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Thermal Profiling Small and Large Devices Packaging

It used to be that device packaging was fairly uniform and consistent and about the same size. As far as doing the thermal profiling for these traditional packages, it was pretty much a walk in the park. However, more recently, an increasing number of boards are populated with micro BGAs, micro CSPs, and micro QFNs, as well as the larger BGAs, CSPs, and QFNs, plus, there is the continually smaller packaging like 0201s for passive devices.

Creating a profile for a particular board like this poses perplexing issues since the larger packages require more heat compared to the smaller ones. Plus, the process engineer is faced with special considerations when it comes to package-on-package (PoP) populated boards. Consequently, the process engineer on the assembly floor must perform a balancing act by using a temperature setting that’s acceptable to both larger and smaller packages, while juggling trade-offs and special considerations.

Fig.1-Voids X-Ray ImageThere are a number of aspects to know about having the right thermal profile to comply with both small and large device packages on your advanced PCBs. A lot of what you need to know can be found in our article, “The Basics of Thermal Profiling for Sensitive Components” published in Circuits Assembly Magazine.”

Meanwhile, here are a few tips and hints that’ll give you a basic understanding of what’s important to know.

  • The most obvious rule in getting the right thermal profile is not to overheat smaller packages to avoid solder bridging. On the other hand, more heat must be applied to larger BGA packages for solid solder joints and avoid creating open solder joints.
  • Before performing profile adjustments, it’s important that x-ray inspection be performed to detect voids and other defects, as shown in Fig. 1 above.
  • So make sure your EMS Provider has latest x-ray equipment possible. It covers about 90 percent of the issues that can be detected as a PCB goes through reflow. X-ray provides a roadmap for adjustment types that may be needed.
  • Once a profile is done, the process engineer makes the necessary adjustments on a particular PCB by adjusting pre-soak, soaking, and reflow temperatures. Plus, the right balance is struck among soak time, peak temperature, and reflow time.
  • Part of soak time review is to investigate differences in solder paste characteristics.
  • Those paste characteristics can drive differences for each profile for both smaller and larger packages.

Today, thermal profiling for advanced PCBs populated with a variety of package sizes calls for extensive experience. Without the critical profiling experience, a right balance of all the critical profile elements may not be possible.

By not hitting the right balance, problems are encountered during testing or while performing QC of these boards. Some problems can be solder balls and delamination on the PCB. Solder balls usually occur when the board is going through an excessive heating rate as the board goes through the reflow oven. The reflow cycle is too long and solder balls form throughout the board. Also, with excessive heat comes a possible delamination issue on the PCBs where the reliability of the board is compromised.