In the first article, we examined some general considerations to make and tips to incorporate into designing PCBs that connect and work together.
In part two, we are going to dig deeper into what to look for when specifying components,
designing the right test protocols and how to best address common problems that crop up.
By the time you’re done reading this, you will be ready and confident to take your design into production with the trusted knowledge and resources of San Francisco Circuits, a professional PCB manufacturing & assembly provider.
Check out the first article:
There are a number of avenues for you to take when researching the various components you will need to assemble your circuit board as defined by your design. Of course, the internet has a wealth of information but can be overwhelming when trying to fight through marketing jargon.
Meeting with manufacturer representatives can be a good way to learn what’s new but, again, you are up against a sales person who, in the end, is trying to get you to buy a product. However, a manufacturer that is willing to share expertise and provide you with insight into industry standards, past experiences and samples can be a great resource.
Partnering with an experienced PCB manufacturing and PCB assembly provider with engineers that have long-standing relationships with component suppliers and an extensive understanding of complex interconnect layouts is crucial to the success of your project.
Successfully taking your intricate PCB designs from layout to manufacturing & assembly is the core value that comes from partnering with San Francisco Circuits for your next project - from flex PCB to bare circuit boards, we are your one-stop shop for advanced printed circuit technologies.
Before you start researching components, be sure that you refer back to the original market research, product plan and operational requirements to help guide your selection criteria. Step away from the minutia of the circuit layout and go back to your overall product goal to determine the attributes and place in the market you are aiming for.
If you are aiming for a user-friendly product, you may want to pay more attention to the user controls/interface components. A high target price product will dictate a far different mix of components than a product aimed at the lower end of the market. Both of these divergent goals create different challenges to come within the target per unit price.
The per-unit target price for the finished product will dictate your overall parts budget. You’ll need to determine where to spend your available budget; there are some areas you won’t want to compromise on price (such as high speed, high reliability, and more sensitive devices) and others where you can save money (such as low-complexity and lower-quality mass produced consumer-based products.)
Here are some of the common components used to connect boards and how you should approach them for the best results:
The amount of power that a thin copper pin can handle is surprisingly little and can quickly become an issue if you are driving high power in a compact design. This is particularly the case in any instance that involves motors, solenoids or bright LEDs and may require multiple pins to achieve the required power rating on a connection.
In general, you want to stay 30% below what is expected at peak.
Also note that voltage tends to drop over long, thin traces or thin pins that have a lot of resistance. Use too little copper in a high current trace, the resistance increases and creates a voltage drop across it. This can also affect a battery’s voltage output.
To combat this, use polygon pours for higher power traces. If board space is at a premium, add more copper per layer.
Transferring high-speed data between boards that have a modular design with several smart boards connecting USB, Ethernet and similar sources, you will have to account for application-specific connectors so you don’t mix signals.
For low speed transfers and less sensitive analog signals, use a combined connection to achieve desired results.
To further isolate and prevent coupling, double the amount of connector pins and place a ground in between each sensitive analog/digital signal.
First, a high yield output requires a manufacturing environment that is clean, precise and repeatable.
Ask your PCB manufacturer for examples of similar projects or any data that they can provide that proves adherence to quality controls. Depending on your application, you will need to test to specifications to meet the requirements of ISO, CE, IPC, IEC and others. These should be outlined in the market or product specification plans.
Boards should be assembled so that connections are straight and flat. Mechanical connections need to be secure and accurate to reduce stress on the components. Any board-to-board connectors that are even slightly mis-aligned can cause failures over time. Spend time making and testing prototypes to ensure the manufacturing process is performing to stated tolerances and can be repeated before starting your output.
There are two kinds of test points: a functional test point for when the board is energized and operating and a “flying probe test” where the board is populated but not energized to check analog components and shorts or open circuits. The functional test, while important in finding faults, is a more expensive route as it will require a detailed plan with operational requirements and established testing thresholds.
Test points are needed for large, complex and sensitive applications. To make this testing process easier, keep the functional testing front of mind in the design process. The design should accommodate test points in easy-to-probe places, such as near the edge of the board so they are more readily accessible. Another approach is to create a “test jib” with a cable assembly that interconnects two boards while exposing the test points. Boards can also be tested separately to ensure they meet individual specifications.
Now that you have an assembled product, it’s time to put those test points in use to determine if the completed product is functioning to specifications.
Flying probe tests are common in post-assembly checks to make sure the right components were used and connections were made. This typically just calls for a test point report with schematic coordinates and bill of materials.
A functional test will measure voltage rails, rise/fall time on trigger lines, analog gains/sensitivities and much more. This will often require a specific testing fixture that utilizes interconnects or test points.
A common methodology is to test continuity between the boards. This is a good place to start since most problems occur here. Next, test all main power and voltage rails. Finally, test the signal integrity with an oscilloscope and tolerances for the main components versus the planned specifications.
Soldering should be automated for high density board applications to ensure connections are made optimally and completely. Flux residue from the soldering process can play a significant role in the performance of your board in terms of resistance. Board to board connections that have several through-hole pins need good flux cleaning since voltage can drop if there isn’t enough copper between connections with a high current path.
Using a solder wave pallet for stability in the soldering process can help ensure solid connections are made during assembly.
Shorts can occur on very high density connectors during the assembly process and need to be tested for during the quality control phase. Good board manufacturers will employ an X-ray machine to verify ball grid array packages.
Noise and inductive coupling issues are hard to trace during the functional test and remediate once a product is in production. As a preventative measure, start with a detailed test plan to lower the risk of these issues.
As an example, think of a board being used for a high-power heating application with a connection that extended to a heater and thermistor for feedback. The heater is Pulse Width Modulated at 100Hz with a varying duty cycles to control the average current through the heating element with power in the 10s of watts. What do you think the affect was on the connection? By examining the signal with an oscilloscope, you can locate power spikes on the thermistor.
Typically, the most common issue that can occur in a PCB interconnection is unintentional radiation and coupling of noise.
There are a few ways to find these issues. First, however, use shielding on connectors when dealing with sensitive and powerful signals. To determine if the signal integrity on an interconnection is bad, use an oscilloscope to find any noise or spikes caused by radiation. A near-field RF probe is the best tool to perform frequency sweeps to determine if a frequency tone is emitting from the connectors. This issue can also affect sensitive electronics nearby.
For example, a board being used for a high-power heating application with a connector that extended to a heater and thermistor was experiencing noise at the connection. By probing with an oscilloscope, you can find where the power spike is emanating from.
Undoubtedly there is a lot to know and plan for when creating a PCB board that needs to connect to another board. But if you keep a few key suggestions in mind, the process can be less stressful and help see you through the process.
For your next PCB fabrication & assembly project, make sure to talk to the engineers at San Francisco Circuits to make your product concept a reality.