Technical Information

Surfaces

Soldering surface coatings

Lead-free surface coatings
The following lead-free coatings for soldering surfaces are available:
HAL lead-free
Chemical Ni/Au
Chemical Tin.
These coatings are RoHS compliant!

RoHS compliance

Comparison of lead-free soldering surface coatings
Available Coatings	HAL lead-free	chem.Ni/Au	chem. Tin
Topography Layer thickness	convex 1 approx. 1-30µ 1	plane *2 approx. 4-6µNi; 0,1µ Au	plan 2 0,8-1,2µ 2
	More details	More details	More details
Shelf life influence
Surface corrosion Diffusion zone growth	not critical not critical *3	not critical not critical *3	not critical critical *3
*Solderability / multiple soldering processes 4**
^1st soldering process	good	good	good
Under O₂ long term	good	good	critical
Under O₂ short term	good	good	critical
Under N₂ long term	good	good	good
Under N₂ short term	good	good	good
Additional applications
Edge connectors	not suitable	limited suitability	limited suitability
Press-Fit Technology	limited suitability	limited suitability	suitable
Aluminium wire bonding	not suitable	suitable *5	not suitable
Keyboard application	not suitable	suitable	unknown
Available Coatings	HAL lead-free	chem. Ni/Au	chem. Tin
Special advantages	robust soldering surface with solder depot, good shelf life	universal plane surface, good shelf life	universal plane surface
Special disadvantages	lack of surface planarity, thus critical for BGA and similar pads, HAL causes high thermal stress	failure risk of “black pads”, high costs	formation of whiskers, multiple soldering may be difficult
Richter Elektronik can recommend the following solutions:
(All evaluations and recom-mendations are based on today’s state of knowledge and are non-committal. The user is supposed to perform his own testing based on his specific conditions and requirements.)	For all PCBs where lack of surface planarity does not cause problems and the thermal stress caused by HAL is not critical.	For demanding SMD boards and / or special require-ments like bonding etc.	For demanding boards with SMD fine-pitch pat-terns, provided that soldering conditions are suitable.

General remarks concerning lead-free soldering surface coatings

*1.	Physically deposited layers, e.g. HAL layers deposited by the application of liquid solder, have a significantly higher thickness, however, they are not plane.
*2.	Chemically (i.e. currentless) 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: Chemically 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.
*5.	The customer must specify the requirements which will be verified by Richter Elektronik.

Leaded surface coating
There are certain application areas where the use of leaded solders is currently still legal. The following coating will be supplied only upon explicit written order.

HAL leaded
In terms of topography, layer thickness, shelf life and solderability this HAL layer is very comparable to the lead-free HAL layer. This soldering surface coating is not RoHS compliant!