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32 SMT Magazine • September 2016 prevented at all costs. To do so, the assemblies need to be destroyed by an International Trade in Arms Regulations (ITAR) certified service pro- vider. This would assure that the scrap boards do not leave the United States. It also assures that a government review of the service provid- er's business practices has met a high-level of scrutiny. Another principal to consider is the recla- mation method itself. Economic circuit destruc- tion could be accomplished in one of two ways: incineration or granulation followed by met- al recovery. Either method effectively secures the security of the design intellectual proper- ty, which again is of paramount importance for military and aerospace applications, but one method proves more advantageous. Incineration is inexpensive; however it makes the metal recovery process more diffi- cult. A hot fire oxidizes valuable base metals such as copper and tin. Once oxidized, a chem- ical or electrochemical process must be used to make the tin and copper reusable. Some would argue the additional process work makes this an unpopular option. The second method is granulation. In this process, a mill is used to grind or crush circuits into a powder like in- termediate. This process has several advantag- es. Granulation obviates the design of the cir- cuit and the identity of the components. Once granulated, calculating the value of this pow- der is very straightforward. The weight per- cent of the base and precious metals (i.e., gold, platinum, silver, and palladium) can be easily measured, thus giving a direct method to de- termine the value recovered from the scrapped assemblies. Another source of scrap and waste from the production of military and aerospace electron- ic circuit assemblies that can be recycled is the solder paste remaining on a stencil when pro- duction switches from one assembly to anoth- er. The paste bead diameter on the last assembly produced needs to be very similar to the bead di- ameter used on the first assembly. Around 1.5–2 cm is the most common process recommenda- tion to reduce defects. When a different assem- bly begins production on a SMT line, obviously the stencil will be changed. The solder paste left on the previously used stencil must be routed to an appropriate reclaim container. During the course of its use, solder paste's ratio of metal to flux increases (paste flux is se- lectively transferred through a stencil aperture at a higher rate than solder powder). This phe- nomenon is referred to as flux drift. If the used paste, subjected to flux drift, is mixed with fresh paste, the resulting homogeneous concoc- tion will be subject to process variation. Any- one familiar with Six Sigma principles knows that process variation leads to defects, which leads to scrap. In summary, the cost of producing advanced technology military or aerospace assemblies is highly dependent on the first pass yield. In the risk of repairing an assembly designed for use in a demanding mission critical or life-saving application, high reliability far outweighs the saving of a flawed circuit. And when scrapping such sensitive materials with design intellectual property that can compromise national securi - ty, one must enlist an ITAR-certified service pro- vider to ensure scrap boards do not leave the United States. Minimizing losses by obtaining the high- est return on scrapped assemblies and unusable solder paste should be a high consideration for military and aerospace circuit assemblers as you begin budget conversations for 2017. SMT Mitch Holtzer is global director of customer technical service (CTS) for Alpha Assembly Solutions. To reach Holtzer, click here. RECYCLING SUBSTRATES AND COMPONENTS IN MIL/AERO ASSEMBLIES " Incineration is inexpensive; however it makes the metal recovery process more difficult. "