Ormecon Introduces Nano-Thin PCB Surface Finish--Organic Metal Nanofinish
March 25, 2008 |Estimated reading time: 8 minutes
Organic metal (OM) is an advanced form of conductive polymers that has metallic properties even though it is characterized as an organic compound. The material contains carbon (C), hydrogen (H), nitrogen (N), oxygen (O) and sulphate (So) as elements and is synthesized and dispersed in the form of 10 nanometer (nm) small primary particles. A strong effect in the prevention of copper (Cu) oxidation was discovered many years ago. The material has been commercially used approximately 10 years for finishing printed circuit boards as a predip in Ormecon's immersion tin process--ORMECON CSN. In this process, the organic metal is used as the Cu surface preparation predip prior to the immersion tin (Sn) deposition.
The process is well established and widely used in the printed circuit board (PCB) industry as one of the top quality alternative finishes required for lead-free electronics manufacturing. The organic metal predip is applied as a 80nm thin adsorbed layer--causing the formation of selectively Cu (+1) and a passivation of Cu. In addition, it takes part as a catalyst to provide electrons for Sn (2+), which is subsequently deposited onto the Cu.
A first process version of a completely new process in which the OM would be the only and final surface finish (not any more only the predip) - a nano finish - was presented for evaluation three years ago and was already suitable for lead-free soldering, but not stable enough to prevent discoloration. The new nanofinish generation, based on the organic metal / silver (Ag) complex, shows high performance with regard to ageing resistance, discoloration and solderability.
The process starts with a combination of a modified acid cleaner, already containing some OM, and a specially adapted micro etch. A predip--conditioning for up to 120 seconds depending on Cu surface type--prepares the boards for the active bath (e.g. OMN 7200, depending on the grade, 35*C for 90 seconds). A final rinse and a dryer complete the process (Figure 1).Figure 1: Process scheme of organic metal based nanofinish.
The organic metal nanofinish is not simply one surface finish process, it comprises four different processes--each of which addresses specific market segments and customer demands. One of the processes is designed to meet demands in the organic solderability preservative (OSP) segment, another in immersion silver, a third in the immersion tin segment and the fourth being the "top grade" that can replace electroless nickel immersion gold (ENIG) or be used as a universal final finish (Table 1).
Table 1: Morphology investigation by SEM and GCM.
Figure 2 shows a scanning electron microscopy (SEM) image of copper pad of a PCB after treatment with the nanofinish. The image shows that the OM-Ag complex is preferably located on the phase boundaries of the Cu crystallites. Most of the visible area is Cu-surface.
Figure 2: SEM image of a PCB after treatment with the organic metal / silver nanoparticle finish.
The electrochemical investigation by a galvanostatic coulometric measurement (GCM) shows that the organic metal based nanofinish has formed a new type of complex (Figure 3). The potential at which this complex is oxidized is significantly different from Ag on Cu alone.
Figure 3: Potential-time-curves for Cu, Ag coated on Cu by immersion and nanofinish.
The finish of the copper surface in dependence on the immersion time in organic metal / silver nanoparticle finish is displayed in Figure 4. The potentials indicate that the amount of free copper surface decreases slowly at the beginning of the process, having the highest coverage rate between 40 and 60 seconds immersion time. After 60 seconds, the rest of the free copper surface is coated slowly. At about 90seconds no (electrochemically accessible) free copper is detectable.
Figure 4: Potential-time curves for a copper surface being coated by organic metal / silver nanofinish for different immersion times.
XPS depth profiles of copper and silver on the organic metal based nanofinish copper surfaces show that the Ag is only nominally detectable to a depth of 2 to 3 nm--before and after reflow. At the outermost surface, the Ag:Cu ratio changes only slightly during the reflow process (becoming smaller). But, from a depth of about 2 nm on, no change in the ratio is detected after the reflow process. The Ag migration appears to not be an issue--the Ag seems to be immobilized in the complex with the OM.
Even more interesting is the ratio of metallic to oxidized copper on the surface of the pad before and after reflow as shown in Figure 5. This ratio did not change during the reflow process--proving the exceptional capability of this new nanofinish in oxidation prevention of Cu.
Figure 5: Ratio of metallic to oxidized copper in the fresh sample (surface).
The surface potentials of copper, oxidized copper, silver on copper after immersion and organic metal/silver nanoparticle finish on copper after immersion were determined using a scanning Kelvin probe (SKP, UBM Messtechnik GmbH). The Kelvin potential is a very reliable indicator of the sensitivity of a surface towards oxidation. Figure 6 shows a copper surface treated with organic metal/silver nanoparticle finish just after finishing.
Figure 6: Copper surface treated with organic metal/silver nanoparticle finish.
The Kelvin potentials of various treated and untreated copper surfaces are summarized in Table 2.
The new nanofinish, composed from an organic metal-Ag nanoparticle size complex, reaches almost the same potential as a pure Ag layer which contains more than 100 times more silver.
With immersion Ag surface finishes, micro voids may occur and are an object of broad investigations and considerations. With the organic metal based nanofinish, no micro voids occur (Figure 7).
Figure 7: Cross section and XRF show no micro voids.
Thermal aging was performed to simulate soldering and storage conditions. To simulate typical soldering conditions, test panels on which the organic metal based nanofinish had been deposited under standard conditions, were tempered up to four times in the RO 300 FC N2 reflow oven from Essemtec. A lead-free soldering profile was chosen with a peak temperature ~ 250*C. To simulate storage conditions, other test panels were aged 4 hours at 155*C.
The performance of copper surfaces treated with organic metal / silver nanofinish and established metallic surface finishes is compared before and after reflow in Table 3.
The organic metal based nanofinish does not show any discoloration during reflow (Figure 8) and the wetting behaviour, according to wetting balance studies, is superior to any metallic surface finish (Table 3). Practical tests in PCB manufacturing and in assembly facilities have confirmed these results. Figure 8 shows a PCB before treatment, directly after the surface finish with the organic metal based nanofinish and the surface after treatment and aging.
Figure 8: No discoloration after reflow.
These solderability results have been confirmed by an external evaluation. A comparison between an established OSP and the organic metal based nanofinish has been performed. For solderability analysis, a wetting balance was used. In Figure 9, the wetting force of the OSP is shown. The wetting force of the organic metal based nanofinish is given in Figure 10.
Figure 9: Wetting force OSP.
Figure 10: Wetting force OMUN.
The wetting force of the the organic metal based nanofinish is with 1.0 mN--almost twice as high as for the OSP (0.55 mN).
An external analysis shows that the lead-free-Sn solder joints (ball grid array or BGA pads) with the nanofinish are of a high quality. The nanofinish solder joints are expected to be reliable long-term.
Figure 11: Perfect solder joint.
Out of the four different nanofinish processes, only OM CSN nanofinish will be discussed with more detail in this article. This process is delivering an only 0.4 um thin pure tin surface finish with the same performance as standard ImSn finishes with 1.1 um pure Sn layer.
The key for the success of this process are two effects. First, the already established OMP predip (as we are using in ORMECON CSN) is reducing the speed at which Sn is diffusing into Cu (and Cu into Sn) by more than half, see Table 4.
Table 4: Sn and Cu diffusion in various ImSn finishes, ORMECON CSN Classic and the new OM CSN nanofinish showing the slowest diffusion speed.
Second, a new post-treatment rinse additive (NPT 7002), based on essentially the same chemistry as the OM nanofinish, is passivating the Sn and, later the intermetallic phase, preventing oxidation.
As a result, the pure Sn layer loss is much slower. Even after the complete loss of the pure Sn layer and the completed formation of the Cu-Sn intermetallic, the solderability is fully preserved because of the oxidation prevention (Table 5).
Table 5: The solderability is fully preserved because of the oxidation prevention.
For experts in ImSn technology, it is surprising that there is almost no discoloration even after four hours at 155*C and after three reflows under lead-free conditions (Figure 12).
Figure 12: OM CSN nanofinish colour fresh (top), after two reflows (bottom left) and after three reflows. Third reflow with solder paste (bottom right)--only slight darkening, no discoloration.
The process is essentially identical with the established ORMECON CSN Classic process, with only two changes--the Sn bath is run at only 53 *C, and the immersion time is only 5 to 6 minutes; and, instead of the standard rinse aid additive, NPT 7002 with the new OM nanofinish formulation is used in one of the rinse steps.
These findings make the ImSn process much more productive because horizontal lines can be shorter and much less expensive; vertical lines will have less Sn tanks; the cycle time can be shortened from 35 to 40 minutes to about 17 minutes; and the total production time is reduced by half--doubling productivity.
With the organic metal based nanofinish, a nanoparticulate complex between the OM and Ag is described for the first time. Although it does not form a continuous nanolayer, the polymer completely and effectively shields the Cu and prevents the substance from being oxidized after at least four lead-free reflows. Aging resistance and wetting (soldering) performance is excellent and enables the replacement of established PCB surface finishes.
The new family of nanofinish processes allows original equipment manufacturers (OEMs), assemblers and PCB manufacturers to produce boards featuring higher productivity, lower failure rates and, most importantly, lower costs.
References
[1]B. Wessling, Handbook of Conducting Polymers (T. Skotheim, R. L. Elsenbaumer, and J. R. Reynolds, eds.). Dekker, New York, (1998).
[2]B. Wessling, Adv. Mater. 6, 3, 226, (1994).
[3] http://www.ormecon.de/(ORMECON CSN, technical information).
[4]N. Arendt, C. Arribas, J. Posdorfer, M. Thun, B. Wessling, OnBoard Technology, 12, (April 2006).
[5]B. Wessling M. Thun, C. Arribas-Sanchez, S. Gleeson, J. Posdorfer, M. Rischka, B. Zeysing, NRL article, (2007).
[6]Dr.-Ing. Manfred Deger, Analytik - Labor - Possendorf, (June 2007).