Producing RoHS-compliant assemblies at SMC: a case study of how one EMS company made the switch.
Successful management of the RoHS-compliant initiative requires engineers from both the OEM and the EMS provider to work together to identify RoHS-compliant substitute parts in existing and new assemblies. In the early months of the transition, parts availability is the most significant challenge for all parties involved. Component materials may need to be changed not only because of the necessity of producing RoHS-compliant parts, but also because existing materials may not be able to bear the higher heat required to solder components in a Pb-free process.
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In 2005, SMC purchased three licenses for the Arrow Electronics database, which enables customer engineers to assist in the identification and procurement of alternative environmentally compliant parts for use in their assemblies. Depending on component availability, the customer may need to wait several months for finished products, or the customer might redesign the assembly using RoHS-compliant parts currently on the market.
Early in 2005, the IT group at SMC redesigned the company's internal component database to identify and sort RoHS-compliant materials. Parts come into the system on reels, in trays, in stick form, in bulk or in bags, and are immediately labeled with internal part numbers that begin with the prefix of "G" (green). Prior to assembly, parts are stored in vertical storage units (VSUs) to simplify identification and to effectively separate and accurately pull inventory.
Orders are entered into the system as total assemblies with an assembly part number. With RoHS-compliant orders, green move tickets travel with batches of assemblies through the system (Figure 2, online), creating a high visual impact and further ensuring separation of these orders. Work instructions are color-coded to help workers differentiate between assemblies during manufacturing. Work instructions and control documents have been updated to comply with ISO and RoHS requirements.
As parts to be used in RoHS-compliant assemblies come into the SMC plant, operators are required to verify and document compliance. Some suppliers are transitioning to Pb-free without changing part numbers, which makes component tracking, dating and marking especially critical within the plant.
To ensure that parts are RoHS-compliant, SMC uses a diagnostic spectrum analyzer to chemically test samples from batches of parts. The quality assurance department also verifies compliance through visual inspection of samples taken at different points in the process. SMC ships certificates of compliance (Figure 3) to its customers with their orders, as specified by the customer. Documentation requirements differ for individual countries and states, ranging from a BoM that lists only the part numbers of components to a lengthy document that lists details of each component that is part of the final assembly.
To eliminate lead and other hazardous substances from PCBs, SMC engineers have changed materials and altered the design process within several areas of the plant.
Traditionally, incoming bare boards are coated with a SnPb material that incorporates a bromine flame retardant. The choice of alternative laminate materials for RoHS-compliant boards depends on the method of soldering and the amount of heat applied during the manufacturing process. RoHS-compliant options include HASL, ENIG (gold), immersion silver, organic solderability protectants and immersion tin. The Pb-based ink used to screen-print bare boards prior to assembly must also be substituted with a non-leaded ink.
In the first assembly step, Pb-free solder paste is applied with a squeegee to specific areas of the board via a custom stencil. The majority of parts are then placed on the board by a component placement machine. Operators then inspect each assembly, attach hand-mounted parts and send the board into the reflow oven.
In 2005, SMC purchased four new reflow ovens to handle RoHS-compliant assemblies. In the reflow ovens, the board and paste are slowly heated and cooled on a conveyor belt that runs through a long oven that is split into a number of zones. RoHS-compliant assemblies require the reflow oven to be set at 220[degrees]F, compared with 186[degrees]F for traditional leaded assemblies. This higher temperature makes the process window tighter, and Pb-free solder does not flow and adhere as well to the board.
Next, if the assembly includes through-hole components, those parts are placed on the board and sent through the wave solder machine. The machine applies flux to the top of the board and solder to the bottom of the board, heating the assembly to 250[degrees]F to adhere the through-hole components. Since the wave solder machine is shared between RoHS-compliant production and regular SnPb production, two pots of solder are kept completely separate in this area of the plant. Between orders of the two types of assemblies, the wave solder machine is completely cleaned. Materials and soldering irons are stored separately as well.
Don Taylor is customer engineering manager at SMC (smcems.com); email@example.com.
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|Title Annotation:||RoHS Transition|
|Date:||Jun 1, 2006|
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