Circuits Assembly - December 2008 - (Page 38) Pb-Free Lessons Learned on the cool side, it may not be fully deactivated in the reflow cycle, thereby posing a potential reliability threat in service. In the context of mixed-metals systems, the paste flux is exposed to peak temperatures above 220°C for at least 30 sec., relatively close to its design specification. If broadly deployed as a SnPb alternative, however, the probability it is exposed to smaller amounts of thermal energy in the reflow process is much higher. To err on the side of safety, I would suggest subjecting the product to SIR or electrochemical migration tests when processed under the coolest anticipated SnPb profile – with an adequate safety margin – before it is introduced into production. Voiding concerns remain relatively the same whether the paste is used as a vehicle to solder mixed metals systems or to address HIP. For the most part, voids are created when the volatile materials in the flux portion of the paste cannot outgas before the metal melts. A flux designed for Pb-free processing can volatilize and vent until 217°C when used with SAC alloys, but its outgassing paths will close off at 183°C if used with SnPb solders. A faster/cooler profile may permit more gasses to be trapped than a slower/hotter profile, but a paste’s behavior with respect to voiding can be formulation-dependent and difficult to predict. The best thing an assembler can do is run the paste candidate at the fastest/coolest thermal profile they expect to use in production, measure the voiding rates and decide if they are acceptable. The third concern was pin-in-paste processing. If using intrusive reflow processes, examine the ability of the paste overprints to pull back to the PTHs. We know different pastes have different sweet spots in the reflow window with respect to pull back, and users should make sure the candidate hybrid paste will support pin-in-paste without leaving random solder balls on the board. And again, we’re not just looking at the high end of the SnPb window like we were with mixed metals; we’re looking at the whole window if we want to qualify these products for general production use. Finally, the newest worry: halogenated materials in the flux. When I mention halogenated materials, I mean non-ionic halogenated compounds, not the ionic halides we are accustomed to in the world of flux. (Halides make great flux activators because they are keen at reducing metal oxides, and our industry has long-established methods of identifying and communicating their presence in soldering fluxes.) Here, I’m referring to those compounds often used as flame-retardants in all types of electronics components, most notably printed circuit boards, molding compounds and connectors. They are sometimes used in small quantities in soldering chemistries to improve a flux’s survivability under extended therCircuits Assembly DECEMBER 2008 mal stresses. They can be found in both SnPb and Pb-free fluxes, but they are not necessarily required for successful soldering. Some fluxes have them; some don’t. Some have such a small amount that they squeak under the 1500 ppm threshold and are therefore considered halogen-free. Given the thermal sustainability demands put on Pb-free fluxes, they are slightly more likely to contain halogens than their SnPb counterparts. If the assembler has no concerns about halogenated materials, this point is entirely moot. But looking down the road at green initiatives and potential environmental legislation surrounding halogenated compounds, qualifying a new process chemistry that contains these materials may be a decision that could backfire in a few years’ time. And let’s not overlook the public relations implications: Who wants to be the target of a one-sided Internet video that opines a product is not “green enough,” but neglects to mention the often irreplaceable safety factor the halogens provide? The lesson to be learned with hybrid solder pastes is to ask a few key questions prior to making the switch: • Can the Pb-free flux provide the necessary electrochemical reliability when processed on the relatively faster, cooler profiles associated with SnPb reflow? • Will the new SnPb paste create more voids than the incumbent one? • Will the solder coalesce properly in the intrusive reflow process? • Does the product contain any materials that are politically controversial or under consideration for near-term elimination? Hybrid pastes now appear to address two widespread issues for assemblers: improving BGA soldering in both mixed metals and SnPb systems. They show promise for improving yield and reliability, but should not be implemented without due consideration and appropriate testing. As engineers, we must make sure that when we fix one problem, we don’t create another. In other words, look before we leap. n circuitsassembly.com 38 http://www.circuitsassembly.com
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