As I talked about in Part 1 of this series, there are many steps required to get a product ready for volume manufacturing. Just because you may have a functionally working prototype, it doesn’t mean your implementation can be manufactured effectively, or even at all. Don't be surprised if someone tells you it will take 6 to 12 months to get your functional prototype ready for mass production! In this article I will outline some valuable DFX principles that will help you reduce that time to get a product ready for production and reach your goal of a high First Pass Yields (FPY) greater than 97%.
VARIANCE IS NOT YOUR FRIEND
The key to any product being manufactured is to remove the variance from your design as much as possible. Variance is the enemy in volume manufacturing, as it leads to more variables, higher risks, and thus lower yields. Any process that is repeated over and over will inherently have some variance each time it is executed. This is called Normal (or Gaussian) distribution.
Humans are inherently Normal (pun intended). So, the first step to improving yield is to remove the variance (i.e. human) and allow for as much automation as possible in the manufacturing, assembling, and testing of your products.
SMD AND THD PARTS
Starting with your part selection, it's crucial to understand for PCBAs there are two main categories of electrical components. What differentiates them is the means by which they are assembled to the circuit board:
Surface Mount Devices (SMD) – Components which are solder assembled to the outer surfaces (top and bottom) of a Printed Circuit Board (PCB)
Thru Hole Devices (THD) – Components which have pins that penetrate through the PCB and are soldered on the opposite side
Many electrical components are available in both technologies, as they each have their specific purposes – but the Surface Mount Technology (SMT) used to assemble SMDs is a much more automated process with drastically higher efficiencies and lower costs than Thru Hole assembly technologies.
So, the first DFX rule for PCBAs is to select Surface Mount packages, exclusively. However, in practice it’s not uncommon to still have some small percentage of Thru Hole components for various reasons, but you should strive to keep this under 10% of the total component count.
SHOW A LITTLE EMPATHY
As mentioned in the previous post, a valuable asset of great engineers is their comprehension of the processes used in other stages of the manufacturing ecosystem. For PCBA SMT, the reflow oven is the lynch pin and understanding how it works will drastically improve your design skills.
In short, a reflow oven has a conveyor belt along with multiple heating elements set at different temperatures to create a specific thermal pattern. As the PCBA travels past these heat sources, the solder will transform from its original paste-like state to a liquid state and then to its final solid state to hold all the components in place. If the heating elements in the reflow oven are not tuned to the specific thermal profile of your PCBA, it will introduce the risk of components receiving too much or too little heat, which results in a low yield due to poor, or “cold”, solder joints.
Even if the thermal pattern is correct, there are still only so many dials and knobs a technician can tune on a reflow oven. So, the majority of DFX efforts should be spent on the component selection and layout of the PCBA, itself. Here are some simple guidelines to help improve your PCBA layout:
- As noted earlier, select a surface mount package for as many components as possible in your circuit. This allows for the highly automated and less variant processes associated with mounting and assembling these parts
- All items have mass, which when combined with the material type result in a specific required amount of applied heat in order for the item to reach the necessary temperatures for the solder to do its job. So, keeping this in mind, it’s imperative to select components and create a layout on your PCB that generates a more uniform thermal profile, so all elements can easily reach their required temperatures. Try to select components of similar mass and volume and place them evenly across your PCB.
- Leaving sufficient space between parts on the PCB is important for multiple reasons.
- It allows pins to have greater access to the heat source, ensuring more robust solder joints
- It reduces risk of a solder short-circuiting between parts
- It allows easier access for test probes
DESIGNING FOR TEST
Remember, the only way to know if the design decisions you make for DFM and DFA are effective is to measure them. Thus, we must also implement good DFT (Design For Test) practices. This means it’s just as important to make the test and measurement stages as simple, efficient, and cost effective as possible, as the other DFX stages.
Testing is no different than assembly when it comes to the use of automation. Relying on people to setup and take test measurements is risky, horribly time inefficient, and expensive. Therefore, each manufacturing line needs one or more automated Functional Circuit Test (FCT) stages. This is the equipment that will test that each instance of your PCBA is assembled correctly and each function is working to its specification. The results of these tests contribute to the final First Pass Yield metric.
A simple step to Design for Testing on your PCBA is to add explicit test points to facilitate the measurement of key signals and outputs of your circuit. All of these test points are to be located as close as possible to their associated source and placed exclusively on the bottom side of your PCBA. A corresponding custom test fixture, often referred to as a “bed-of-nails” is used to hold the PCBA under test in place. An array of electrically conductive pogo pins mounted in the test fixture make contact to the PCBA at the exact locations of the test points. These pogo pins are connected to the appropriate automated test equipment to make and record all of the test measurements. Software packages are used to customize and execute these tests, again removing the human element
GET OUT OF YOUR BUBBLE
If there’s one thing to take away from this article is to ensure you never design a product in a bubble. It’s imperative that you talk to your manufacturer early about your design and the best ways for them to assemble and test it.
Remember, the fundamental philosophy of DFX is you are making design decisions to help someone else do their job better.
That someone else more often than not is your Contract Manufacturer. So, while there are plenty of DFX guidelines published for you to start with on your own, your CM reserves the right to tune those guidelines to better match their specific processes and equipment capabilities. Therefore, the earlier you incorporate their DFX feedback into your design, the faster you will be able to get ready for production with a greatly reduced risk and a very high yield.