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Minimizing scrap and waste from the process of assembling military and aerospace electronics should always be a top priority for design and production engineers.
However, the critical reliability requirements of these types of assemblies and the risk/reward proposition of manual or semi-automatic rework/repair make the decision to scrap defective assemblies easy. When lives, military mission success and millions of dollars of equipment value are at stake, reworking a $200 assembly with different touch-up fluxes or manual soldering with cored wire may create an unacceptable result when a FEMA is analyzed.
So once the decision has been made to scrap a quantity of defective boards that were destined for the cockpit of a large volume stealth fighter, there is only one question that needs to be answered: How can the value of the substrate and components used to create the defective assembly be recovered without compromising the top secret design of the circuit?
Keeping the scrapped materials within the United States is an obvious first principle. Selling the scrap to a broker or dealer could easily result in the scrapped circuit board being shipped to a foreign country for disassembly or disposal. Obviously, this outcome should be prevented at all costs. To do so, the assemblies need to be destroyed by an International Trade in Arms Regulations (ITAR) certified service provider. This would assure that the scrap boards do not leave the United States. It also assures that a government review of the service provider's business practices has met a high-level of scrutiny.
Another principle to consider is the reclamation method itself. Economic circuit destruction could be accomplished in one of two ways: incineration or granulation followed by metal recovery. Either method effectively secures the security of the design intellectual property, 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 difficult. A hot fire oxidizes valuable base metals such as copper and tin. Once oxidized, a chemical 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 intermediate. This process has several advantages. Granulation obviates the design of the circuit and the identity of the components. Once granulated, calculating the value of this powder is very straightforward. The weight percent of the base and precious metals (i.e., gold, platinum, silver, and palladium) can be easily measured, thus giving a direct method to determine the value recovered from the scrapped assemblies.
Another source of scrap and waste from the production of military and aerospace electronic circuit assemblies that can be recycled is the solder paste remaining on a stencil when production switches from one assembly to another. The paste bead diameter on the last assembly produced needs to be very similar to the bead diameter used on the first assembly. Around 1.5–2 cm is the most common process recommendation to reduce defects. When a different assembly 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.
To read this entire article, which appeared in the September 2016 issue of SMT Magazine, click here.