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Hitachi

Corporate InformationResearch & Development

Photo: IKEDA Osamu
IKEDA Osamu
Senior Researcher

Solder has been used for joining electronic components since time immemorial. In former times, lead was a mainstream ingredient of solder; however, in consideration of its negative impact on people's health and surrounding environments, lead is being systematically removed from soldering processes, and "lead-free" processes are being introduced.

However, in regard to solder joints of power-electronics products used in motorcars, for example, the temperature of the soldered components during operation of such products becomes high. Accordingly, it has been a technical challenge to replace the lead in such soldered joints.

Aiming at that challenge, Hitachi has been promoting the switch to lead-free processes right from the start, and we have developed a joining technology based on lead-free solder that doesn't degrade under high temperature. We are currently applying that lead-free technology to power-electronics products.

The switch to lead-free solder is progressing

It seems that the switch to lead-free solder is progressing well, right?

Photo: IKEDA Osamu

IKEDAThat's right. For many years, solder containing lead has been used. Such lead-containing solder is easy to use and provides a good joint. In July 2006, however, the "Restriction on Hazardous Substances Directive" (RoHS, for short) came into effect in the EU, and since then, "lead-free transformation"—namely, soldering processes have been shifting from ones using lead-containing solder to "lead-free" ones—has been ongoing. The RoHS Directive is a regulation stating that the materials (including solder) used in the components of electrical products must not contain any hazardous elements.

However, in regard to "high-lead solder," which contains lead (characterized by a high melting point) as its main constituent, no other solder has been developed to completely replace it, so the RoHS Directive does not regulate this kind of solder at present.

That means it is difficult to replace high-lead solder with lead-free solder, right?

IKEDAYes, that's what it means. High-lead solder has two key features: a high melting point and excellent heat resistance. Its excellent heat resistance means that it can even be used at high temperatures. As for its high melting point, it was worldwide knowledge that although other metals have high melting points, they could not compete with lead in terms of material cost and ease of use. However, since we cannot afford to continually use materials harmful to people and the environment forever, the efforts toward lead-free transformation in whatever cases possible are presently continuing.

Although high-lead solder has a high melting point, are there any solders with low melting points?

IKEDAAs for solder, high-melting-point ones and low-melting-point ones are used according to place of usage.

For example, you've probably seen the green printed circuit boards (PCBs) in mobile phones and PCs. As well as being used for joining electronic components on those circuit boards, solder is also used for joining parts within those electronic components. In that case, high-melting-point solder is used for joining the parts within electronic components, while low-melting-point solder is used for joining the electronic components to the PCB. The reason is as follows: If the same kind of solder is used in both cases, when the electronic components are joined to the PCB, the solder within them will melt, and the components will fail.

Photo 1: A substrate on which electronic components are joined by solder
A substrate on which electronic components are joined by solder

IKEDAIn the case of power-electronics products used in motorcars, etc., soldered components work in a state of continuous high temperature owing to the heat generated by power semiconductors. As a result, it is necessary to use a solder with heat resistance up to considerably higher temperatures than those manageable by solders used for joining electronic components on PCBs. Aiming for "complete lead-free transformation of power-electronics products," we have started research on solder materials that can replace lead.

Figure 1: Temperature of soldered joints during use of a product
Temperature of soldered joints during use of a product

Successes and failures associated with investigation of solder materials

During your research on solder materials, what sort of things did you tackle?

Photo: IKEDA Osamu

IKEDAFirst of all, we investigated solders containing a mixture of tin and copper. Tin is a common ingredient of lead-free solders. As a main ingredient of solder used for joining electronic components on PCBs, it is characterized by a low melting point. So we decided to mix copper—which has high a melting point—to tin.

When a joint is made with solder composed of tin mixed with copper, the tin (with low melting point) melts first. After that, as the joining process proceeds, the tin and copper react, and a tin-copper compound is formed. That compound has a higher melting point than that of tin. We thought that although the state of the solder is one of a low melting point at first, during the joining process, the melting point of the solder as a whole would increase. That was a new concept that had not been considered up until then.

Figure 2: Joint mechanism of solder blending tin and copper
Joint mechanism of solder blending tin and copper

Was that new concept a success?

IKEDAThe idea that the melting point increases after joining had been confirmed in regard to solder for joining the parts within electronic components, and the Japan Electronics and Information Technology Industries Association (JEITA), the body responsible for setting standards for solders, has recognized that idea.

However, in regard to our target, namely, power-electronics products, it is unfortunate that the idea has not gone down well. In the case of power-electronics products, each semiconductor device is about 1 cm square, and many devices must be joined precisely without any gaps. However, our solder was not filled properly, so voids were generated within the solder.

As it turned out, at Hitachi, we gave up on applying this joint technology to power-electronics products.

Nonetheless, because of this trial-and-error process, we were able to reconsider what points should be considered in the case of power-electronics products.

Investigation taking a different approach-a road opens up from failure

What is the key point in regard to power-electronics products?

IKEDAThe key point is that power-electronics products operate in a high temperature range, namely, 150℃ to 200℃. Generally, as for the reason that lead-free solder with tin as a main constituent has not been applied to power-electronics products, under continual use at high temperature, the reaction between the solder and the joint components continues, holes appear in the solder, and the components become disconnected; in other words, there is a danger that the characteristics of the solder will be degraded. If that reaction were suppressed, lead-free solder could be applied to power-electronics products. So we decided to start validating solders after changing our mindset and taking a different approach.

What kind of approach did your investigations take?

IKEDAFirst, we considered solders with other constituents. As for the method I previously mentioned, it involves a mechanism by which a chemical compound is formed by the reaction of tin and copper during the joining process. This time we decided to add a tin-copper compound to solder from the beginning.

On top of that, we came up with a new idea in regard to the joint surface of the components. That is, we coated the joint surface with nickel. In that way, the tin-copper compound is attracted to the nickel-coated joint surface. This process forms a barrier layer, so we thought that the reaction between the components and solder could be inhibited by that barrier layer.

Figure 3: Joint mechanism of solder containing compound of tin and copper
Joint mechanism of solder containing compound of tin and copper

What was the outcome of the investigation?

IKEDAAs you can see from this photo, we confirmed that the tin-copper compound is attracted to the nickel-coated joint surface; in other words, we verified the planned outcome.

Photo 2: Solder joint containing a compound of tin and copper
Solder joint containing a compound of tin and copper

IKEDAIncidentally, on taking that approach, we gained knowledge from our failures when we investigated solders containing a mixture of tin and copper.

As I explained before, the tin-copper compound is attracted to the nickel-coated joint surface, this is that of a phenomenon by which "superfluous constituents" from among those of the solder are attracted to joint surface. When we investigated the solder composed of a mixture of tin and copper, we understood that the superfluous constituents are attracted to joint surface as a result of including a certain amount of copper in the solder (which is normally not present).

When we attained that knowledge, we were able to improve the heat resistance and create a solder joint as planned. On achieving that result, we realized that a route to applying lead-free solder to power-electronics products had finally opened up.

Toward application to power-electronics products

Did application to products go smoothly?

IKEDAWhether we were lucky or unlucky, we had to verify applications in products under the toughest conditions. As for the first products we investigated, unfortunately, we were unable to apply lead-free solder to it.

It operated under high temperature (200℃), so the solder joint required high reliability. Although they were performed under really severe conditions, tests on reliability (namely, maintaining a joint for 1000 hours at 200℃ and maintaining a joint under temperature variations due to cooling to below zero and heating to 200℃) were passed without any problems. However, in the case that lead-free solder is applied to actual products, we had to perform tests that pass current through semiconductor devices—namely, "power-cycle tests." In regard to those tests (in which only the semiconductor element is heated when the current is switched on, and the device cools down when the current is switched off), we were not able to pass the target. Without passing that test, we could not apply lead-free solder to that products.

Applying lead-free solder to products faced a different type of difficulty from verifying materials, right?

IKEDAThat's right. However, without giving up hope in spite of those failures, we investigated applications in an IGBT (insulated-gate bipolar transistor) module—which operates under a slightly lower temperature environment. An IGBT module acts as a core of an inverter for controlling motors in electric vehicles and hybrid vehicles. We thought we could find applications in products operated at a utilization temperature of 150℃, which is lower than that of the products we first investigated.

Figure 4: IGBT module for a car
IGBT module for a car

IKEDAAs a result of that investigation, we were able to apply the lead-free solder to these modules. These IGBT modules were a first successful application of Hitachi's world-leading lead-free transformation. And presently, they are being applied as IGBT modules for motor vehicles.

Creating a lead-free IGBT module is impressive, isn't it?

Photo: IKEDA Osamu

IKEDAAlthough we took quite a difficult road to get this point, when we were able to apply lead-free solder to IGBT modules, we realized that our efforts had been rewarded.

Triggered by this adoption of lead-free solder to a single product, application of solder containing a tin-copper compound was expanded to products like other electric vehicles and IGBT modules for wind generators.

At present, our development target has shifted to products that demand higher heat resistance and to products that demand higher reliability (such as trains and high-power-consumption industrial equipment), so we are aiming at creating solder that is applicable to such products.

Aiming to create a solder that surpasses solder

Looking back on your research, how do you feel now?

IKEDAI think I've been lucky for two reasons.

As for the first reason, the timing of my involvement in research on solder was just right. Although I have been researching solders ever since I joined Hitachi, I started my research just when new efforts targeting lead-free conversion of high-temperature-system solders started. I was glad to be involved in the research area that no one had researched before. All the same, when I started that research, I knew little of the common knowledge about the world of solders. Although I had studied metallic materials during my student days, I didn't know much about solders. So, in a way, I did various things the hard way. Even though I had many failures, I was able to link together the knowledge I had acquired up till then.

As for the second reason, I was blessed with good senior. The idea of mixing tin and copper originally came from a senior, but I was given the chance to investigate it. I am ever grateful to my senior for thinking, "We'd like to do something a little bit more interesting."

Moreover, just when I started work at Hitachi, Hitachi Group was taking on the leadership of an international collaborative research project concerning lead-free transformation, called "Intelligent Manufacturing Systems - Next Generation Environment-Friendly Soldering Technology" or IMS-EFSOT for short. At that time, my senior was playing a central role in that project. On seeing my senior strolling on the world's stage, I was greatly inspired.

Tell us about your future targets.

Photo: IKEDA Osamu

IKEDAAs for the world of power-electronics products, we are aiming to develop new solder materials that not only "do not use lead" but also "transcend lead." In the case of leaded solder, it has been used in various products in an all-conquering manner, and there seems to be no comparable material to it. Having said that, since we have come to understand that "Such and such a material works for such and such a product," I want to develop solder with a longer lifetime and higher reliability than leaded solder and apply it to just the right products in just the right places. And I'll be glad to see products applying our lead-free-solder technology being used around the world.

(Publication: September 6, 2013)

Notification

  • Publication: September 6, 2013
  • Professional affiliation and official position are at the time of publication.
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