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Lead-free PWB final finishes are used to provide a solderable and coplanar surface for component attachment. Through the MFG aging test, creep corrosion results can be studied for these materials.
By C. Xu, J. Franey, D. Fleming, and W. Reents, ALCATEL-LUCENT
As the electronics industry moves to lead-free assembly and finer-pitch circuits, widely used printed wiring board (PWB) hot air solder leveling (HASL) tin/lead (SnPb) finish has been replaced with lead-free and coplanar PWB finishes such as organic solderability preservative (OSP), immersion silver (ImAg), electroless nickel immersion gold (ENIG), and immersion tin (ImSn). SnPb HASL offers excellent corrosion protection of the underlying copper and has inherent corrosion resistance. Lead-free board finishes provide reduced corrosion protection to the underlying copper due to their thin coating. For ImAg, the coating material also can corrode in more aggressive environments, which has become an issue for products deployed under high levels of sulfur-containing pollutants. In those corrosive environments, creep corrosion can lead to product failures in short service life (1-5 years). In some of the worst cases, creep corrosion failures within one year of product deployment have been reported. This has prompted an industry-wide effort to understand creep corrosion, although minimal progress has been made, due to an inability to reproduce creep corrosion in the lab using realistic accelerated aging tests. We report that creep corrosion is highly sensitive to the surface chemical properties, which must be considered when simulating creep corrosion in the laboratory. Searching for creep-corrosion-resistant ImAg or other PWB final finishes should be done on an assembled board with solder mask and fluxes. The MFG test provides a viable and realistic accelerated aging test for creep corrosion.
Lead-free PWB final finishes were developed with the primary design objective of providing a solderable and coplanar surface for the component attachment during electronic circuit assembly. While short-term corrosion resistance of board finishes is important for maintaining solderability shelf-life up to 12 months, long-term corrosion resistance was neither required nor considered.1-3 The long-term corrosion resistance in the field during the service life of the device has not been an issue for the traditional HASL finish, as it protect underlying copper due to its thick coating and inherent corrosion resistance. Extensive testing and reliability assessments have been performed on the four lead-free PWB finishes. However, little attention has been paid to the corrosion resistance of the lead-free PWB finishes once they are field-deployed. Recently, the corrosion resistance of PWB finishes generated considerable interest due to premature field failures observed in various parts of the world. Works on the product failure in the field due to corrosion and simulating those failures in the lab was conducted.4-7 Two types of failures were identified: creep corrosion and flaking of corrosion products.6 In both cases, the semi-conducting to conducting corrosion products can cause either intermittent or permanent short circuits in the electronics.
Figure 1. Samples after 40 days MFG test using international conditions on ImAg (top) and bare Cu (bottom).
Mixed flowing gas (MFG) containing H2S, SO2, NO2, and Cl2 has been widely used to simulate environmental conditions in the field and perform accelerated test on electronic devices. While corrosion product flaking can be reproduced easily in the MFG test, creep corrosion has not been reproduced consistently. For instance, samples prepared using clean IPC-B25 comb patterns showed minor to no creep corrosion after extended MFG exposures using the international condition, even though severe flaking of corrosion products was observed.6 Figure 1 shows samples with ImAg-plated Cu trace (top) and Cu trace only (bottom) after 40 days MFG exposure using international condition.6 Thick corrosion products (mostly copper sulfide) as well as flaking of the corrosion products were observed in both cases but no signs of creep corrosion across the FR-4 surface between the traces were seen in either case. However, creep corrosion readily occurs in the MFG test if certain types of contamination are present on the surface. In Figure 2, an ImAg-plated IPC-B25 comb coupon after two days MFG exposure shows no creep corrosion in most areas, although severe localized corrosion was seen on ImAg-plated traces. In the center of the coupon, creep corrosion was observed within a droplet-shaped area and a short circuit of 1.3 kΩ was measured. This indicates that creep corrosion is highly surface-specific and sensitive to surface chemical properties. Clean FR-4 surfaces do not support creep corrosion, while contaminated FR-4 surfaces do. Certain surface characteristics are required for the corrosion product to creep.
In PCB assembly, flux residue is a common surface contaminant. It bears the question whether flux residues could also promote creep corrosion, as in the case of certain types of surface contamination. If true, it may explain the difference between creep corrosion observed on assembled PCBs in the field and inconsistent evidence of creep corrosion on lab samples, which have not seen assembly fluxes.
Figure 2. Creep corrosion over contaminated surface resulting in short circuit of 1.3kΩ
To examine the possible effect of flux residues on creep corrosion, an ImAg-plated populated PCB designed for consumer electronics was assembled via selective wave soldering, as flux residues on the board indicate. It was subjected to five days of MFG exposure in the lab. Three distinctly different areas were observed (Figure 3). In the right top corner, vias were selectively soldered and completely covered by the solder. Consequently, no corrosion was seen in this area. The left bottom corner was not soldered and the Ag finish on the vias is completely exposed. Severe corrosion was observed in this area, but no creep corrosion. The third area is the boundary between unsoldered and wave-soldered regions. Severe creep corrosion occured in this boundary area, as did short circuits due to the creep corrosion.
This pattern of no corrosion in the soldered area, creep corrosion in the boundary area, and corrosion without creep in the non-soldered area was observed repeatedly. Apparently, flux has migrated from the soldered area to the adjacent boundary area during selective wave soldering and left residues in the boundary area. The combination of exposed Ag and flux residues in the boundary area allows the Ag to be corroded in the first place, and then the corrosion products migrate on the flux residue-covered soldermask surface. On the other hand, the Ag-plated vias in areas far away from the soldered area was corroded without signs of creep corrosion, as the clean soldermask surface does not support corrosion product creep. This result further demonstrates that creep corrosion is surface-specific; only certain types of surfaces can support creep corrosion. The “wrong” combination of soldermask, fluxes, and process conditions can produce a creepable surface for ImAg-plated circuit boards. Searching for creep-corrosion-resistant ImAg or other PWB final finishes should be done on an assembled board with soldermask and fluxes. The MFG test provides a viable and realistic accelerated aging test for creep corrosion.
The flux residue-assisted creep corrosion is not only limited to ImAg-plated PCBs. Similar results are also observed for OSP8 after five days MFG exposure using the international conditions.6 Two circuit boards have OSP final finishes but were assembled with different types of solder paste, one with a solder paste containing organic acid flux, the other with paste containing rosin-based flux. After five days MFG exposure, severe creep corrosion was observed on the board associated with the organic acid flux. On the other hand, the board assembled using paste containing rosin-based flux only showed localized corrosion in the exposed OSP areas without any significant creep corrosion. Furthermore, the degree of creep corrosion depends on the cleaning operation after assembly and correlates with board cleanliness. This drastic difference between boards assembled with different type of fluxes demonstrates the important role that flux residues have on creep corrosion. To further delineate the effect of various assembly fluxes on the creep corrosion, we performed a systematic study involving eight different wave soldering flux and six different solder paste (to be presented at APEX).
The occurrence and severity of creep corrosion on PCBs is highly sensitive to the surface condition. While clean FR-4 surfaces do not support creep corrosion, contaminated PCBs can be highly active for promoting creep corrosion. The corrosion test using mixed flowing gases provides a realistic accelerated test for equipment deployed in various environments encountered in the current global market places. No condensing condition is required for simulating the product failure due to creep corrosion in the laboratory. SMT
Figure 3. ImAg board from a consumer product after five days MFG exposure using international conditions.
- For a complete list of references, contact the authors.
C. Xu, J. Franey, D. Fleming, and W. Reents, Alcatel-Lucent, may be contacted via C. Xu at 600-700 Mountain Avenue, Room: 1E-234, Murray Hill, NJ 07974; email@example.com. Results will be presented at the APEX 2009 Technical Conference in Las Vegas.