Design-for-Assembly Is Crucial to Design Success

Have you ever tried to loosen or remove a screw from an assembly but found that your screwdriver could not make a proper vertical alignment with the slot in the fastener head? Perhaps the access space above the screw did not allow for the length of the screwdriver handle so you could not even begin to insert the blade of the tool into the fastener slot.

Not only is this maddening if you do not own an offset screwdriver, but it is also a poor design if the assembly beneath the fastened-downed plate was meant to be accessed as a requirement for field setup or option selections. You have just become a victim of poor design-for-assembly (DFA) planning.

When products are being designed, the engineers must also consider product characteristics beyond functions, user interfaces, appearances, or operating conditions. If DFA considerations are not included in product concept or design review meetings, a perfectly functioning product delivered from engineering to manufacturing may not be producible. Let me give an example that gave one manufacturing team fits trying to create a work-around solution until a new printed circuit board could be fabricated and stuffed.

Without DFA, a product may not be producible.

Thru-hole challenge
In direct-to-enclosure mounted printed circuit cards, there are thru-holes in the PCB material designed to accommodate mounting hardware such as screws and stand-offs. The idea being that the fasteners will hold the printed circuit card securely to the enclosure and will not flex during use or transport. With surface mount components, there is very little tolerance for board flexure as the added stress of bending the board near the components may damage solder connections and or cause micro cracks in the components themselves.

So the placement of these holes and their proximity to components is critical for both short- and long-term reliability. The assembly operation of fastening these boards without over-tightening the fasteners may require a torque screwdriver so torsion parameters of the fastener are not exceeded. These special screwdrivers are bulky and require some operating space in order to get the vertical entry into the slots of the fasteners. If the designer of the enclosure did not confer with the designer of the printed circuit card as to how the card would be mounted and where the fastening holes would be drilled, then there is a high likelihood that there will be assembly issues.

Here is the rub: Often, a design engineer may give the mechanical engineer a board outline with dimensions including the height for the tallest component, but neglects to specify where mounting holes will be in the circuit card. The mechanical cad people will ask how the board will be mounted and may prepare the enclosure with fastening positions in mind, but may not necessarily appreciate the tools required for the fastening operations. The axiom here is that too much information is better than too little.

Access problem
In our real-life snafu example above, the enclosure designer selected a clamshell type 1RU (Rack Unit) and strengthened the top edges by adding ¼" flanged bends. The added bends were not a problem for the prototype assemblers in engineering who did not care about fastener torque requirements. The product was passed onto manufacturing where it was discovered that the torque screwdrivers could not access the fasteners because of the top bends in the enclosure.

This is just one simple example that is meant to highlight the need for thorough communications during the design process that is inclusive of design for assembly issues. In that light, it is best to have manufacturing people involved in concurrent engineering design reviews. Chances are the production floor representative would have mentioned the tools clearance requirements as part of the design considerations.

The work-around turned out to be a procedure where the enclosure bend had to be manually notched to allow for the required production tool access. Unfortunately, there was a PEM nut on the bend that was placed in the same location to help secure the front panel to the enclosure. The front panel had to be redesigned with a new PEM location. The original panels had to be scrapped, and the enclosure design had to be modified, also reflecting the new PEM location to match the front panel changes.

DFA is no less essential for a good design than the design of the hardware or the software. Get your factory people, including contract manufacturers, involved as early in the design cycle as possible. In doing so, you will avoid many issues that can hinder your company's progress.

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