CREEP CORROSION IN PCB

EXPERIMENT RESULTS AND REAL CONCLUSIONS OF CREEP CORROSION ON LEADFREE PCB IN HIGH SULFUR ENVIRONMENTS

Eng Ofer Nisanovich B.sc. ISRAEL

Jan 2021

INTRODUCTION

Copper is the most widely used metallization material for printed circuit boards (PCB). While copper can have good solderability, it oxidizes rapidly when exposed to the environment. This oxidation quickly reduces the wettability* and solderability**. Surface finishes are commonly used to protect the copper metallization of the PCB from oxidation, as well as from contaminants and damage from mishandling prior to assembly. (2) The surface finish protects the base material (copper), enabling good solderability of the PCB even after a period of storage. Some surface finishes also serve as a barrier layer to reduce interdiffusion between the solder and the underlying base material, since the base material may dissolve and diffuse into the solder and form intermetallic compounds (IMCs). These IMCs are typically brittle and their excessive growth can degrade the reliability of solder joints.(3) The formation of an intermetallic compound is a diffusion-controlled process. Some surface finishes inhibit solid-state intermetallic compound layer growth, which may occur during high-temperature operations. As the electronics industry has transitioned to lead-free electronics, both to comply with government legislation and to be compatible with the supply chain several surface finishes have become replacements for traditional tin-lead finishes. The key lead-free finishes are immersion gold over electroless or electrolytic nickel (ENIG), organic solderability preservative (OSP), immersion tin (ImSn), and immersion silver (ImAg). This paper evaluates ImAg as a circuit board surface finish in terms of its ease of use, wettability, solderability, shelf life, appearance, solder joint strength, intermetallic and void formation, reliability, and cost. A comparison is made with other commonly used lead-free finishes which may dissolve and diffuse into the solder and form intermetallic compounds (IMCs). These IMCs are typically brittle and their excessive growth can degrade the reliability of solder joints. (3) The formation of an intermetallic compound is a diffusion-controlled process. Some surface finishes inhibit solid-state intermetallic compound layer growth, which may occur during high-temperature operations. As the electronics industry has transitioned to lead-free electronics, both to comply with government legislation and to be compatible with supply chain infrastructure,(2) Several surface finishes have become replacements for traditional tin-lead finishes. The key lead-free finishes are immersion gold over electroless or electrolytic nickel (ENIG), organic solderability preservative (OSP), immersion tin (ImSn), and immersion silver (ImAg). This paper evaluates ImAg as a circuit board surface finish in terms of its ease of use, wettability, solderability, shelf life, appearance, solder joint strength, intermetallic and void formation, reliability, and cost. A comparison is made with other commonly used lead-free finishes.

LEAD-FREE SURFACE FINISHES

In lead-free electronics, tin-lead finishes have been replaced by OSP, ENIG, ImSn, and ImAg. Each of these finishes has certain advantages and disadvantages. (4–8) The OSP finish is an organic treatment applied over clean copper pads, which protects the copper from being corroded or oxidized. The typical OSP process consists of, first, acidic cleaning of the bare copper, then micro etching, air knifing, applying the OSP, another air knifing, and a drying process. In the first step, the acidic cleaning solution removes surface oils and solder mask residues from the exposed copper surface; the cleaner prepares the surface to ensure controlled, uniform etching in subsequent steps. Next, the board is processed through a micro etch solution that typically consists of dilute acids such as hydrochloric, sulfuric, or acetic acid. The etching of the existing copper surfaces removes any remaining contaminants and chemically roughens the surface of the copper to promote adhesion. Air knifing then removes excess solution from the panel to limit oxide formation on the copper surfaces prior to the OSP coating. Next, a protective OSP layer is selectively deposited on the exposed copper surface in a water and acid bath. The protective layer adheres to the copper to preserve the solderability of the copper surface for future assembly. Air knifing is then used again to remove excess OSP from the panel and to promote even coating across the entire printed wiring board (PWB) surface. Finally, warm-air drying cures the OSP coating and helps to remove any residual moisture from the board. The OSP coating is nearly invisible and may be applied either as a thick benzimidazole (0.1 lm to 0.5 lm) or thin imidazole (30 A ° to 100 A ° ) layer. Due to the thinness of the OSP, the finish may also be prone to mechanical damage (e.g., surface scratches) during board handling operations, which could expose the underlying metal and impact pad solderability. Therefore care must be taken to avoid surface defects. (10) The ENIG process involves the deposition of an initial layer of nickel onto the exposed copper surface of the PWB, followed by the deposition of a thin, protective layer of gold. The ENIG process generally involves cleaning the bare copper, micro etching, applying a catalyst, acid dipping, and electroless nickel and immersion gold deposition. The cleaning and micro etching steps in the ENIG process are the same as those in the OSP process. After cleaning and micro etching, the board is exposed to the catalyst, which often consists of a palladium salt in an acidic solution. Palladium ions are deposited onto the surface of the PWB in a displacement reaction, effectively exchanging the surface copper atoms for palladium atoms, forming a catalytic layer for subsequent nickel plating. The acid dip, usually a dilute sulfuric or hydrochloric acid, removes any residual catalyst to prohibit plating on the non-copper surfaces of the PWB. Once the catalysts are in place, an electroless nickel solution is used to plate a layer of nickel onto the surface of the catalyst-covered areas in a high temperature, acidic bath. The electroless nickel solution contains a source of nickel ions and phosphorous-containing reducing agent, usually sodium hypophosphite. (11) In the presence of the catalyst, the reducing agent provides electrons to the positively charged nickel ions, causing the reduction of the nickel and the deposition of elemental nickel onto the exposed catalyst. Phosphorous is co-deposited with the nickel with a concentration typically between 2% and 15% by weight, and the resulting nickel–phosphorous alloy forms a corrosion-resistant layer that protects the underlying copper. (12,13) The thickness of the nickel layer of PCBs is typically 3 lm to 5 lm. The immersion gold plating bath then applies a protective layer of pure gold onto the surface of the nickel. A chemical displacement reaction occurs, depositing the thin layer of gold onto the metal surface while displacing nickel ions into the solution. Because the reaction is driven by

the electrochemical potential difference between the two metals, the reaction ceases when all of the surface nickel has been replaced by a gold layer, typically with a thickness of 0.05 lm to 0.2 lm. (9) The process of ENIG is more complicated and expensive than OSP since it involves two separate deposition steps, each of which employs a precious metal, a catalytic process, and a chemical reaction involved in the deposition of nickel and gold layers. Gold is considered an excellent choice as a surface finish since it dissolves readily into the solder and does not easily tarnish or oxidize. Since copper has a tendency to diffuse into the gold, reach the surface, and oxidize to form an unsolderable surface, the gold layer cannot be plated directly onto copper. Therefore, nickel is used as a barrier layer between the gold and the copper, since it plates easily onto copper and the gold plates easily onto nickel. Nickel is unsuitable for use as a surface finish layer since it oxidizes quickly in air to form an unsolderable surface. Because the immersion gold layer is very dense, it offers solderability protection similar to the thicker electroless and electrolytic platings. The gold layer provides a shield to stop the nickel from oxidizing in the air and offers an excellent surface for retaining solderability. ENIG is good for fine-pitch product applications. (14) However, black pad and embrittlement of gold intermetallic compounds have been reported as a threat to the reliability of electronics assemblies, causing brittle failures at the interface. (15) Although it has been claimed that a concentration of gold in a solder joint below 3% by weight is a good rule of thumb for eliminating the possibility of gold embrittlement. Brittle fracture failures were observed when the gold concentration was below that threshold.(16,17) If an electroless plating of palladium (Pd) is applied between the electroless nickel plating and the immersion gold plating, then the finish will become Ni/Pd/Au (ENEPIG), which is also being used in the industry. The electroless palladium process is similar to that of electroless nickel. The palladium layer reduces the formation of the black pad. However, the palladium layer does not prevent the interfacial brittle failures of solder joints in four-point bending.(18) The immersion tin process utilizes a displacement reaction between the copper surface and the tin ions in the solution to deposit a layer of tin onto the copper surface of the PWB. The ImSn process generally involves cleaning the bare copper, microteaching, pre dipping, immersion tin deposition, and a drying process.

After cleaning and micro etching, as in the OSP process, etched panels are then processed through a pre-dip solution that is chemically similar to the tin bath, thus protecting the plating bath from drag-in chemicals from the previous etching process. The pre-dip is designed to activate the copper surface for a uniform finish and help with the adhesion of tin to the copper surface. The heated tin bath for immersion deposits a thin layer of the tin onto the exposed copper circuitry through a chemical displacement reaction that deposits tin ions while displacing copper ions into the plating solution. The bath is considered self-limiting because plating continues only until all the copper surfaces have been coated with a tin deposit. The presence of a complexing agent, thiourea, alters the chemical activity of the metals, tin, and copper, in the solution, and thus enables tin to be deposited onto the copper surface by displacing copper. (19,20) Water rinses follow each of the steps described above, with the exception of the pre-dip. Following the tin bath, a drying process removes any residual moisture from the board to prevent staining and ensure high metal quality. (9) A typical ImSn finish thickness is from 0.76 lm to 1.27 lm. (21) Since tin is a major component of lead-free solders, the metallurgy of the ImSn finish is well suited to lead-free applications. Immersion tin is good for fine-pitch product applications. (14) However, tin whisker formation can occur on the surface of pure tin, which may cause reliability risks to the electronics. (22,23) Furthermore, ImSn can attack and dissolve solder mask materials easily to undercut copper traces. (21 )The formation of Cu-Sn IMCs between the copper pad and the tin finish will consume the tin finish. So the excessive growth of Cu-Sn IMCs will degrade the wettability and solderability of the ImSn finish. (24,25)

ABSTRACT

This article (experiment) will describe an actual creep corrosion phenomena. It will analyze the results of a test that was performed on several PCBA devices, which were coated with Immersion silver (ImAg), and were subject to sulfur compound contamination . It will show the initial analysis and confirmation of the failure mechanism, as well as an experiment, that has succeeded in recreating the phenomena under lab conditions. It will also present several solutions that will prevent or dramatically reduce the vulnerability of PCBA with RoHS compliant coating when they will be subjected to a sulfur-rich environment.

INTRODUCTION

We found several faulty units that suffered from corrosion and black powder residue. This corrosion compound covered all Vias and resulted in several short circuits within the PCB electronic device. The residues were primary located in the Vias and on several pad surfaces (plated with ImAg).We have obtained from the metallographic spectrum analysis, using SEM (BSE)/EDS, evidence of sulfuric residue as well as a high We believe that the sulfur molecules were from an external source, probably a highly contaminated environment with sulfuric particles. We needed to understand how the copper reached the outer layer and formed this new compound, as well as the failure mechanism and the appearance of corrosion surfaces.

Preliminary assumptions

From other studies on this phenomenon and the outcome of the EDS analysis, we came to the understanding that this is creep corrosion which begins by the growth of dendrites [1] and of Cu2S compounds. Due to the fact that not all 100% of the inner via surfaces were successfully coated, causes the copper to be exposed to the outer environment. We can understand how Cu was found in the spectrum. But in order to complete the mechanism we needed to find the 3rd element. Once again, previous studies provide a good and acceptable assumption in the form of relative humidity in the surrounding environment the subjected device. Oxygen in the water vapor together with the sulfur and copper provide an acid compound that started the creep corrosion. At this point, our main question was why prior to the RoHS PCB fabrication (HASL) trend, this phenomenon was rarely seen. The answer leads us to look at the ImAg as possibly contributing to the accelerated failure. We have deiced to engage in a comprehensive experiment between 5 different coating materials: ImAg, ENIG, OSP, HASL, and LF HASL. We also compared PCBs with conformal coated (Humiseal1b31) to those without. Each group also was both with and without solder mask plugged vias. This way we would have the full range of possibilities which are standard today in the industry and we would be able to choose between the best perform technology.

Experimental Detail

 We produced 32 assembled PCB units (8 layers, FR4 base material, 1.6 mm thickness, 10*10 cm) We divided the batch and used 4 different coatings: ImAg, ENIG, HASL, and LF HASL. Each 8 unit group was divided into 2 subgroups of 4 units each, one with conformal coating and one without.. These subgroups were further divided into plugged and unplugged vias. This resulted in 16 different groups of 2 units that were distinguished from each other by at least one characteristic. This wide range of possibilities provided us with the capability to find the best technology in PCB coating and fabrication that would support and provide good protection against sulfuric corrosion while being feasible and available in the PCB fabrication industry without having to use expensive or unproven technology. As there is no formal test procedure or standard that specifies the conditions required for a sulfuric acid contamination experiment, we designed a special system that provided humid air with sulfuric particles. The system was divided into 4 sealed plastic containers that were connected to a vapor generator, that provided air with R.H >98%. Each container included a small 200cc vessel filled with pure sulfur We continuously measured the humidity and temperature in the containers, using a special humidity and temperature gauge that sampled the internal environment conditions every 30 minutes We ran the experiment for 3 weeks. After this period we sent the units for spectrum SEM (BSE)/EDS analysis and received the data. We analyzed the data and presented it in 8 different layouts which show separately the impact of each parameter.

Conclusion

The environmental conditions of the PCBA combined with poor surface finish constitute the most likely reason for the creep corrosion when these products are exposed to high sulfur environments with elevated humidity. The creep corrosion product is primarily Cu2S which is produced by the galvanic driven attack of the copper beneath the edge of the solder mask as shown in figure 8a. Test methods are being developed to replicate creep corrosion so the mechanism can be better understood and the effectiveness of corrective actions can be tested prior to their implementation. After careful review of the parameters from our experiment, we have concluded that the ENIG finish included via plug process is very resistant to this creep corrosion. In addition, the chance for failure can be reduced through changes in the PCB layout. Design recommendations include: plugging all non-test vias with solder mask, use of non-solder mask defined (NSMD)test vias and pads, spacing these sufficiently apart, and using solder paste to cover all remaining metal features on the PCB.