Remember these three experiences, in addition to using heat sinks, you can also solve chip cooling problems

Latest update time：2019-02-13

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In some chip applications, such as voltage regulators, high heat generation is inevitable when the device is working. The exposed pad package is a thermally enhanced standard size IC package. Its advantage is that in addition to using bulky heat sinks, the heat dissipation function can also be achieved from the standard PCB layout and soldering process.

The exposed pad is usually exposed on the bottom of the package. This provides a very low thermal resistance ( θ _JC ) path between the die and the chip. Figure 1 below shows one of the exposed pad designs.

Figure 1, schematic diagram of the internal structure of the exposed pad chip

According to experience, some cases of defective parts claiming "parts overheating" were ultimately found not to be circuit design issues, but rather poor heat dissipation caused by poor soldering of exposed pads.

Here are some tips to help engineers take care of the chip's heat dissipation at the PCB layout and soldering levels.

Tip 1 :

Note whether the manufacturer recommends using the "Solder Mask Defined (SMD)" or "Non-Solder Mask Defined (NSMD)" soldering method.

The two defined welding methods have their own characteristics, as shown in Figure 2 below:

Figure 2: Comparison of “Non-Solder Mask Definition” and “Solder Mask Definition” welding methods (Image source: http://www.onsemi.com/pub/Collateral/AND9107-D.PDF)

Both methods have their own advantages: Under "Solder Mask definition", the solder mask will cover part of the pads to reduce the risk of short circuit with the adjacent pads after the solder paste is squeezed out after the parts are in contact; in "Non-Solder Mask definition", the opening is larger, the solder paste fully covers the PCB pads and the stress concentration is lower. According to sharing with engineers, there are certain problems when using "Non-Solder Mask definition", because they are worried that too much solder paste will cause a short circuit without a Solder Mask, which will lead to insufficient solder paste and poor heat dissipation. Using a soldering template can help engineers put solder paste on the PCB pad more effectively and accurately. There are also soldering templates with a variety of package sizes available on the market.

Tip 2:

The size of the designed pad must comply with the requirements in the data sheet

Basically, engineers need to understand that the PCB pad needs to meet the size requirements in the data sheet. But at the same time, if the PCB board layout allows, designing a larger PCB pad surface area can also help increase the chip's heat dissipation performance. Figure 3 is one example where the manufacturer recommends increasing the copper area to enhance the heat dissipation benefit of the exposed pad.

Figure 3. Example of copper area larger than exposed pad area (Image source: http://www.ti.com/lit/an/sloa120/sloa120.pdf )

Tip 3:

Make good use of thermal vias

Adding thermal vias from the pad area of the exposed pad to the other side of the PCB can effectively dissipate heat. The thermal via application can be designed inside the PCB thermal pad or outside the pad (that is, on the copper surface without solder paste). In the example in Figure 4, the manufacturer recommends that the thermal via be arranged outside the pad.

Figure 4, an example of the manufacturer recommending that thermal vias be placed outside the pad ( Source: http://www.ti.com/lit/an/sloa120/sloa120.pdf)

Warm Tips

If heat dissipation vias are used in pads, the soldering method should also be coordinated accordingly to prevent solder paste from accidentally flowing into the holes and blocking ventilation, which will not achieve the heat dissipation effect. Engineers can consider peelable or washable solder masks to protect the non-zero soldering area, including "exposed copper area" and "heat dissipation vias outside pads", to prevent vias from being blocked or overheated during wave soldering or PCB coating processes after assembly.

Conclusion

In addition to the external heat sink, the chip design with exposed pad package can provide engineers with an additional heat dissipation channel and increase the flexibility of product design. However, it is necessary to use appropriate soldering techniques, otherwise the exposed pad cannot be used to achieve the desired heat dissipation effect.

Here are some practical lessons learned:

1. Pay attention to whether the manufacturer recommends using the "Solder Mask Definition" or "Non-Solder Mask Definition" welding method. When using "Non-Solder Mask Definition", engineers can consider using a welding template to apply solder paste more effectively and accurately;

2. If the PCB layout allows, the PCB pad can be larger than the actual area. If the PCB layout space is limited, remember to at least meet the pad requirements in the data sheet;

3. Heat dissipation vias can be used inside or outside the pad. When using heat dissipation vias inside the pad, engineers can consider using peelable or washable solder masks to protect the "exposed copper area" and "heat dissipation vias outside the pad" during wave soldering or PCB coating process after assembly to prevent the vias from being blocked or overheated.