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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.

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.

15 comments on “Design-for-Assembly Is Crucial to Design Success

  1. FLYINGSCOT
    February 19, 2013

    I feel for the design team when they screw ups occur.  They are frustrating when they happen and sometimes hard to avoid.  We had a similar one where we build the PCBs inside plastic watertight enclosure which cost a lot of dosh.  Later we found that we were not sure of the version of a key chip inside the enclosure but opening the enclosure would wreck it.  Workaround was to rent a fancy boroscope and drill a tiny hole in the enclosure to peek at the chip version.  We then gooped the hole with fancy epoxy that was watertight.

  2. dalexander
    February 19, 2013

    @Flyingscot…Wow! What a great example. If the encapsulant is the same formula as commonly used with conformal coatings on PCBs, it must have been like drilling through amber. How did you all know where to do the drilling such that you saw just the target chip, or was the encapsulant over the entire enclosure and there was open space above the PCB? Then the bore scope would have to have been flexible and wide angle to view the target chip. I'm curious as to the details. Can you say more? I suppose you could have used a stuffing diagram along with the PCB CAD to plot the incursion site and align the drill accordingly, but I am really curious on how long the process took and the methods and equipemnt used.

  3. dalexander
    February 19, 2013

    @Flyingscot, I just reread your post and saw that the enclosure was made of plastic with some kind of humiseal and that breaking the seal would compromise the enclosure. I now understand that the PCB was mounted inside the enclosure and that it was not in itself conformally coated. This means a flexible bore scope could see a large region of the PCB and zoom in. I had one of these FO scopes at Microsoft and it was really fun to play with. It was outfitted with a camera and I could see under BGA packages, inside of switches, and scan for corrosion on connector leads. 

  4. William K.
    February 19, 2013

    Stories like this prove that good communication and broader skill sets are quite valuable. It also shows that designers must be familiar with the manufacturing capabilities and limitations. Those capabilities and limitations change with time, so a smart designer will keep up the communications with the manufacturing group. That is one more reason for keeping the manufacturing part of the business at home, where communications are simpler and can be much fas6ter, and sometimes a lot more truthful.

  5. dalexander
    February 19, 2013

    @William, I like the term “Business at Home.” There are many times where I had to leave the company to go to a sheet metal or machine shop to watch the production process and learn what needed to be changed to better accomodate volume production. I also spent a lot of time at contract manufacturers to watch the intial assembly proceses, especially where 2nd operations like hand insertions added cost to the fab process. By being close to the outside contractors, we were able to keep an eye on quality and statistical process controls and react accordingly. There are definite advantages to keeping “Business at home.”

  6. elctrnx_lyf
    February 20, 2013

    This is one of the important reason why it is important to have engineers who have system level thinking. It is very important to have the manufacturing team as part of all product concept discussions.

  7. Brian Fuller
    February 20, 2013

    This is a great thread, folks. I'd be interested in hearing some real-world company examples. What companies do you consider to be doing DFA well? Success here is not easy because it requires technological support (obviously) but cultural acceptance, and the latter is always tricky. 

  8. syedzunair
    February 20, 2013

    Designers must be familiar with the limitations and the capabilities of the manufacturing departments. Any designs must only be approved after a careful scrutiny from the production. Communication is the key here and must be done effectively. 

  9. syedzunair
    February 20, 2013

    elctrnx_lyf:

    I would say that it is important to ensure collaboration amongst the departments. If they continue working in silos the issues will continue to exist. They must learn how to function as a single unit with a broader goal. 

  10. William K.
    February 20, 2013

    Syedzunair, You Are Exactly correct! The alternative is a manufacturing department that is able to do everything. Those are fairly rare.

  11. William K.
    February 20, 2013

    @Doug, it is more difficult to watch a manufacturing process on the other side of the world. That 15 hour plane ride each way is a bit of a deterrent, as well. Not to mention the language barrier.

  12. dalexander
    February 20, 2013

    @William, I was sent by our company to scope out the Philips (Phone division) in Scotland as a possible remote factory site for a project we were building for the U.K. The plan was to set up a factory within Philip's factory and staff it with Quality Assurance people from our US Based company. So, you are right about trying to maintain harmony between an engineering operation in one country and a factory in another. As concerns DFM, DFA, and DFT our plan was to equip the factory with the product's manufacturing needs in mind. That way DFA, DFT, and DFM as built into the entire operation. It was a short run, single customer operation so newer product introductions were not a factor in this particular equation. When working with remote CMs, we always asked for an equipment facility list and process and procedure standards before selecting the factory. We also did site visits and surveys to check for calibration compliance and ESD safeguards. There is never an initial 100% compatibility as unique fixtures and training always seem to be part of the manufacturing process. We also had the factory hire local labor that reported to our operations people so there would be no conflict of interest on quality or yield reports. But there is no doubt about it, when the cat is away, the mouse will play.

  13. syedzunair
    February 21, 2013

    Yes, those are pretty difficult to find these days. 

  14. FreeBird
    February 21, 2013

    Douglas: it sounds like the example above happened within an organization that still did its own manufacturing. If that is the case, better communication is the solution. But what about outsourcing? I've had the impression for awhile that the DFX function belongs to the EMS. Does that work any better or worse than in-house?

  15. dalexander
    February 22, 2013

    @Freebird, you are correct. We did our own manufacturing, but at the system levels only. The PCB stuffing and machining and sheet metal fab all were outsourced. So, we had not so much of a vertical integration thing going as much as a hybrid manufacturing operation. We did not have a Turnkey Assembly House because our product line was more characterized as a low volume, high mix. The end product was almost custom for each delivery. Our internal BOM management was based upon configurable BOMs. I liken this to a Restaraunt model, where all the food stores are in the kitchen, but the customer can pick and choose from a menu to suit his or her particular tastes. Our storehouse had sub assemblies that could be mixed and matched depending upon frequency, bit rate, modulation, power, antenna size, cable or wave guide interfaces, single or redundant circuitry, software options for alerts and alarms, mobile vs. stationary…etc. So, the sales people would sit down with the customer and fill in a sales order check list reflecting all of the options above. A data entry clears would plug in all the selections and the BOM configurator would spit out the resultant materials requirements. The order would be included in a master schedule for MRP, and we're off to the races. Our procurement people would order components, modules, and assemblies accordingly. The various qualified suppliers would handle their respective parts of the order and ship the goods to our factory for final assembly and test. As you can imagine, the BOM configuration software was critical to the whole operation and saved us tons of time over having to enter each new Top Level Product into the Item Master with unique part numbers. We cross referenced the Sales Order to the Customer's specifications and serial numbers and that is how we tracked field units and created reorders quickly. Pretty nifty, Huh?

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