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Connect the Dots
By Matt Stevenson
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Connect the Dots: Avoiding Five Common Pitfalls of Parts
Ill-fitting parts can frequently cause delays and cost overruns, and undermine PCB performance, durability, and overall quality of the board. These poor results can be avoided. Here are five methods designers can implement to avoid common, parts-related manufacturability pitfalls.
1. Pay close attention to pinhole size
It’s important to check component physical dimensions, consider dimension tolerances, and account for variation that can affect fit. Pins can be the wrong size or have the wrong spacing, and components can be much larger than their footprint or land pattern might indicate.
Alternate or replacement components can be on the wrong end of the tolerance range. It just takes a few mils before things don't fit and assembly starts to go wrong. Finding a good alternate part can be a challenge, which is why datasheets are invaluable.
In addition to watching part sizes, pay close attention to the minimum, nominal, and maximum material conditions for the original part.
2. Use the product datasheet when designing the land pattern
One of the most frustrating mismatches with alternate through-hole parts is when the land pattern matches, but the pin size is off.
If hole sizes are too tight, pins may not fit through the holes. Or, if pins do go into the holes, they may not solder well. Solder will need to flow up through the gap between the pin and the hole barrel. If there is not enough space to allow solder mass to flow through the hole, the circuit board will absorb heat from the molten solder and cause the solder to solidify part way up the hole. This “cold” solder joint can result in premature failure of your circuit.
When designing the land pattern, the pin size and tolerance range for components can be found in the product datasheet. Use that information to plan the proper hole size. Component holes should be sized correctly to allow between 12- and 16-mils diameter larger than the component pin at maximum material condition (MMC).
MMC is the condition where the hole is drilled at the low end of the tolerance range, and the pin measures at the high end of the tolerance range. Pin locations should be placed at nominal location, or the basic dimension shown on the datasheet.
3. Let the datasheet tell the real story
Third-party CAD libraries can contain millions of different parts, so discrepancies are inevitable. When the datasheet and the library part don't match, address the delta before submitting the design. Always check any library part for accuracy before using it the first time.
The datasheet for a part usually tells the real story—a long story, but one worth reading. Some datasheets can run up to 200 pages, but only a few lines provide the information needed to make the crucial decisions about sizing. It's important to read and comprehend the key parts of datasheets so problems in CAD don't lead to the wrong sizing and spacing on the PCB.
4. Pay attention to pinouts when using alternate vendor parts
Even if pin size and through-hole size are a confirmed match, and even if solder joints appear sound, a part can still not work as expected. Similar parts with the same footprint might look like they should act identically, but they won't always have the same pinout. Each transistor has a gate, drain, and source, but different manufacturers can vary in what goes where. For example, a Motorola part can differ from a Texas Instruments part.
When it comes to generics, there is even more potential for variance. The same basic component will come in multiple packages. Sometimes the variations are tossed into the back of a datasheet as an afterthought, but these can be critical. Similarly named packages can even come in different widths.
5. Be aware of mechanical fit.
It's not just the footprint and through-holes that are important; physical size of a component can keep parts from fitting into designated spaces. MMC body size should be the rule, so pay close attention to the tolerance range.
As parts get larger or are sourced from multiple vendors, footprint size may need to expand considerably to accommodate all of the dimension and tolerance variables. When combining multiple part body dimensions, always take the largest dimension or it may become more possible for parts to end up smashed together on the board.
Maintaining awareness of these key areas will help ensure parts-related manufacturability issues are avoided.
This column originally appeared in the June 2023 issue of Design007 Magazine.
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Connect the Dots: Controlled Impedance and Calculations for Microstrip Structures
Connect the Dots: Best Practices for Ensuring PCB Design Manufacturability
Connect the Dots: Choosing the Right CAD Tool
Connect the Dots: Best Practices for Prototyping
Connect the Dots: Best Drilling Practices for Better PCB Manufacturing
Connect the Dots: Playing the ISO ‘Game’ for Better Quality
Connect the Dots: What is an Annular Ring?