Research progress in improving package reliability of power semiconductor devices

The topic of today's report is power module package connection reliability improvement research.My name is mei yunhui, from tianjin university.Today we are going to share two parts.The first is an increase in resistance to temperature shock aging.The second is the ability to resist power cycle aging.Why share these two parts? Because for power module, these two points are important indicators to evaluate the anti-aging ability of power module.Therefore, for power devices, the development of third-generation semiconductor devices and the emergence of high-performance devices represented by silicon carbide have led to the continuous improvement of power density and application frequency, which will bring thermal impact and further aggravate the aging rate of devices.Therefore, we need to have a vigorous discussion in this regard.

First, the development of new energy automobile industry, including electric vehicles, rail transit.New energy vehicles include not only electric cars, but also hydrogen-fueled cars.More and more new forms of energy, such as aerospace and wind power generation, are being applied to traditional industries.All of these new forms of energy will require power electronics technology to convert other energy into electricity, or to convert electricity into other forms.Whether the power module can work reliably and stably directly affects the reliable operation of the whole power conversion device.The reliability of power modules must be guaranteed and improved.

The first key question, let's look at this picture.This is a cross section of a typical IGBT module, and you can see that the IGBTmodule and diode chip include several connections, one is the connection between the chip and the substrate, the supplementary connection between the substrate and the baseplate, to establish an effective cooling channel.In the two connections, the most discussed is the first-layer chip connection, which is crucial for the chip's heat dissipation.But the connection between the substrate and baseplate is equally important.Because the connection between large area substrate and baseplate is more likely to fail. Why? Because of reflow welding.The process of reflow welding is realized by melting solder joints. Melt of solder joints, for example, connect the chip to the substrate through a melting point reflux process, which melts it and solidifiesit to form a connection.When we connect substrate to the baseplate again, it is necessary to reduce the reflow welding temperature to prevent the first solder joint connection between the chip and the substrate from solder remelting.The melt of these solders will cause the solder to become more brittle and prone to failure, which is not allowed, as well as lead to chip displacement and soon.Therefore, a solder with a lower melting point is usually selected. For example, Sn,Ag3.0 and Cu0.5 are a relatively common solder.

At the same time, the melting point of the solders connected with the substrate is not only low, but also large. Therefore, under the working conditions of long-term high temperature and coupled vibration, For example, when working incomplex conditions like cars,high-temperature fatigue creep and failure are easy to occur, resulting in the failure of the large-area connection between the substrate and the baseplate.

We list some typical IGBT modules that show substrate and baseplate packaging failures.As you can see, there are two DBC boards in the form of half a bar pasted on the bottom board of such a large area, and there are six DBC boards pasted on the form of six units pasted on the whole bridge.From the above, it can be found that with the aging time, the large area of interconnect between the substrate and the baseplate rapidly fails.We can see that the layered image continues to expand from the edge to the center, which can easily lead to a rapid increase in the thermal resistance of the package.This thermal resistance will greatly affect the overall heat dissipation degree of the module, thereby further accelerate the aging and failure of the module, and finally lead to the layered fracture of the solder, seriously restrict the use of the module in high temperature application, complex working conditions, vibration and other fields.

We think there are many ways to improve it, such as using better lead-free solder, etc., but the improvement is still limited.We are based on a technology we developed, which is the technology of nano silver solder paste, because the characteristic of silver solder paste is low temperature forming (different from solder joint) and high temperature using.Why use at high temperature? because it is formed at low temperature not by melting the silver, but by sintering it in the solidphase.Therefore, after forming a connection in this form, the melting point forit to melt is the melting point of silver, which is 960℃, making the choice of secondary solder joint will be greater.Why? Because the solder with the original melting point of more than 300 degrees was selected for the first welding, which become the solder with melting point of more than 900 degrees was selected, so the second welding can choose higher temperature solder, can greatly improve its temperature resistance performance, improve its high temperature reliability. In addition, some direct choice of low-temperature sintered nano silver solder paste as a secondary welding is also possible, and can be formed in one time, which has been reported in foreign countries.However, its cost may be a better plan for the automotive sector, but for other plans with more stringent cost control, we hope to come next, high temperature and low cost lead-free solder, high temperature solder spot, solder with lead, etc., so as to reduce its cost under the premise of ensuring high temperature and reliability.

Therefore, in order to verify the feasibility of this idea, we conducted aging experiments in the laboratory with modules prepared in batches. Including ABCD four types of modules, the module A is one of the traditional business module, Connect with high temperature solder once, connect with low temperature solder twice, The chip connections of B, C and D are all made of sintered nano silver. Basically, low temperature solder, high temperature lead-free solder and high temperature lead solder are selected for the secondary welding connection to compare and verify it.The main purpose of BCD and A is to verify an effect of sintered silver solder on life improvement. The comparison of BCD to BCD is to verify the life of different secondary solder.

The first step is to prepare different IGBT modules, characterize and analyze its static and dynamic electrical and thermal properties, and finally explore its thermal and aging properties after high and low temperature impact.

The process of module packaging can be directly completed in our laboratory, because our laboratory has a relatively complete research production line. You can see a  process which is preparation process, the 1200V/50A sample module was finally formed.

This isa routine static i-v curve test of different ABCD modules at low temperature 25℃ and high temperature 125℃.We can see that its static parameters have avery high degree of coincidence.This also illustrates two points. First, compared with the commercial scheme of module A, the performance of module BCD does not degradedue to packaging.Secondly, the modules packaging in the scientific research process line will not cause degradation of chip performance due to technological problems, showing a good static output, which indicates that the difference of substrate connection materials has no effect on static electrical characteristics.

The dynamic switching performance was tested in a double pulse test equipment.The bus voltage is 600V and the collector current is 50A, such a test condition.After testing, the result is consistent with the static performance.Dynamic parameters are also very consistent, high coincidence.It also confirms the two points mentioned just now. First, the use of different substrate connection materials does not lead to the degradation of chip performance, which is feasible.Second, our own research process line process capability has not caused the performance of the chip itself degradation.The whole packaging process does not lead to the degradation of chip performance in advance, which indicates that the overall packaging scheme, and packaging process are feasible.

Next wetested the thermal resistance performance of the module.This is a thermal resistance test equipment developed by us, which can test IGBT, also can test different thermal resistance of silicon carbide devices, and also can test the finished module, also can test the semi-finished packaging module.We developed this device to reduce the cost of the entire test, which is very cheap.At the same time, we used gate voltage as the temperature sensitive value, because take the gate voltage as the sensitive value, the K coefficient value of theentire test voltage is nearly -10℃/W, the accuracy will be improved by fourtimes than the traditional pressure measurement value using VCE, because the corresponding K coefficient of VCE is only -2.5℃/W.

Then we first compared the heat tests of each module.Module A is commercial, directly purchased, IGBT module used by the manufacturer.Its steady state thermal resistance was 0.407℃/W, and the BCD modules using sintered silverreduced to 0.337℃/W, and the thermal resistance decreased by17.2%,and the thermal resistance performance of BCD module varies greatly, that is to say,mainly due to the use of sintered silver as the chip connection material in the chip layer, its overall thermal resistance decreases by 17.2%.In addition, itstransient thermal resistance also varies significantly. As you can see, at the beginning of the curve, the overall value of transient thermal resistance is smaller than that of commercial module A.Because the thermal conductivity of sintered silver is greater than 240W/(m·K).In fact, today's sintered silver has a higher thermal conductivity than 240, depending on the process of sintering silver.At present, the method of low-temperature sintering nano-silver without pressure is adopted in this research scheme.Here we have a report on our previous work, you can read it again if you are interested.

Next,we will share the thermal shock performance, which is often referred to as highand low temperature shock aging performance.This is an important test standardand test method for evaluating the reliability of secondary welded joints.We chose the test condition of low temperature -40℃ and high temperature 150℃, which is also a test condition based on the test capability of our experimental equipment.The conversion time of high andlow temperature is less than 5 minutes, and the moisturizing time of high andlow temperature is 30 minutes each.To further verify the ability of two aspects, the first is to verify the ability of secondary welding joint to resist the interaction impact of high and low temperature, the second is to verify the creep and some phenomena of low temperature embrittlement that occur when it is kept at high temperature and low temperature for 30 minutesres pectively.The standard used is a 20% variation in the overall test thermal resistance of the test module compared to the initial test value as the aging criterion.

This is a test result of anti-shock aging.As you can see, compared with each module of ABCD, the commercial module A triggered aging between 400 hours and 600 hours, which means the commercial module failed between 400 hours and 600 hours.As for the BCD module, which uses sintered silver as the connection layer of the chip and different secondary solder as the connection scheme between the substrate and the bottom plate, it can be found that although the performance of B  that is the module that uses low-temperature solder, has been improved, but the improvement is not obvious. That's going from 400 to 600 increase to 800.But for C high temperature lead-free solder modules, the module overall aging reaches to 1000 hours.For modules D with lead solder at high temperature, it is still some distance from failure at 1000 hours, an increase of about 15%.It is shown that when sintered silver is used as the chip connection material and high temperature lead solder is used as the secondary welding connection of the module, the overall aging resistance to temperature shock is greatly improved, which meets the application requirements of strict working conditions.

According to the different aging time, different modules, and the degradation of the secondary welding interface, we used ultrasonic scanning mirror to conduct a non-destructive scan of the welding layer.As you can see, the scan results of the substrate after comparing more than one thousand hours of cycles show that during the thermal shock process, the thermo-mechanical stress at the corners of the interconnect layer is the largest, cracks appear first, and continue to extend inward.For the best performing module D, after more than 1,000 hours of welding, the entire welding layer is not obvious, almost all of the black area, thanks to the use of high temperature solder, interconnect interface layer cracking effectively is suppressed.

This is our summary of the experiment at this stage.

1.The module D which is that the current assisted sintered silver is used as the connection material of the chip, and the high temperature and low cost sn5pb92.5ag2.5 solder is used as the connection material of the substrate,Its static and dynamic electrical properties are very consistent, and there is no chip degradation, which proves the feasibility of the module process.

2. Compared with commercial IGBT module, the thermal resistance of module D is reduced by 17.2%.

3. The thermal shock resistance of module D increased from 400-600 hours in the original commercial module to more than 1000 hours,which successfully improved the thermal mechanical fatigue resistance of the module.

Next, the second part is the anti - power cycle aging capacity improvement attempt.

Why share this?In addition to secondary welding, also mention the lead bonding part.It also belongs to the small bonding part, its solder joints are smaller, one - wire welding, especially for SiC chips.Because the area of SiC chip is 1/2 that of Si chip, the young's modulus of SiC chip is 3 times that of Si chip, and the poisson ratio is 1.6 times that of Si chip, so the risk of SiC lead bond failure is greater.Because it has a smaller area, it can draw fewer lines.The unit line requires more power to flow, the current will be greater, the current density will be higher.Meanwhile, the young's modulus of SiC chips is much higher than that of Si chips.So at the same current capacity, you have to withstand more stress.Moreover, the current density of SiC chips is higher, so the state of mechanical stress on the entire SiC chip bonding line is much higher than that of Si chips.So the risk of failure is very high.

This phenomenon has also received wide spread attention and reports recently.With Germany as the representative, they have a higher drift rate in the solder layer and a very large decline in the service life. It has been reported that the service life of SiC modules with the same function is 30% less than that of Si modules, which is relatively optimistic. There are reports that the highest is reduced to 10% of the original, which is a 90% reduction, depending on the temperature range for different process cycles.The large and small alternating range directly affects the difference of life multiple.

We have also done our own research in this regard. This is a diode of SiC. There are obvious black spots in the center area of bonding points on the failure chip surface.This indicates that when the failure area of the bonding point becomes larger and larger, the current passing through the effective connection area will become larger and larger to a certain extent, which will lead to the overcurrent burning out and cause the failure.

We carried out a simulation analysis.Sintered silveris used as the connection layer of the chip, instead of the solder layer of the traditional high temperature high lead or high temperature lead-free solder used in primary welding.First of all, the maximum temperature gradient and maximum temperature of the bond can be reduced byusing the characteristic of the largest silver.In such a simulated amplifier, the maximum temperature can be reduced by 6℃, and the temperature gradient can be effectively alleviated, thanks to the increase of thermal conductivity in the direction of longitudinal thickness, the heat dissipation condition and the maximum temperature can be improved.At the same time,the thermal conductivity of its horizontal plane is also very high, which makesits diffusion capacity on the plane very strong, and makes the temperature gradient between chips and chips smaller when the thermal bonding between the bonding line points of chips is more obvious, which is conducive to the reduction of its maximum temperature point.

In this view,  we developed 1200-v / 1700-v SiC SBD as a continuous flow diode, such a hybrid module of SiC IGBT, to improve the ability of bonding line to resist power cycle.IGBT commercial modules of conventional solder silicon are compared.This is the result of the comparison between the two modules we prepared and the commercial module shown in the table. The main features of the two modules are that silver and non-silver are used as the connection layer of the chip, both of which are ROM chips. The 1700-v adopts the CRRC series. Special thanks to CRRC friends for their support of our chip research.The CRRC chip we bought is still good.

By controlling the sintering atmosphere and temperature, our low-temperature sintered nano-silver achieves the high strength connection of "chip/nano-silver solder paste/bare copper substrate".For large area chip shear strength can reach more than 30MPa.This is about our process. I won't go into too much detail.

Module packaging process is all completed in the laboratory research process line.1200-v / 300-a uses rohm SiC SBD chip, and 1700-v / 300-a uses  CRRC SiC SBD chip as the research object.

The static i-v curve is studied and tested as shown in the figure. The repeatability is very good, and the saturation voltage drop, including the diode positive wizard, will be very overlapping, indicating that the packaging method can be used and the chip does not degrade.

At the same time, the leakage current static facility is also tested.Why do we want to do module B, why do we want to use 1700-vSiC SBD to compare the anti-power cycle ability, the selection of domestic SiC chip schottky diode as a secondary diode, why use it?Because considering the fact that foreign diode, its leakage current is very large.We can see that the leakage current at 150℃ can reach a very high value, which is twice the leakage current of the silicon module. This not only brings the loss of leakage, but also the overall secondary secondary flow, in the reverse blocking, diode high temperature, SiC SBD blocking capacity is atrisk.Because for 1200-v, if you block 1200-v, its blocking threshold is between 1200-1400, so its performance is not good.Therefore, we chose 1700-v SiC SBD instead of 1200-v forcomparison with hybrid module.

At 150℃, the first leakage can be significantly reduced, reduce the loss of leakage, reduce the risk of operation;Second, the use of 1700-V instead of 1200-V, it has sufficient pressure protection.Because of1700-V, with stand voltage can reach 1700, 1800 or even 1900.This margin is also related to the current level of device development.When 1700-v is used, its leakage, reliability, and security are greatly improved.For Si's 1700-v / 1200-v diodes, it increases the cost by increasing the voltage, but for SiC SBD, the increase of cost in 1700-v / 1200-vis not obvious, which makes the scheme feasible.

About dynamic switch testing.Under such a test condition, high temperature and low temperature tests are carried out for SiC mixing module respectively.

We can see that in this way, we can reduce the peak current of the mixing module from 165A to 146A, reduce the phenomenon of reverse overrush, and almost have no effect of reverse recovery current with SBD.The switching transient voltage peaks of hybrid modules A and B are reduced by 40V and 52V, respectively.The excellent reverse recovery capability and low overshoot turn-off characteristics further validate the feasibility of our entire process package scheme, which will not cause its electrical degeneration due to the selection and change of these schemes.

At the same time, compared with the commercial IGBT module, the reverse current of the hybrid module is improved slightly.However, for SiC IGBT, the reduction in reverse recovery is very significant, which is bound to significantly reduce switching losses.

Compared with the traditional full Si module, the opening loss of hybrid module is reduced by 30.7%, the shutting loss is reduced by 5.3%, the reverse recovery loss is reduced by 88.6%, and the total switching loss is reduced by 28.8%.Therefore, the 1700V SiC hybrid module has very low switching losses.

Next, Acomparison is made between the mixed modules A and B and the commercial IGBTmodules.For the mixing module, the steady-state thermal resistance of the mixing module encapsulated bynon-pressure sintered nano silver is 0.10℃/W, which is 9.9% lower than the Si module connected with high lead solder.Because its thermal resistance depends on how it dissipates heat.The thermal resistance contrast depends on the area of the chip, The area of different chips is different.So the area is not the same,its own heat dissipation not only depends on the connection material of the packaging, its heat dissipation resistance will be different.

Second,the steady state thermal resistance of the diode of hybrid module B is 0.102℃/W, which is 33.7% lower than Si.The thermal resistance of the diode decreases significantly per unit of the chip itself.Because the thermal resistance of the diode itself is larger, the variation is greater.

This is a power cycle test. Constant junction temperature mode, from 150℃ to 50℃, 3 seconds on, 3 seconds off, a very fast power cycle test, mainly used to evaluate the reliability of the module.Its failure can be judged when its saturation pressure increases by 5% or its thermal resistance increases by 20%.In general, industry standards require such atest to pass 30K cycles, The average failure life of commercial modules is generally up to 60K cycles, so it is a certain margin.

For the IBGT chip, after the mixed module adopts sintered silver, the power cycle is increased to 95K times, reduction of failure saturation pressure is increased by 5%, which is an increase of 1.5 times.Unpressed sintered silver can improve such an effect as compared to high lead and high temperature solders.

Its saturation pressure drop rose by 5.14%, and its thermal resistance was not yet reached 20%.Therefore, low temperature sintered silver is helpful to improve the bonding reliability of IGBT.

At the same time, for the SiC ROM SBD of 1200V/50A and  Si diode of 1200V/50A, we can find that under the same test conditions, there is a significant difference between the two.

The solder bonded SiC SBD failed at 8000 cycles, when the forward voltage increased by 0.12v, 2.6% higher than the sintered silver bonded SiC SBD.8000 times with solder will fail, Si is much higher than this, using sintered silver life is much higher than that of Si. About 10,000 times, the use of sintered silver began to fail, It can be said that there is a certain improvement effect, but it is not very obvious.

But at least from the accelerated aging, about 20% increase in the effect, it shows that it has a certain improvement effect.In the actual use process, it's also a kind of help to its life. Next, we will further study the enhancement of aging effect of SiC modules and the enhancement of anti-power cyclic aging effect of SiC chips. We hope to have an opportunity to discuss and communicate in the future.

This is a summary of the power module upgrade.