Assembly Process Design

It is common for a product design to be split into logical sub-assemblies based on how the product design team separates functional blocks of the product. Obvious sub-assemblies include: 

  • Printed Circuit Board Assemblies (PCBA) 
  • Electrical control boxes 
  • Cables 
  • Plumbing and tubing 

Much of the organization falls out of the way the product was designed and the design iterations. By the time the Beta systems are built and tested, some modifications have been made to the sub-assembly definitions based on the Beta build experience. All this effort is necessary, but seldom does the product design team – under pressure to release a design for manufacturing – go back and look at the process critically to see if there are better ways to build. That effort is for the manufacturing engineering team. 

Other considerations for the process design should include: 

  • Logical flow – sometimes it might not be immediately obvious where to start an assembly. A logical assembly process becomes evident when any one component must be installed before or after another which defines an order of operations for the assembly. Imagine, for example, a PCBA is mounted on the underside of a bracket that is installed in an enclosure. If you need to plug cables into to the PCBA, it can be difficult or impossible to see where each cable needs to insert on the board without having an unobstructed view of it. Inserting the cables before the bracket is installed simplifies the assembly and mitigates the risk of cables being plugged into the wrong locations. Another example would be firmware loading on microcontroller installed on PCBA’s. Typically, firmware would get loaded via a programming fixture onto a PCBA, but if the PCBA is installed into an enclosure, access to it is obstructed by other components, firmware loading becomes more difficult.  
  • Maintenance, Service and Repair – are there components that must be calibrated or serviced on a recurring basis? Are they easy to identify and remove? Is there a spare part, an FRU (Field Replaceable Unit) required? The parts needed for Field Service or Depot Repair should be sub-assemblies in the product and should be easy to access and replace. Also consider that these FRUs should be stand-alone products with packaging and instructions. 
  • Risk of building in defective parts – we must assume that the final system testing is adequate (or we will modify the test procedures) but consider the expense if you wait until the end of the build to test the product and find a defect in a low-level assembly (e.g., a PCBA). A risk analysis on each sub assembly would help identify the risk of a product failing or needing addition calibration and would force Engineering to evaluate the risk of failure of an assembly versus the expense of replacing the assembly in a finished product. Should we have sub-assembly testing to find and replace a defective component before a non-reversable process or there is significant value added? Some products have portions of product potted for environmental reasons; if the electronics embedded in the potting do not work, it could cause the manufacturer to scrap the whole product because it cannot be repaired. 
  • Consider the processes that will be used for an assembly. In many cases, it is best to separate mechanical assembly from electrical assembly because of the different skill sets and equipment involved. If any assemblies need a clean environment like a flow bench or clean room, it is important to consider how the assembly is designed and handled once no longer in the clean environment. In other cases, assemblies can be broken out depending on the type of assembly e.g., soldering/potting/conformal coating that require special set up and could damage other components.  
  • Tools, Fixtures – consider the tools required for assembly and how they will be used. Be sure that there is room to manipulate the tools. It is sometimes easy to forget during product design that a product must physically be put together. What seems easy in CAD (given that parts can pass through others) cannot be physically accomplished in Production. Design and implement fixtures where necessary to reduce the risk of mis-assembly or incorrect alignment. 

Batch build or Cell build? 

Product Resources uses both Batch and Cell manufacturing strategies depending on the product volume and schedule. For a single shipment of product, usually based on a PO-to-PO basis, a batch process is typically chosen. For continuous manufacturing, typically via a supply contract, we can take the time to setup a Work Cell. The initial setup costs are easy to justify on a multi-year contract. 

Batch assembly is the traditional method of building a product. Purchased material is booked into the stockroom. Then material in the stockroom is kitted with all the drawings and procedures, then moved from the stockroom to a workstation. Inventory transactions track each piece of material. The assembly is built; if any material is damaged additional transactions and interactions with the stockroom are necessary. Once complete, the finished assembly is moved back into the stockroom and another inventory transaction is made. This is an efficient process, especially when using computers and automated data input (e.g., barcode scanners), but there is a considerable number of touches and inventory transfers on the material. 

Compare this method to a Work Cell. A Work Cell is an assigned space with dedicated workstations and inventory for a particular product. There is considerable setup at the outset of manufacturing, but there is considerable transactional efficiency. 

When purchased material is received, it is moved to an inventory location within the Work Cell rather than into inventory. Transacting the material out of inventory and into WIP is eliminated. When required, the Work Cell personnel pick material directly from the cell’s inventory location and use it immediately. As the products are completed and exit the cell, inventory is automatically adjusted. 

Work Cells can incorporate mechanical, electrical, plumbing assembly and test operations in one area, or there may be multiple cells with some focusing on assembly and other on test and calibration. 

The choice of Batch or Cell build is not one or the other, there are many instances of batch-built assemblies feeding inventory for a cell-built product. Having the flexibility to use one or more methods can be advantageous.