Reflow oven for PCB assembly
Back when I was first learning about electronics and circuit design, I often heard older engineers say that the best engineers are also good technicians. In other words, knowing the performance objectives for your board and creating a circuit layout to meet them was not enough. Additionally, you had to understand how your board was built and why, as well. Needless to say, I have found this to be true as there are a myriad of problems that can arise during manufacturing if they are not taken into consideration during design.
One of the greatest problems or threats to PCBs during manufacturing and operation is heat. This is a bit ironic as heat is an integral part of the manufacturing process, especially during assembly when your components are being soldered to the board. In fact, for surface mount technologies, a reflow oven is used and the board is subjected to high temperatures for a significant period of time.
Accounting for the temperatures that your board will be subjected to is an aspect of good PCB design that requires an understanding of how heat is distributed and the rate at which your board’s temperature changes or thermal diffusivity. This parameter depends primarily on the material of your board and its thickness and is most often determined by applying laser flash analysis (LFA). Let’s see how LSA for manufacturing is done for PCBs, after first clearly defining thermal diffusivity, which is defined to determine. Then we will be prepared to put forth a method for incorporating this thermal management into the design process.
Understanding the Thermal Diffusivity of PCBAs
Thermal management is too often not a major consideration unless dealing with high power PCBs, where removing heat or thermal dissipation is critical. This is indeed very important; however, it is only one aspect of good thermal management. To gain a better, more comprehensive perspective it is probably informative to define a few terms.
PCB Thermal Management terms:
🔥 Thermal Dissipation
Thermal dissipation is the process of removing excess heat from a circuit board. There are a number of techniques for achieving this; including the use of heat sinks for high power components and strategically located thermal vias. Dissipation is the primary thermal concern during operation.
🔥 Thermal Distribution
For PCB assembly, elevating the temperature of your board is a necessity. If you use unleaded solder, these temperatures may be approximately 250°C (482°F). In contrast to PCB operation where removing excess heat as fast as possible is the goal, for manufacturing ensuring even distribution of heat is the objective to attain good quality solder joints.
🔥 Thermal Diffusivity
Unless artificially forced or pumped, heat always travels from the higher temperature to the lower. Thermal diffusivity is the rate at which this transfer occurs for the board.
From these definitions, we can see that thermal diffusivity is an important parameter for dissipation, during operation, and distribution, during manufacturing. Now, let’s see how this parameter is determined for manufacturing.
Laser Flash Analysis for PCB Manufacturing?
Laser Flash Analysis (LFA) apparatus
In the figure above, an example of the apparatus used to perform LFA is shown. The sample in the diagram represents the board material. This test is typically performed in an enclosed machine and the results are analyzed by software. The parameter of concern is the thermal diffusivity, which can be determined using the following equation:
(1) 𝛼 = k / (⍴cp)
where 𝛼 is the thermal diffusivity,
k is the thermal conductivity,
⍴ is the material density and
cp is the specific heat capacity.
All of the variables of Eq. (1) are temperature dependent. This allows for the determination of changes as the temperature is increased. With this data in hand, accurate thermal analysis and therefore, effective thermal management can be achieved for your design.
Designing for Laser Flash Analysis in Manufacturing of PCBAs
The results from an LSA can be used to ensure that your chosen board materials and your PCB layout are optimized for your manufacturing process. Additionally, a comprehensive thermal management strategy that includes electrical-thermal cosimulation can be done using Cadence’s Celsius Thermal Solver.
Cadence Celsius Thermal Solver
As shown above, Celsius allows you to evaluate your board’s heat distribution which can be also be coupled with electrical analysis to plan your dissipation strategy, as well.
Cadence’s PCB Design and Analysis platform is an advanced comprehensive system design tool that provides you with integrated design, SI/PI analysis and thermal management. With Allegro, you can optimize your design for manufacturing and get your boards built faster and to the highest quality.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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