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 FAQ's

  . COMPONENTS (8 questions)
  . HAND SOLDERING AND DE-SOLDERING (3 questions)
  . INSPECTION (3 questions)
  . PRINTED BOARD DESIGN (3 questions)
  . PRINTED CIRCUIT BOARDS (6 questions)
  . REFLOW SOLDERING (5 questions)
  . REWORK (1 questions)
  . SCREEN PRINTING (2 questions)
  . WAVE SOLDERING (4 questions)
  . X-RAY INSPECTION (1 questions)





REFLOW SOLDERING

1 . Can I use the same lead-free temperature profile for second side reflow on a double sided product?
2 . How can I justify the use of nitrogen gas during reflow?
3 . I want to profile my reflow oven to try and reduce reflow temperatures where should I place my thermocouples?
4 . If I change my reflow extraction system or my settings do I need to perform further profiles?
5 . We have recently introduced surface mount components into our products. We are suffering a high level of rework on 0.020" devices due to solder shorts. We also have to introduce lead-free. The assembly is stencil printing of paste using 0.008" stencil and reflow soldering.

 

Can I use the same lead-free 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 your product reaches reflow temperature. Often the two sides of the board are different in terms of the component types used, side two is often more complex than side one. 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. Don’t be a lazy engineer be a great one!!!

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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 materials. When you compare boards reflowed in air and nitrogen there is a visual benefit. However 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 OSP copper based board finishes. This will improve other manufacturing yields and save enough money per board to pay for gas usage. Nitrogen suppliers like BOC can provide running costs for reflow and wave.

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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 theoretically 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. Obviously large components like QFPs and BGA will affect the temperature more than small transistors. If you are going to be running pin in paste measurements, attachment should be made under the component body just like BGAs.

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If I change my reflow extraction system or my settings do I need to perform further profiles?
Yes you will. 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. Its good practice to determine the degree of temperature variation in your oven and then set the frequency of profiles on your oven.

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We have recently introduced surface mount components into our products. We are suffering a high level of rework on 0.020" devices due to solder shorts. We also have to introduce lead-free. The assembly is stencil printing of paste using 0.008" stencil and reflow soldering.
The stencil being used is too thick causing too much paste to be applied. The stencil thickness should be reduced to 0.006" possible even 0.005” with a possible aperture reduction of each direction.

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