Reflow Profile Optimization

It is quite common that this is not carried out, which can lead to perceived solderability issues and/or yield fallout.

Also, many customers deliberately try to keep a low peak temperature in order to protect other sensitive components.

In order to adjust the “achieved” profile to the “target profile”, (as shown above,) simply increase the temperature settings in the first one or two zones in the reflow oven. This allows a good temperature equalising / soak time, which crucially increases the time above molten at the DirectFET MOSFET, WITHOUT increasing the PEAK temperature achieved.

• Convection heating relies on indirect thermal transfer from circulated air to the board and components
• Good convection ovens offer almost 100% thermal transfer by this means
• Poor (older design ovens) may have up to 30% thermal transfer by infra-red

• Infra-red heating relies on indirect thermal transfer from heater platens running in the infra-red light spectrum - the heat is radiated
• Infra-red ovens are susceptible to uneven heating as a result of black-bodied infra-red absorption
• This means that black components will absorb heat easily but silver ones (such as DirectFET!) do not!

• Different components will absorb heat energy at different rates, e.g. black components will absorb heat faster than silver components.
• Customer may be running a ‘cool’ profile (low peak temperature) due to sensitive components.
• Different surface finishes require different levels of soldering energy to complete the joint, e.g. HASL board finish requires less energy than Entec.
• Low thermal mass component may be located next to a high thermal mass component that sucks up all the heat/soldering energy.

• HASL is the easiest finish to solder to since one side of the solder joint has already been made - the surface finish is the same as the board attach solder.
• NiAu and NiAg require more energy to make the solder joint than HASL.
• Organic pad finishes require even more energy than the other options (this is disliked by many solder paste vendors).

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  Reflow Gases
  The chamber of the reflow oven is usually filled with:
  • AIR – reflow chamber is open to ambient or
  • N₂ – reflow chamber is purged with nitrogen.
    • N₂ has many benefits such as the reduction of voiding however it can increase the chance of component tombstoning. + nitrogen purge MUST be used when a bare copper board finish has been spec’d to reduce oxidization of the pads

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  X-Ray Examples