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What is the relationship between the IGBT package and solder paste layer void

in determining the safe operation of the entire device. Since the IGBT adopts stacked packaging technology, this technology not only improves the packaging density, but also shortens the interconnection length of the wires between chips, thereby improving the operating speed of the device. But because of this structure, the reliability of IGBT has been seriously challenged. What is the relationship between the IGBT package and solder paste layer void? In this article, you can find the answer.

Table of Contents

What is the manufacturing process of IGBT module?

Here is a brief demonstration of the manufacturing process of the IGBT module.

Screen printing➔Automatic mouting/placement➔Vacuum reflow soldering➔Ultrasonic cleaning➔Defect detection (X-ray)➔Automatic wire bonding➔Laser marking➔Shell plastic sealing➔Housing potting and curing➔Terminal forming➔Functional testing

IGBT packaging technology

As an important core device of power electronics, IGBT’s reliability is the most important factor in determining the safe operation of the entire device. Since the IGBT adopts stacked packaging technology, this technology not only improves the packaging density, but also shortens the interconnection length of the wires between chips, thereby improving the operating speed of the device.

But because of this structure, the reliability of IGBT has been seriously challenged. Since the IGBT is mainly used to switch the current, it will generate a large power loss, so heat dissipation is an important factor affecting its reliability.

What is the relationship between the IGBT package and solder paste layer void?

Solder layer voids are an important factor affecting the reliability of IGBTs

Solder layer voids are an important factor affecting the reliability of IGBTs
Solder layer voids are an important factor affecting the reliability of IGBTs

The failure of the IGBT module package level mainly occurs at the connection of the bonding wire, the chip welding place and the substrate welding place. In particular, the two solder joints are the main heat transfer channels of the IGBT, and the welding quality of the solder joints is the most important factor affecting its reliability.

Studies have shown that the voids in the solder layer will affect the thermal cycle of temperature and reduce the heat dissipation performance of the device, which will also promote the rise of temperature, and affect the thermal cycle during the working process. Then, it causes the local temperature to be too high, and accelerates the damage of the module.

A survey shows that for every 10°C rise in operating temperature, the failure rate caused by temperature doubles. Moreover, there is a hysteresis phenomenon between the stress and the strain. During the continuous temperature cycle, the shape of the material changes in real-time, which increases the thermal fatigue of the solder layer. Therefore, for IGBT packages, the most important thing is to reduce voids in the solder layer.

The reason why the IGBT soldering layer is prone to voids

The generation of voids in the solder layer is mainly caused by the volatiles in the solder material remaining in the solder layer, and the IGBT chips are usually relatively large, with a length of 10mm-20mm, and the size of the DBC is usually 20mm-40mm. The large welding area which makes it very difficult for the volatiles in the welding materials to volatilize.

Therefore, the void of the IGBT solder layer has become a problem that engineers try to solve. For the high reliability requirements of IGBTs, the void rate must be an important control factor in the packaging process. Generally, IGBTs used in small household appliances and general electrical equipment require a void rate of <5%. For rail transit, aerospace and other fields, the void rate requirements are even more stringent, and even need to be below 0.1%.

Relationship between IGBT package form and void

1) Form of welding material
1. Solder tab: It is the most widely used solder in the field of IGBT.
Advantages: fewer organic components are used, and the void is low.

Disadvantages:

  • Inconvenient to operate;
  • Each product requires a specific size of solder tab.
Solder tab - It is the most widely used solder in the field of IGBT.
Solder tab – It is the most widely used solder in the field of IGBT.

2. Solder paste: It is a measure that has gradually emerged in the field of IGBT packaging in recent years.
Advantages: easy operation, low cost, suitable for welding of products of various specifications.

Disadvantages: 

  • Volatilization of a large amount of organic components can easily cause voids;
  • Residues must be cleaned.

Using solder paste soldering, in order to promote the volatilization of volatiles in the solder paste, the solder paste is usually printed into several rectangular arrays, especially the solder paste printed between the substrate and DBC. The channels between the solder paste will facilitate the evaporation of volatiles discharge. However, this requires a good match between the volatility and wettability of the solder paste and the process, otherwise it is easy to generate voids in the trench.

Also read: Top 10 Best solder paste manufacturers in the World 

Solder paste - It is a measure that has gradually emerged in the field of IGBT packaging in recent years.
Solder paste – It is a measure that has gradually emerged in the field of IGBT packaging in recent years.

2) Welding process

1. One-time packaging: DBC/substrate and DBC/chip are placed with solder at the same time, and the soldering is realized at the same time through one furnace.

Advantages: high efficiency;
Disadvantage: The two layers of solder melt at the same time, and the ceramic chip floats between the upper and lower layers of molten solder, which increases the difficulty of void discharge.

2.Ladder packaging: first solder the DBC/substrate with high-melting-point solder, and then use low-melting-point solder for secondary soldering between the DBC/chip (during the secondary soldering, the solder in the first soldering does not melt).

Advantages: Avoid DBC floating between molten solder, low void rate;
Disadvantages: low production efficiency.

3) Selection of welding furnace

IGBT products with high requirements must be welded in a vacuum furnace. Besides, the vacuum negative pressure is conducive to the discharge of volatiles in the solder paste. But the vacuum degree and vacuum timing should match the solder paste, otherwise, it is not easy to achieve the desired effect.

In addition to the welding process, a low-viscosity two-component high-transparency silicone gel can be cured at room temperature and has the characteristics of moisture resistance, shock resistance, temperature resistance, and self-healing. It is mainly for sealing and potting of IGBT modules and other precision electronic components.

●Soft and elastic colloid after curing, jelly-like gel, good adhesion and sealing performance to many substrates;
●High and low temperature resistance (-60°C-240°C), and has excellent electrical insulation properties;
●Highly transparent, easy to repair, and can heal automatically after the colloid is broken.

Conclusion

After the above illustration, you can know something about the relationship between the IGBT packaging and the solder paste layer void. As you can see, the void issue of solder paste is essential for IGBT packaging. Once we cannot better handle with the void issue, it might greatly impact the IGBT packaging performance. IBE, since we have established in 2005, has accumulated more than 17 years experience on PCB and PCB assembly, Our one-stop services including design, manufacturing, assembly, testing and delivery promise to offer a satisfactory service for you.

FAQ

GBT (Insulated Gate Bipolar Transistor), an insulated gate bipolar transistor, is a composite fully-controlled voltage-driven power semiconductor device composed of BJT (bipolar transistor) and MOS (insulated gate field effect transistor).

The IGBT has the advantages of low driving power and low saturation voltage. It is very suitable for the conversion system with a DC voltage of 600V and above, such as AC motors, switching power supplies, lighting circuits, etc.

The generation of voids in the solder layer is mainly caused by the volatiles in the solder material remaining in the solder layer, and the IGBT chips are usually relatively large, with a length of 10mm-20mm, and the size of the DBC is usually 20mm-40mm. 

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