs. A two part laminate jig can
sandwich the flexible, solder paste is dispensed at high speed, components
placed and then reflowed. Using a cover plate over the flexible as part of a two
piece jig prevents screen printing from being used.  


Dear Bob 

What is the benefit of using 62% tin 2% silver alloy paste during reflow?

Today the benefit is a slightly lower temperature for reflow. The eutectic
alloy, 63% tin 37% has a reflow temperature between 183-184oC. The silver
bearing alloy reflows at 179oC. The four or five degrees can make a difference
allowing upper reflow temperatures to be lowered.


Dear Bob 

Can I use the same temperature profile for second side reflow on a double sided
product?

Yes you can, but when does reflow take place on your existing process? You must
fully understand your process and when does your product reach reflow
temperature. In the case of double sided reflow the first side components can
soak up heat and slow down reflow. This may still allow reflow using the same
profile but the joints may only just reflow causing a lot of voiding.


Dear Bob 

I have changed my solder paste to a finer particle size due to my fine pitch
requirements. I now have problems with solder balls which I did not originally
have.

Just remember that when you reduce the size of solder particles in a paste for
fine pitch printing you are increasing the metal surface area. In combination
with a low residue paste it may be just too much for the paste activators. Try
running some comparisons of you new and old paste using your existing reflow
settings.


Dear Bob 

What are the main problems with reflowing copper finish boards?

Despite the literature from the protective finish manufacturers heat does effect
the copper circuit. It increases the time to obtain full wetting hence
increasing the possibility of poor yield. Double sided reflow again increases
the surface oxides and poor yield. You need to confirm the quality of the basic
coating with your supplier and consider the other process stages which effect
solderability. Cleaning the board, washing off poor paste prints, poor board
handling, the original surface preparation of the copper before treatment all
can effect reflow yields.


>From the desk of Bob Willis in the Old Country, England.
Tel: 01245 351502
Fax: 01245 496123
Email: [log in to unmask]