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The Function of Nitrogen in Reflow Soldering


With the  increasing use  of  copper-OSP boards the  use  of  nitrogen in reflow soldering is often demanded. This in view of the  wave soldering process that  has to  follow reflow soldering.  During reflow the  OSP that  protect the  solderability of the  copper surface will vanish since the  OSP film will be evaporated during the  reflow process.  In order to  prevent oxidation of   the  copper surface during reflow soldering the use of  nitrogen might be mandatory.

The effect of nitrogen on the reflow process 

For reflow soldering alone the  use  of  nitrogen is often not  necessary. When the solderability of the  components and the board is according to  the  necessary level, the  solderpaste should be able  to  create sound soldered joints if the  correct heating profile is used.

The addition of  nitrogen is in that  case  for  the  reflow process not  necessary.  If however nitrogen is used  it will support

the process since during the  reflow process now further oxidation of  the  joint surfaces and the solderpaste will be decreased. The nitrogen supports the  wetting capability of the  solderpaste due  to this  minimised oxidation during soldering.

Another effect of  nitrogen is that  it changes the  surface tension of  the  solder compared to  soldering in an air environment. Depending on  the  solder alloy this can  give  a higher or a lower surface tension. A higher surface tension might assist in the  alignment of  the  components during soldering.

As long  as the  wetting of   the  component joint surfaces is not  initiated and the solderpaste is completely molten this  can cause  a movement of  the  component. The component will "float" at the solder globule surface. This globule is formed during melting of  the  solderpaste when the  wetting of  the  joint surfaces is not directly initiated during the  melting and confluence of  the  solderpaste.

As long  as the  component is still  in contact with the  solder, wetting will again (partly) compensate for  this  component movement as the  surface tension of   the solder will try to  keep the  wetted joint surface in contact with the  solder and the other joining part  (solderpad). Depending on  the  mass  of  the  component and the displacement that  has taken place before wetting took place, the  component might after soldering not  be completely in contact with the  solder pad.  However as long as the  joint is sound and the  gap to  the adjacent component is still sufficient such displacements can often be allowed.

MyLinda is the author of this solution article.

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