Technical Information
Electroless Sn
Soldering pad cross-section
| Topography | plane *2 |
| Layer thickness | 0,8 - 1,2 µ *2 |
| Surface corrosion | not critical |
| Diffusion zone growth | critical *3 |
| Solderability 1st soldering process | good |
| Under O2 long term | critical |
| Under O2 short term | critical |
| Under N2 long term | good |
| Under N2 short term | good |
| Edge connectors | suitable |
| Press-Fit technology | suitable |
| Aluminum wire bonding | not suitable |
| Keyboard applications | unknown |
| Special advantages | Universal plane surface |
| Special disadvantages | Formation of whiskers, multiple soldering may be difficult |
| Richter Elektronik can recommend the following solutions: (Based on current knowledge, without obligation) |
For demanding boards with SMD fine-pitch patterns, provided that soldering conditions are suitable. |
Chemically (i.e. electroless) deposited metal layers as e.g. Ni/Au, tin or silver, cannot be grown to any desired thickness due to the nature of the process which will come to a standstill once the noble metal has fully covered the ignoble copper. This process creates a plane surface which is identical to the topography of the underlying copper surface.
3*
Independent of the deposition method, generally a so-called diffusion layer is formed between two layers of different metals. This barrier layer is created by an exchange of atoms between the metals of different nobility. There will be a “mixing” of the base copper and the metal of the “ennobled” soldering surface. The thickness of this diffusion layer depends on the type of metals involved as well as on the temperature conditions and the time for which the temperature conditions affect the layer system. Higher temperatures and longer residence times lead to a growth of the diffusion layer (e.g. during storage and soldering).
4*
The following can be derived from the items 1 to 3:
Electroless deposited layers are plane but very limited in thickness. This results in a high danger of loss of solderability as the diffusion layer may penetrate the entire thickness of the coating under certain circumstances in which case the plane surface “ennobled” for soldering is not solderable any longer. On the other hand, a physically deposited soldering surface has a much higher thickness which under normal circumstances is several times thicker than even a seriously grown diffusion layer. However, it is to be considered that this coating has the disadvantage of a lacking planarity.