A popular song “Summer” by War during the late 1960s and 70s, the adolescence of rock and roll, proclaimed “Summertime is the best time of year any place.” I, for one, completely agree because sun and fun seem synonymous at times. However, too much sun can quickly change a fun time into a dangerous one. This is truer in PCB assembly than anywhere else. For example, too much heat applied to your board during the soldering process can damage components and weaken or deform your PCB.
PCB thermal design for operation is a well-known consideration for engineers and board designers. Ensuring that your board dissipates heat and components don’t overheat are crucial factors that must be solved during design, typically using heat sinks and cooling fans. Although often overlooked, thermal design factors can also affect PCB manufacturing and therefore warrant similar attention. Let’s examine some common manufacturing issues that should be considered during PCB thermal design for manufacturing as well as possible solutions for mitigation.
PCB Thermal Design for Manufacturing Issues
Two of the most important steps in PCB manufacturing are reflow and rework, which are performed during assembly. Jointly, these soldering steps ensure that your surface mount devices (SMDs) are securely mounted to the board. Once your components are placed at specific locations on the PCB, solder paste is applied and the joints are heated so that the solder easily flows to fill pads and form a secure connection. This process is known as solder reflow and performed in a reflow oven. During this step, the temperature variance across your PCB may be as high as 95°C. Rework involves correcting any errors that may have occurred during reflow. These errors may include misaligned components (components that may have repositioned, resulting in an insecure mounting), tombstoning (where one side of a component is unattached) or other unacceptable connections. This soldering process significantly raises board temperature.
Successful board manufacturing depends on the ability of board materials to withstand the temperatures applied during the soldering process. However, this ability is not simply a function of material properties but also depends on your implementation of PCB thermal design for manufacturing. Issues may arise in these areas:
- Thermal relief – The path for heat dissipation during PCB assembly.
- Solder parts – Depending upon the type of solder used, the temperature at the solder joints of your components varies.
- Thermal stress – Excessive heat or repeated heating of the board applies stress to the board.
Fortunately, certain PCB thermal design actions can be applied to mitigate these potential problem areas.
PCB Thermal Design for Manufacturing Solutions
Although the PCB thermal issues discussed above will not result in component destruction or board failures in the field, they must be addressed before your PCB can be manufactured. Your contract manufacturer (CM) must, therefore, correct any design issues before board assembly. This process will result in your boards getting built; however, you will most likely face manufacturing delays and additional costs. A better option is to consider the impact of thermal issues on your board’s manufacturing process and apply the following PCB thermal design for manufacturing solutions as part of your design for assembly (DFA):
|PCB Thermal Design Solutions for Manufacturing|
|Employ good component placement and spacing||Ensure power components are placed and spaced such that heat is distributed across the board rather than concentrated in one area. Observe spacing rules and component heat dissipation tips.|
|Place components on one side of the board only, if possible||This reduces the number of times the board must be heated, which applies less stress.|
|Consider coefficient of temperature expansion (CTE) when selecting board finish type|
Different finishes have different tolerances to temperature variation. Choosing a finish with a high CTE makes the board less susceptible to cracking at elevated temperatures.
|Make good use of thermal reliefs||Use an adequate number of vias directly under the component to dissipate the heat as fast as possible.|
|Use adequately sized thermal reliefs||Ensure thermal reliefs are sized to provide good connectivity with pads and planes.|
|Increase copper weights to aid thermal relief||Higher copper weights dissipate heat faster (start at 1oz and increase if necessary).|
|Choose board size to provide larger heat distribution area||Consider board area and thickness (larger areas can withstand higher temperatures).|
|Set layer number and sizes to help with high-temperature stackups||This is important for highly complex designs as microvias and stacked vias are subject to expansion.|
The PCB thermal design solutions above can be applied during, and as a part of, the PCB design process. The best way to incorporate these solutions is to partner with your CM early in the design process. This allows you to synchronize your PCB design intent with the manufacturing capabilities and processes that will be used to develop your boards.
By applying the above PCB thermal design solutions and including your CM in your board design, you can prevent increased temperatures from being applied to your PCBs and endangering the board manufacturing process.
|Tempo‘s Custom PCB Manufacturing Service|
Tempo Automation, the leading turnkey PCB prototype manufacturer in the industry, will partner with you from Day 1 of your design to ensure that you are aware of all necessary thermal issues that may impact the manufacturing of your PCBs.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on PCB thermal design or how to minimize thermal issues during your board’s manufacturing process, contact us.