Dr. Bernhard Wessling
Ormecon Chemie
D-22949 Ammersbek
Germany
reprint available on request
Sonderdrucke auf Anfrage verfügbar
Alternative surface finish technologies are necessary as Hot Air Levelling cannot meet future demands, neither technologically nor ecologically.
Whereas most alternatives are based on conventional chemistry (gold, silver, palladium, conventional tin), but have been fine-tuned and modernized, we are presenting a completely new chemical principle: the Organic Metal.
An Organic Metal is an organic compound (i.e. a carbon chemistry based synthetic compound) which has metallic properties. Our Organic Metal, ORMECON®, is a polyphenylenamine-para-toluene-sulfonic acid polysalt (structure: see Fig. 1).
A metal is characterized by the mechanism by which electrons can move. Metallic electrons are „free", they are placed in an elongated „metallic band", which consists of an overlead of the binding electron energy band with the non-binding band. Such a feature is principally not limited to inorganic elemental metals, but can also be realized with organic compound and polymers. However, all of such attempts ended up with compounds which were unstable, insoluble and unloadable - hence totally useless.
After 20 years of basic and apply oriented research, our company has succeeded in designing a polymeric composition, which is highly stable, economically producible and can be processed. However, as insolubility and unmolderability are principle properties connected with the metallic property, processing could not be performed by dissolution or molding techniques, but by completely new developed dispersion technologies.
Polyphenylenamine provides a unique property set:
It is a metal and, in the galvanic series, it is placed between silver and copper, it is therefore more noble than copper, iron and all other non-noble metals, but less noble than silver and the noble metals.
It can be reproducibly reduced and oxidized, i.e. it can act as a catalyst.
The raw powder is moderately conductive (5 S/cm). New research results indicate the possibility of achieving conductivity values about 100 times higher than that, and we expect that even higher values can be found.
Some dispersions are higher in conductivity than the raw material.
Dilute dispersions are transparent green, the same is polyphenylenamine in pure thin layers or coatings.
The conductivity can be tuned, if necessary, over about 10 orders of magnitude.
These principle features allow for a completely new process resulting in a new surface finish of printed circuit boards:
The Organic Metal ennobles and passivates copper. Fig. 2 shows the potential shift induced by the Organic Metal. The potential shift ranges at about 800 mV and is responsible for that the oxidation of copper can be performed only at an equivalently higher oxidation potential, i.e. under much harsher conditions.
The deposition of tin on this modified ennobled and passivated surface is catalized by the Organic Metal.
Fig. 3 shows how even a wafer-thin coating of copper deposited from aqueous dispersions of the Organic Metal prevents the oxidation of copper: a copper plate which was etched in 10% sulfuric acid, is rinsed with clean water: when drying in open air, it oxidizes and corrodes within seconds (Fig. 3a). If on the other side copper is being dipped in the aqueous dispersion of the Organic Metal after etching and washing, and is even rinsed again afterwards, one would think there is no coating, as nothing can be seen. But after a few seconds and even more after minutes and hours it is clear: the copper surface does not change its colour, it does not get oxidized.
The coating cannot be seen as it is only about 80 nm thin. Such fine nanostructures can only be detected with tunnelmicroscopy or XPS1. Fig. 4 is an XPS spectrum which shows the presence of the Organic Metal and the induced chemical change of a copper, the precise oxidation to Cu(+I).
To our regret, this perfectly passivated copper surface is not solderable, especially not after ageing at 4h/155°C. Solderability is more than only prevention of copper oxidation. It also asks for wettability by solder tin in the wave soldering step.
We have therefore investigated if the deposition of tin on such a Organic Metal pretreated and passivated / ennobled copper surface was possible.
Tin deposition on Organic Metal-passivated copper
In a first phase, we analyzed the tin deposition using a well known conventional immersion tin formulation. It is basically known that the electroless chemical deposition of tin is a depletion reaction whereby copper has to go into solution. As copper is more noble than tin, this step cannot proceed deliberately, but is only possible because copper is being complexed (with thiourea) which has a lower potential.
Our analysis comprised the quantitative analysis of tin (the tin decrease) and of copper (copper increase) in dependence of the area of copper being tinned. Parallel to that, at points of every 0.5 m² copper area tinned, test PCB samples were tinned and their solderability was assessed2. Fig. 5.1 shows the result. When a conventional copper surface is tinned with the well-known previous generation immersion tin formulation, neither the tin decrease nor the copper increase are linear at any phase of the test regime; above 0.2 m² the non-linear concentration dependence becomes dramatic: there is almost no tin deposition anymore, and only very few copper goes into solution. This is also the area above which solderability cannot be guaranteed anymore (many holes unfilled), above 0.6 m² no solderability at all can be observed (no hole is wet, or filled by solder tin).
The reason for this is that beginning with a very low concentration of copper in the immersion tin solution, not only pure tin is being deposited, but also copper-tin alloys (cf. Fig. 6 left part). At higher copper contents, only the copper tin alloys are deposited.
We have therefore designed a new process, in which every step is optimized with regard to the previous and the following step. The process sequence is shown in Fig. 7 following this process, there is a totally different concentration dependence: both ions Cu(II) and Sn(II) are completely linear over the whole range of copper area to be tinned. Our conclusion was: independent of the copper concentration (which is dependent on the copper area having passed the tinning process), only pure tin is deposited. The solderability was guaranteed up to 0.7 m² Cu/l tin bath, equivalent down to a tin concentration of 12 g/l.
Further optimization: higher efficiency of the immersion tin formulation
However, it was unclear why - although the concentration curve was linear over the whole range - solderability was limited to 0.7 m²/l. We suspected that the reason for this was the relation of the slope of the Cu and Sn concentration. In the ORMECON-CSN process (Fig. 4.2) it was about 1.6:1, which was neither 1:1 nor 2:1. But as a chemical electron transfer would either involve a 1- or a 2-electron-transfer, one of the two relationships should be observed. Because something in between was found instead, this was the indication for that the conventional tin chemistry could not be applied if we really wanted to design a modern tin surface finish based on Organic Metal. We decided to try to find a tin formulation allowing for a 2:1 slope relation.
In co-operation with Florida CirTech, we developed a completely new tin formulation which now - as shown in Fig. 5.3 - has a nicely linear concentration increase for copper and a nicely linear concentration decrease for tin and a practically linear Cu:Sn relation of 2.2:1.
It is a matter of debate whether this relationship is optimal and why we tried to design this process instead of a 1:1 process. People might think that a relation of 1:1 should theoretically be correct and therefore to be preferred3. We doubt this. We are of the opinion that a 2-electron process of this kind is not directly feasable, but would involve the action of oxygene as the removal of the second electron from Cu(I) needs a higher oxidation potential. This is a risk not only with regard to further detrimental oxidation reactions, but also with regard to the reproducibility of a chemical process and the reproducibility of the tin deposit.
We think that an optimal tin deposition should only involve the reactions
Cu(0) ---> Cu(I) + e
Sn2+ + 2e ---> Sn(0)
therefore only 1 electron from copper to tin, hence 2 Cu atoms per 1 Sn atom. Such a relation would lead to a 2:1 slope relationship which more or less exactly we now do find with our new and improved immersion tin generation ORMECON CSN 7001 (or: OMIKRON+).
These results are in strong correlation with our knowledge grown from XPS in Fig. 3 where the Organic Metal induced a precise oxidation to Cu(I) as the first step of the passivation reaction.
Obviously, the Organic Metal provides exactly the correct copper ions, namely Cu(I), which during the oxidation and complexation reaction can tranfer exactly one electron to Sn(II), see Fig. 6.
Now, one might be afraid that due to a Cu:Sn relation of 2:1 much more Cu is produced than with previous immerison tin formulations which might eventually reduce the application range or process window of such tin bath.
The contrary is the case: even at 1m² per liter tin bath and more can be nicely tinned, with no degradation of the solderability results at all. At least up to 1 m² Cu/l tin bath, all plates are 100% solderable after 4 or even 8 hours ageing at 155°C, followed by three reflow cycles and the wave solder.
The effects of ageing have been electrochemically analized. Fig. 7 shows the content of pure tin and Cu6Sn5 in a tin layer which was deposited by a new tin formulation without previous pre-treatment with the Organic Metal. The sample was taken after 0.5 m² Cu/l tin bath and 20 minutes tinning time. Fig. 8.2 shows the same sample after 4h/155°C ageing: the tin phase was almost completely transformed into Cu6Sn5, the sample is not solderable (more precisely: less than 50% of the holes were wet and filled with silver tin). Fig. 8.1 shows a sample taken at 0.9 m² Cu/l, which was tinned from a bath containing now 10 g Cu and 16g Sn/l, a bath concentration which would generally be considered as not anymore useful because of having too much copper and being too low in tin. But in contrast, the content of the pure tin phase is even higher than for the sample in Fig. 6.1. After subsequent ageing only half of the pure tin phase gets lost and transformed into Cu6Sn5 and a pure tin phase equivalent to 0.4 µm stays unchanged, which leads to a 100% solderability of the sample.
The economical result is equivalent with a double efficiency increase compared to previous tin formulations: not only the composition is less expensive, but the efficiency of the tinning bath is at least 100% higher than any earlier immersion tin formulation, provided that the tin deposition is performed after appropriate treatment of the copper with the aqueous Organic Metal dispersion.
The tinning time can be lower to less than 7 minutes and the tin phase is equivalent to 0.8 µm tin.
More than 20 factories in Europe, U.S. and Asia are testing the new process, partially also in continuous production scale. Horizontal use is also possible.
With the absolutely planar new surface finish, the production of very dense PCBs (fine pitch structures) is possible without the need to take silver or gold as surface finish. The Organic Metal moderated tin deposition is a new high tech, high performing surface finish technology at costs lower than those of hot air levelling.
more information: tin oxide formation with and without Organic Metal pretreatment
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