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Checking Out Thermal Profiles For Small PCBs

You may be one of those OEMs that mostly deals in large, traditional size PCBs and hardly ever develops a small sized product or systems. But get ready for them. Based on technology trends and market demands, smaller PCBs are increasingly being developed spurred on by the escalating demands from mil/aero, industrial, computer, communications, and medical electronics markets.

One key point to know is creating the right thermal profile for these small PCBs during assembly is of utmost importance. As we’ve said before, no two thermal profiles are created equal. It’s important to create a unique profile for each board because no two boards are alike. It’s especially critical due to their size to have a one of a kind thermal profile for smaller boards since they can only withstand a lower duration peak temperature cycle than the larger PCBs when subjected to reflow.

Click on this link to check out our article in Circuits Assembly Magazine to get more details on the importance of the right thermal profile for small PCBs.

Meanwhile, here are a few tips and hints to help you get a good handle on thermal profiles and small PCBs.

  • Profiles for lead-free PCBs require higher peak temperature than profiles for tin-lead PCBs.
  • Each board side requires a unique profile on two-sided PCBs.
  • Thermocouples must be placed on small PCBs in a number of distinct and correct ways.
  • Thermocouples are placed in areas where critical BGA, QFN, DFN, CSP or any other temperature sensitive components are located.
  • Small PCBs like 2-x 2-inches or less shown in the image below must undergo reflow at least twice to get an optimized profile.
  • It is particularly important to strategically place thermocouples on micro BGAs and other small packaged devices to obtain accurate temperature readings.

As you move into small PCBs for your products or even if youíre there already, it might be a good idea to get together with your contract manufacturer or EMS Provider to get a feel of how theyíre doing their thermal profiles. Hereís why. An inexperienced process engineer can inadvertently create certain problems unless key thermocouple placement steps are taken.

In effect, an inaccurate thermal profile results if placement is incorrect. Temperature readings will be imprecise, and the reflow profile will need to be revisited. Process issues, like voids, shorts, or cold solders will occur if thermocouples are randomly placed on a board without considering proper placement. If they are erroneously placed on a board, the benefits of efficient temperature readings are lost, thus introducing probable damage to a board during reflow.