I was finally taking a trip to China; something I had been planning for years. I was about halfway through the flight when all of a sudden, my vacation excitement turned to dread. I realized that back at home, where I am PCB designer, there might have been a critical board mistake during assembly. I was regretting not specifying that reflow profile. Then again, maybe I was regretting over specifying it? Any mistakes on my board were made days before I boarded this flight and with proper planning, I could have avoided this feeling of dread. There are two popular techniques and philosophies used when designing and manufacturing PCBs. The first is taking complete ownership of a PCB design by specifying every aspect of the board construction and assembly. The alternative is allowing the contract manufacturer to specify some of the construction and assembly process. Each option has its time and place, so let’s explore which is better under different circumstances.
When is being in control the way to go?
Most of the time, specifying every aspect of a board’s design is the safer and better option. Controlling the entire design package allows a company the most freedom and flexibility with where the PCB is manufactured and the quality of its assembly. When one is scaling up production or looking to use offshore cost reduction, it’s common to change board manufacturers and assembly houses. Allowing a contract manufacturer (CM) the freedom to select their own material and process will make transferring the Printed Circuit Board Assembly (PCBA) more difficult. This is because the resulting product at the new assembly house is likely to vary from the build at the initial assembly house. While this might not illicit mid-vacation dread, the certification issues that come with it will. Often, if a board changes, the certifications associated to that product must be re-verified. This can be costly and delay product releases.
From the sounds of it, you probably think that fully specifying how a board is constructed and assembled is always the best answer. However, choosing the appropriate board materials, determining the appropriate flux, tuning the reflow profiles and solder paste mask, developing the PCB penalization, etc. can be time consuming and difficult. There are some scenarios where these factors detract from the benefits discussed above. Let’s discuss the scenarios where you should and should not take full ownership of the PCBA.
Do you really want to be thinking about PCB specifications when you’re taking in this view?
Designing for UL Certification
When you’re applying for UL certification, it is important to fully specify every aspect of the PCBA to avoid costly delays and unexpected expenses. Anyone who has ever done it will tell you getting UL certification is a difficult and time-consuming process. Passing UL often requires subjecting the product to an array of tests. Allowing a manufacturer the ability to choose materials for the design may mean these tests will fail because they don’t follow the specifications necessary to pass. Depending on the safety level of the product, certification can take months to complete and possibly several iterations before the product finally passes. In this case, the only real option for a PCB designer is to learn what safety specifications must be met and use this to determine most, if not all, aspects of the PCB design. For example, a common concern in UL testing is flammability (UL94). Often specifying the right dielectric material for the PCB substrate is critical for these applications. Allowing a PCB manufacturer the freedom to choose any fiberglass material will likely result in additional UL testing cycles. It is prudent to outline design specification from the start and avoid any issues during certification. Sometimes it is hard to determine how much to specify. Let’s look at a case that is somewhat of a gray area.
Designing for Controlled Impedance and User Input
When you’re designing PCBs for the IoT market, you’ll need to use some discretion with how many specifications you outline. Most IoT designs will use Bluetooth or Wifi to report sensor data or some other function. This means that there is typically a PCB antenna that requires controlled impedance. Similarly, capacitive sensors must have strictly controlled capacitive profiles for optimum performance. Both of these are greatly affected by board construction. In general, this is done by simply using a tool to estimate the target impedance and requesting that it be controlled in the manufacturing notes for the PCB. It is also common to ask for a coupon and the board house’s test results. These verify that the controlled impedance is met to the product’s standards. For these cases, selecting every aspect of the board design and assembly is a bit of a gray area.
The best practice for these scenarios is to strictly specify the board requirements (such as the impedance) and to evaluate the PCB after it is assembled. This is sometimes called an audit. Simulations cannot perfectly estimate PCB parameters, therefore critical requirements must be physically measured and fine-tuned by the board manufacturer. So, it is better to give manufacturers the freedom to choose their process materials and assembly flow so that they can meet your strict end-product specifications. As you can see, how much you should specify in this scenario is not 100% clear. On one hand you will strictly specify the board behavior, but on the other hand, you allow the CM the ability to use whatever methods are necessary to achieve the goal.
Specifying what food you want can sometimes be as tricky as specifying process materials
Developing Quick Turn Prototypes and Proof of Concepts
The startup world has a phrase, “Move fast and break things”. The core of this idea is that a startup company doesn’t really know if they have a good product or idea until the market validates it. As a result, startup companies move fast to try and get a close approximation of the product to test the market. This is often called an MVP (Minimum Viable Product). Moving fast also often means moving cheap. In this case, it is more important to get a working board fast than to have an easily transferable board with tight quality constraints. It doesn’t make sense to spend time making a tier-1 level package for a board until the product is ready for production. You’re best off leveraging contract manufacturers as much as possible so the engineers can focus on building out the critical components of the product.
The Key Takeaways
Despite the many advances in PCB layout software, developing solid PCB designs is still hard. Sometimes allowing a contract manufacturer to make critical design decisions for the construction and assembly of a board is the easier solution. However, doing so may jeopardize the quality and certification of the product. Professional PCB design software, like Altium Designer, and add-ons like Altium Vault, can help you specify board construction and assembly. This will reduce, if not eliminate, any mid-vacation dread, and make the transition from working prototype to selling product that much easier.
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