Surface-Mount Component Packages - Which is best?

Surface-mount PCB components are available in multiple industry standardized packages. These packages are defined by the Institute for Interconnecting and Packaging Electronic Circuits, more widely known in the industry as IPC (and formerly known as the Institute of Printed Circuits).

Each standardized package has a unique footprint, or land pattern, and follows nomenclature that indicates an acronym for the family name followed by the number of pins on the device. For example, the surface-mount package “LQFP-44” indicates a Low-profile Quad Flat Package with 44 pins. Some more common IPC packages for ICs are SOT23-5, TSSOP-16, SOIC-8, and QFN-8. These are just a few examples. The list goes on and on, and continues to grow as new sizes and shapes of chip packaging are created to accommodate newer technologies.

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Once a team has completed the specification for a circuit board, the design cycle begins. The engineers are selecting new parts to achieve the required design functionality, and are fervently creating them for placement into the schematic and PCB. Inevitably, someone on the team will open a datasheet to create a new part and discover that the same part is available in not one, but several different IPC packages.

Consider this I2C bus voltage-level translator — Texas Instruments model PCA9306. This part is available in four different 8-pin packages: SSOP-8, VSSOP-8, X2SON-8, and DSBGA-8. Which part package is the best?

At first glance, the smallest package seems optimal, as the PCB is already cluttered with parts, and using a smaller part will leave some precious space on the PCB for routing traces. But a deeper look may reveal some other perspectives.

The best choice depends on how you perceive the trade-offs of each IPC package in the context of your organization. Viewing the possible outcomes through different lenses can offer valuable insight when selecting which package is best for the circuit board.

Here are some perspectives to consider:

• Engineering considerations

  • One package may already exist in the company’s component library. (Faster design results with reduced design risk)

  • If the design is only a prototype, can the engineer switch out the part in the lab, or will another batch of boards need to be assembled to test an alternate?

  • One package may offer better thermal qualities than another.

  • Does the height of the package impact your ability to apply a heatsink to another nearby component?

• Procurement considerations

  • Is the pin pitch so tight that the clearance between the copper of each pad exceeds the capabilities of your PCB manufacturer? A tighter pin pitch (and pad-to-pad clearance) may increase the cost and delivery time of your PCBs. (While that might be acceptable for prototypes, are you willing to accept the increase on larger production runs?)

  • Is one package more readily available through distribution than another? (One of my favorite websites for checking part availability across various electronic component distributors is Octopart.com.)

  • How does the unit price compare between the different package options?

  • In today’s tumultuous procurement space, a part that’s widely available today may be out of stock tomorrow. Does an alternate manufacturer offer a compatible part in the package you are considering? Always be prepared for the unanticipated fire.

• Manufacturing considerations

  • Often, the smallest package is a newer technology with uniquely shaped pads which are very close together. This may present soldering challenges for the CM who is assembling your boards, resulting in unexpected shorts underneath the part.

  • Will assemblers need to touch-up the part after it’s placed and soldered by machine?

    • A leaded part can be reworked or installed from the bench, but a BGA (ball grid array) is best installed by a pick-and-place machine. Similarly, hand soldering a part with a center pad may present difficulties if you don’t have the right tools for the job.

    • A smaller part may allow more space for a hand solder gun, while a larger part may result in the solder gun damaging nearby components. (Watch out, ceramic caps!)

  • If working with a polarized part, does the package have a keying pin that ensures it can only be installed at one orientation angle? Or can the part be unintentionally rotated, while still being properly soldered to the PCB? Diodes are often available in two pin packages, which can result in backwards installation if care is not taken to communicate proper orientation to the CM. However, diodes in SOT23-3 packages have a third pin, ensuring that the part can only be installed in the proper orientation.

  • The cost of a part that cannot be reworked is not limited to the single part. You’ve also lost the PCB, all other parts soldered to the PCB, and the time spent building it on the line.

• Marketing considerations

  • Is the final product photogenic? Does it look like it was designed in the 1980’s, or does it utilize modern technologies?

When selecting the IPC package for a surface-mount component, the smallest package is not always the best choice for a design. Often, a deeper understanding of an organization can lead to a part package that is more optimal.

Our staff at the Engineering Design Group has a history of working closely with the different teams across an organization. But we understand that every company is unique. Whether you seek to shave a few seconds off your product cycle time, reduce BOM cost to increase profit margins, or to show your customers that you are on the leading edge of an industry — We are here to help.