Environmental concerns over the use of Lead (Pb) in electronics has led to a shift to using alternative lead-free finishes for electronic components and circuit boards. One such finish, Tin (Sn), is currently used extensively for electronic components and circuit board plating.
Tin is deposited onto copper using an electroless chemical plating process. The purpose of the plating is to extend the shelf life of bare copper by slowing the oxidation process. However, it has a limited shelf life of around a year at ambient conditions. Complicating the shelf-life issue, global shortages of certain electronic components has been driving the use of reclaimed components or components that have been in storage for many years. Tin plating that has aged more than a year starts to rapidly deteriorate not only due to oxidation of the tin, but also due to copper-tin (Sn-Cu) intermetallic formation, both of which will cause severe solderability issues.
While minor oxidation can usually be overcome by low to medium activity soldering flux, the Sn-Cu intermetallic formation becomes a much more serious problem, that has traditionally led to scrapping circuit boards and components that have yielded to excessive intermetallic growth. The alternative to scrapping was to send the components or circuit boards back to a plater for strip-and-replate. This was not always feasible and almost always cost prohibitive.
Recently though, a novel new approach has been developed for dealing with extreme oxidation and intermetallic growth of tin plating over copper. The process was developed under a joint effort of a major flux supplier, Superior Flux, and a component tinning and re-balling service, SemiPack Services Inc. The process was developed such that component scrap, or expensive strip-and-replate, can be avoided.
The process involves using a (1) copper-filled flux material, that can be applied using basic hand-fluxing methods, (2) a typical SMT process reflow oven for heating and activation, and (3) SMT component hand-cleaning processes consisting of heated DI water and neutral cleaning chemistry (4) followed by an IPA final cleaning.
Tin-plated copper will degrade over time due to the formation of copper-tin intermetallic compounds (IMCs) at the interface between the tin plating and the base copper. Copper and tin form 2 primary IMCs, Cu3Sn and Cu6Sn5 . During the tin plating process, a layer of Cu6Sn5 forms between the tin plating and the copper base metal. After plating, and at storage temperature, Cu3Sn forms because of a solid-state diffusion reaction between the Cu base metal and the Cu6Sn5 that was formed during the plating process.
As the solid-state diffusion process continues, the tin plating is ‘consumed’, and the Cu3Sn IMCs will eventually present themselves at the surface (Figure 1). This can occur in as little as 6 months depending on storage conditions. The Cu-Sn IMCs will oxidize readily when exposed at the surface of the plating. The oxidized IMCs ultimately result in very poor solderability, even when using the most aggressive soldering flux.
The solderability issue is two-fold, intermetallic breakthrough, and oxidation. Both issues must be addressed to fix the solderability problem. Figure 2 shows pads from a BGA that exhibit the issue of intermetallic breakthrough and oxidation. This component came from a lot of 10,000 parts that were not solderable through the BGA re-ball process and were therefore dispositioned as scrap.
The phase diagram for Cu-Sn (Figure 3) shows these IMCs, their corresponding liquidus temperatures, and how they transition to different phases. This is important to not only understand why the solderability issue occurs, but also to understand the solution to the problem. Note specifically, that according to the Cu-Sn phase diagram, the liquidus temperature of Cu6Sn5 is around 227°C, while the liquidus temperature of Cu3Sn is above 415°C. Therefore, at a typical soldering temperature range of 220 to 260°C, Cu6Sn5 is the direct reaction product between Cu and liquid Sn, while the Cu3Sn can only be obtained in the solid-state reaction between Cu and Cu6Sn5. The melting points of Sn and Cu can also be interpreted from the phase diagram to be 232oC and 1085oC respectively .
Cu3Sn not only inhibits good solder wetting but also interferes with soldering due to its high melting point of around 415°C. The addition of Cu promotes the growth of Cu6Sn5 which in turn promotes the formation of a good solder joint since this IMC layer melts at typical solder reflow temperatures. The Cu3Sn IMC layer has such a high melting point that it won’t support good wetting at typical soldering temperatures .
Looking at the phase diagram for Cu-Sn, for typical soldering temperatures at or below 260oC, Cu6Sn5 is formed by the reaction between liquid Sn and solid Cu. The Cu is essentially dissolving into the Sn. For temperatures below the melting point of Sn such as typical storage temperatures, Cu3Sn can only be formed by the solid-state diffusion reaction between the solid Cu and the Cu6Sn5, since both Sn and Cu and their IMCs, having melting points well above the storage temperature.
Every tin-plated copper alloy experience the formation of copper-tin intermetallic compounds (Cu6Sn5 and Cu3Sn) at the interface of the tin and the base metal. Many dissimilar metals (tin and copper) in close contact will diffuse and form intermetallic compounds. The compound initially forms at the interface of the plating and the base metal and grows until eventually all the tin is consumed. Figure 1 provides a schematic illustration of the process. Copper-tin intermetallic compounds are easily oxidized. The oxidized intermetallic compound can adversely affect contact resistance and solderability.
Application Process – Superior Oxide Cleaner Flux Material
The Superior Oxide Cleaner process is unique in that no specialized plating equipment is required. The process involves using a copper-filled flux material that will prepare the damaged plating for soldering. A thin layer of the material is applied to the affected surfaces using a brush or lint-free swab. After application, the affected parts are placed into a typical SMT reflow oven running a typical tin-lead reflow profile. Figure 4 shows the DSC-TGA curve for the Oxide Cleaner. It shows how the Oxide Cleaner material responds to temperature over the SMT reflow process temperature range.
After the reflow process, the material is mostly charred and requires thorough removal. The process residue can be removed by soaking in a heated DI water neutral pH cleaning chemistry, followed by a hot DI water rinse, and finally an IPA scrub.
The Oxide Cleaner material works by breaking up and displacing the oxidation layer, and by diffusing additional copper into the affected Cu-Sn intermetallic layer. The process essentially creates a displacement reaction of forming a copper surface that is not oxidized and removing the IMC. The end result is an active copper surface that readily responds to the tin-lead soldering process.
Results and Conclusions
It was once thought that intermetallic breakthrough was an issue that could not be corrected and resulted in scrapping parts and PWBs. With the Oxide Cleaner process, this is no longer the case. Not only can parts and PWBs be salvaged, but it can be done using a conventional SMT convection reflow process. Specialized stripping and plating processes are not required.
The process was validated using 3rd party micro-section analysis of the solder joints to confirm the solder joint integrity. Figure 5 shows representative micro-sections of BGA spheres soldered to a BGA package after the Oxide Cleaner recovery process. The intermetallic layer is shown to be typical structure and thickness, uniform and continuous, as would be expected on a BGA package not suffering from the intermetallic breakthrough issue. The expected lifetime of the refinished components, using this restoration process, will be comparable to the original component, prior to the intermetallic damage.
Recently, this process was utilized to recover over $3.4 million worth of non-balled BGA components damage by intermetallic breakthrough, that was the result of an insufficient tin plating that was compromised, due to storage of more than a year. Using this process, the parts were inexpensively recovered and later assembled into a high reliability product.
Dave Hillman, Collins Aerospace – Micro-section analysis and content review
Tina Reese, SemiPack Services Inc – BGA re-ball process development
Richard Kenyon, SemiPack Services Inc – Reflow profile and cleaning process development
Mary Bloyd – CEO, SemiPack Services Inc
Scott Bloyd, Executive VP, SemiPack Services Inc