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Zulki’s PCB Nuggets: Putting the Heat on for Thermal Profiling

A unique thermal profile is designed for each PCB job undergoing conventional SMT assembly, as virtually every PCB assembly professional knows. But what about a PCB assembly project involving both conventional rigid board and an extraordinarily small rigid or rigid-flex circuit undergoing microelectronics assembly? In short, that’s called PCB hybrid assembly, which requires two separate, unique, and distinctly different thermal profiles.

Here’s where extensive experience dealing with these unique thermal profiles comes into play. Combining SMT and microelectronics assembly with separate thermal profiles for each is of particular importance to OEMs designing advanced wearable, IoT, and other portable products. It’s important for these and other OEMs to get a handle on the number of factors going into developing the right thermal profiles for SMT and microelectronics assembly. Failing or falling short on any of the steps in creating the right thermal profile for either or both PCB assembly processes can prove costly in dollars and time to revenue to an OEM.

Let’s first do a quick review of the various aspects of a thermal profile for SMT assembly. Thermal profiling is a major part of the SMT manufacturing process and is basically a recipe for preparing an SMT component-populated PCB for the IR reflow oven.

Three steps are involved. First, solder paste is applied to the PCB by screen printing it through a stencil, otherwise known as stencil printing. Next, a pick-and-place machine takes components from a reel, tube, or tray and automatically places them on the board. Third, the PCB goes through either an IR reflow oven or through vapor phase.

When creating a thermal profile, a solder sample board is used and is designed based on the solder paste specifications of a particular manufacturer. Thermocouples are attached to different parts of the PCB. The idea here is to record temperatures at different spots to create a custom thermal profile, which includes three different area segments; those are the soak period, peak temperature, and cool-off period. As shown in Figure 1, zones 3–5 are soak periods, zones 6– 8 are peak temperatures, and zones 9–10 are cool-off periods.

Figure 1: A unique thermal profile graph, showing zones 1–10 at the bottom.

A thermal profile also depends on a few things to be in place. It must factor in the number of copper layers on the board, especially power and ground. It must also take into account component density at the top and bottom layer of the board. Plus, it has to account for whether it’s a leaded or lead-free profile.

Solder paste melting temperature for leaded and lead-free are vastly different, and that must be taken into account. A unique thermal profile is thus created for each assembly by keeping into account factors like the number of components and their types, the number of layers, and the board thickness.

Thermal Profiles for Microelectronics

There are several factors that must be considered in the development of a thermal profile for PCB microelectronics assembly. First, there are the thermal profile requirements associated with the particular die being used. Then, there is the die’s moisture sensitivity level that must be accounted for to assure it is preserved at the optimal level without moisture seeping into the package. Sometimes, these dies need to be baked at certain temperature profiles for a specific period of time.

You also have to look at the substrate’s thermal profile and glass transition temperature (Tg) requirements to determine if PCB material as FR-4, Rogers, or Megtron 6 are used. Now, we have to check out the optimal epoxy to be used in a particular project. The question is whether or not it is going to be conductive or non-conductive. The right selection is made based on the nature of the project and type of die to be used.

Lastly, all Tg and thermal profiles are important because these profiles, and Tg levels need to be compatible as far as operating temperature ranges and heat transfer levels so that die attach and wire bonding are properly performed during microelectronics assembly.

Zulki Khan is the president and founder of NexLogic Technologies Inc.