Perfect thermocouple placement is necessary for optimized profiles.
Growing numbers of printed circuit boards are now denser and smaller to comply with wireless and portable applications, bringing a new round of challenges to create the proper thermal profiles. Even the smallest boards come in a vast number of sizes, shapes and thicknesses, and each requires a unique thermal profile for a perfect assembly process.
No two boards are alike. Consequently, it is of utmost importance to develop a one-of-a-kind thermal profile for each board. Size matters: Smaller PCBs subjected to reflow withstand a lower duration peak temperature cycle than do larger ones. And some boards are more populated with components than others. With compact designs growing in popularity, most PCBs have both sides populated with ball grid arrays (BGA), µBGAs, chip-scale packages (CSP), µCSPs, quad-flat no-lead packages (QFN), dual-flat no-lead packages (DFN), and other devices.
Thermal profiles take into account the size of the PCB and the number of critical components to be installed, along with several other factors. Profiles for SnPb and Pb-free are different because Pb-free solder requires a higher peak temperature. Each board side requires a unique profile. The thermal profile for the bottom side is developed first. The reasoning is this: When the bottom side is profiled and undergoes reflow, less heat is applied to the topside of the board since it is not yet populated. When the topside is profiled and the bottom side is already profiled, more heat is required so that components on the topside are properly reflowed and to make certain that the desired soldering temperature is achieved for that side of the board.
Thermocouples in the right place. Small PCBs might be processed in panels. Because the boards are smaller, the boards are presented in array form so they can be run through the SMT assembly line more efficiently. The number of boards per panel can vary. [Ed.: For more on arrays, see “The PCB Array, and Why We Use It,” PCD&F, June 2012.] Creating a profile for a specific small board can be tricky because thermocouples need to be placed on the boards in a number of distinct and correct ways.
For larger boards, placing thermocouples at the right place isn’t such a problem because a baseline profile is needed for any size board. This profile is the startup profile for a given assembly. This profile might be defined based on the process engineer’s experience.
For boards in array or panel form, however, thermocouples are placed in the areas where critical components are located (Figure 1). These components are BGAs, QFNs, DFNs, CSPs or any other temperature-sensitive components that cannot withhold temperature above certain threshold limits, based on manufacturer’s specification defined in a datasheet. Figure 3 shows thermocouple (TC) placements on different boards. The TCs are placed on the PCB where a dummy component is placed before sending the board through the oven. Data are gathered and a profile generated after the board comes out of the oven. The profile is then evaluated and adjusted as needed.
Sometimes, certain components require special attention during reflow due to temperature requirements, as well as limitations for a particular device or component. Some parts cannot exceed a specific temperature range. In particular, µBGAs and µCSPs require lower temperatures compared to larger packages because of their lack of size. The number of balls on the µBGAs and µCSPs is fewer; therefore, less heat is necessary to establish a good profile.
As for thermocouple placement, if a particular PCB has, for example, two BGAs in the board and comes in a panel of four small boards, thermocouples need to be placed in those eight locations of those four boards. Optionally, thermocouples could be placed in strategic areas where thermal readings are critical. For the most part, these areas are where BGAs, CSPs, QFNs and other critical devices are located.
Thermocouples provide the actual temperature reading from the specific areas where they are placed on the PCB. These readings indicate if the reflow profile is optimized or in need of adjustments. Figure 2 shows the actual peak temperature of U1 at 241.51ºC. This profile is for a Pb-free assembly, and the manufacturer’s suggested peak temperature range is between 230ºC and 250ºC. With a little bit of a tweak of the temperature settings on Zones 2, 3, 6 and 8, the predicted peak temperature is now at 244.28ºC. Different contract manufacturers have different requirements when generating reflow profiles for each assembly. Experience and the solder paste manufacturer’s reflow profile guidelines play a big role.
Thermocouples on micro devices. Thermocouples are placed on such devices as µBGAs and µCSPs to achieve the optimum profile for that given board and to ensure proper wetting occurs. The goal is not to overheat these parts or the PCB, lest they be damaged from the excessive temperature. By getting the most accurate readings on the areas where the critical components are placed, avoid overheating and damaging them.
PCBs measuring 2" x 2" or less are particularly challenging when it comes to thermocouple placement. If that board has more than three areas requiring temperature readings for a correct profile, there is no space on that small board to place three or more thermocouples. That particular PCB has to undergo reflow at least twice to get an optimized profile.
This challenge becomes compounded if the customer doesn’t provide a solder sample. As a result, the process engineer has to use a good board to develop the profile. That particular board is run through reflow once, rather than running a solder sample board multiple times inside the reflow oven. Getting a solder sample from the customer is a key item when preparing a reflow profile on any given board.
It is particularly important to strategically place thermocouples on µBGAs and other smaller packaged devices to get accurate temperature readings (Figure 3). The temperature reading plays an important role for thermal profile development. These readings tell the process engineer if his or her profile needs adjustment or not. Temperature readings are compared to the solder paste manufacturer’s recommended application reading and reflow guidelines. The target is to get as close to the manufacturer’s reflow guidelines as possible, in order to have a 100% accurate board assembly.
By placing thermocouples on those components, data are collected to determine if certain areas are being overheated or whether some areas aren’t getting enough heat. Therefore, some adjustments are needed on the temperature settings. Applying too much heat causes component failure or causes flux to evaporate from the solder paste, making the assembly more prone to brittle joints that can create a latent defect in the field.
Also, if no temperature readings are taken on these critical components, the profile will not be optimized for a particular board. Various process issues may occur, resulting in unnecessary rework and project delays. For instance, a voiding – where a BGA is not 100% collapsed and a gap exists between the BGA and PCB – is one of the more common defects. The reason could be a lack of uniform localized heating on the BGA balls.
Another consideration when deciding where to place thermocouples on a particular panelized board is the component placement. If components are located on the edge or middle of a large conventional board, thermocouples are placed in those areas to obtain actual temperature readings and adjust accordingly. On a small, panelized board, the thermocouples are still placed at locations where critical components are placed. Subsequently, thermocouple placement is evenly distributed on the panelized PCBs to be manufactured.
If placement is incorrect, an inaccurate thermal profile results. Temperature readings will be imprecise, and the reflow profile will need to be revisited. Process issues, such as voids, shorts or cold solder, will occur if thermocouples are randomly placed on a board without considering proper placement. If erroneously placed, the benefits of efficient temperature readings are lost, and the board could be damaged during reflow.
With excessive heat come possible delamination issues on the PCBs where the board reliability is compromised. For small boards, in particular, delaminating is a problem that can arise by not creating the right thermal profile. If temperature settings are too high, “bubbling” on the board can occur (Figure 4). Bubbling is a form of delamination that causes the top layer of the PCB to rise up like a bubble and creates reliability issues. Various areas on the board can delaminate, which decreases the integrity of the PCB. Damaging costly components on the board is another problem. Tombstoning is yet another issue that can arise when too much heat is applied to the board. This is where one side of the component is pulled, causing the other end to stand up.
It’s worth noting there are sensitive components that cannot handle high temperatures and may fail during testing. They are regarded as sensitive based on their susceptibility to heat. Some devices like a µBGA, for example, cannot be exposed to temperature exceeding 230°C. Thermocouples are placed on the areas where these components are located on the board. Once data are collected, the profile is evaluated and temperature settings adjusted accordingly until an optimum reflow profile is achieved.
Simon Ilustre is a process engineer at NexLogic Technologies (nexlogic.com); firstname.lastname@example.org.