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Posted on 29 June 2019

Cleaning of Power Module Substrates

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Contaminants must be removed from the surfaces

For energy-efficiency reasons, modules produced today have ever increasing performance requirements with greater packaging density (for instance in automotive or industrial applications). Accordingly, even the slightest contaminants remaining on the surface impede the reliability required in these critical and highly sensitive applications.

By Thomas Kucharek, Team Leader, Application Technology ZESTRON Europe

 

Cleaning modules for power electronics/IGBT modules is a must in a solder paste based process. Which requirements have to be met and which advantages can be realized by an optimized  cleaning process?

In the power module manufacturing process, contaminants typically remain on the substrate and chip surfaces. To guarantee the highest process reliability, these contaminants must be removed from the surfaces through a cleaning process. Previously, flammable solvents were used in the cleaning process as no alternatives were available. Today however, cleaning with waterbased media is the standard.

The need for cleaning in power module production

There are several process flows for power module production and thus different stations, where cleaning is usually required. The production process flow may differ due to the field of application, such as for automobile and industrial operation, as well as due to the design and functionality defined by the end customer.

Usually, the first production step is the die attach to the copper substrate by soldering, for example of IGBTs and diodes (Figure 1). Afterwards, production flows may vary. In any case, there are two points where cleaning is recommended. The first point would be after die attach by soldering to prepare the modules for wire bonding and the second would be after heat sink soldering before the modules are further processed, i.e. moulded, wire bonded and/or connected to frames.

Irrespective from the process flow and final module structure, contaminants such as oxide films and flux spatter remain on the chip and substrate surface after soldering. These residues have a negative impact on subsequent processes such as wire bonding or moulding. Wire bond adhesion is often impaired when flux splatter remains on surfaces. Residues on the substrate and chip surfaces can impair achievable shear values for copper, diodes and IGBTs. Since these values are often used for the cleanliness qualification, it is necessary to completely remove all residues in a suitable cleaning process.

Cleaning applications in power module production

Advantages of an optimized cleaning process

While traditionally, solvents were used to clean power electronics, water-based cleaning electronics provide excellent cleaning performance while ensuring material compatibility and long-term reliability of the modules at the same time. As a prime side-effect, they provide significant better VOC (Volatile Organic Compound) and health and safety data.

Production practice confirmed several advantages of optimized cleaning processes in the fields of wire bonding quality and reliability (shear tests and power cycling), moulding and material compatibility, which shall now be further described.

 Wire bonding quality – Shear tests

There are two main issues which influence the quality of wire bonding. The first are flux residues on the substrate after soldering and especially flux spatter remaining on the chip surfaces (Figure 2). Bonding on uncleaned chips leads to insufficient quality and often results in either unnecessary high bonding power, leading to heel cracks or even chip defects, or lift offs due to poor wire connection.

Diode uncleaned and cleaned

Another quality feature for power modules are visually flawless and spot-free substrates and chip surfaces. However, the soldering process or unsuitable cleaning agents, among other things, can lead to heavily oxidized parts. These oxide layers on the surface also lead to problems during bonding and can impair the production yield (Figure 3).

Bonds on oxide layer, bond on a copper substrate activated by cleaning

A suitable cleaning process must therefore achieve excellent cleaning performance to remove all flux residues from soldering and be able to activate oxidized surfaces. With a cleaning process specifically adjusted for power modules, the surfaces are optimally prepared for bonding. Production practice proved that water-based, pH-neutral and surfactant-free cleaning media can achieve this relative to formerly used solvents. They deliver good bond yields as well as visually flawless parts.

The quality of the wire bonds is usually evaluated by shear testing. A stable process with high shear values is required for production yield. Modules failing to pass shear tests can have an immense effect on efficiency and costs, especially if the substrates are already soldered to the heat sink.

Zestron investigated the effect of cleaning on shear values in an internal study based on several customer projects. In a first step, results showed that an optimized cleaning process leads to a significant increase of shear values compared to uncleaned substrates (Figure 4). The second part of the study investigated different cleaning technologies (MPC and surfactants) regarding their ability to prepare the surfaces for the bonding process. Thereby, it was proved that water-based cleaning agents lead to higher shear values than conventional surfactants, as they do not leave any residues on the surfaces after rinsing.

Shear-value comparisons: uncleaned power module; modules cleaned before bonding

Depending on the overall production process settings, even plasma treatment, which has been typically necessary, can be saved if a suitable waterbased process is used. This in turn leads to significant cost reduction potential.

Long-term reliability – Power cycling results

The aim of cleaning is always to guarantee the modules' long-term reliability. The widely used method to qualify the long-term reliability or lifetime is powercycling. The modules are stressed based on specific life cycle models of their dedicated application with their lifetime being measured in cycles.

If the modules have not been sufficiently cleaned and contaminants still remain on the chip and substrate surfaces, they often fail to pass power cycling tests. The most common failures are bond liftoffs caused by poor bond connections due to residues. Once a bond connection fails, the stress on the others is raised, usually leading to an exponentially increasing number of bond failures finally resulting in the breakdown of the power module.

Cleaning the modules prior to bonding has a significant impact on the power cycling results. If the residues from soldering are removed completely in an optimized process and the bond quality is thereby improved, long-term reliability is increased showing in excellent power cycling results. In comparison, modules cleaned in a conventional process statistically show shorter life times.

Moulding adhesion

The cleaning process also impacts the quality of the moulding. In many cases, there is a moulding applied to the bonded substrates to protect them from environmental influences. Contaminants play a decisive role with regards to moulding adhesion and thus also reliability. Flux residues on the substrate surface reduce the adhesion forces of the moulding, which in turn may lead to delamination. They may also lead to electrochemical migration beneath the moulding and thus to field failures. In the course of several cleaning projects it was proven that cleaning increases the adhesion forces while limiting delamination defects and thereby improves the moulding reliability.

Material compatibility

When looking for a cleaning process for power electronics, the cleaning performance is usually the main fact of interest with regards to bonding and production yield. However, another important aspect is often neglected in the first place: the compatibility of the cleaning process with the power modules’ materials.

Cleaning agents for power modules must meet exceptional requirements in this area. Thus, the cleaning agent selection is critical to ensure it does not affect various chip passivations or the substrate surface. Unsuitable cleaning agents attack the passivations thus leading to the impairment of the chip function (Figure 4).

It is therefore recommended to test not only the cleaning performance but also to ensure the material compatibility of the cleaning agent before a process is implemented.

Glass passivation of a thyristor: attacked by an unsuitable cleaning agent and optimally cleaned

Summary

Cleaning power modules before bonding is a must to ensure their long-term reliability. Adjusted processes with cleaning agents specifically developed for power modules ensure highest cleanliness levels and thus increase bond and mould quality. At the same time, they ensure compatibility with chip passivations and substrate materials. Significantly improved results of the shear test and power cycling qualification finally lead to an increase in production yield. Zestron has already implemented several high volume processes in this area for automotive and industrial applications and is ready to support you with the selection or optimization of your power electronics cleaning application.

 

 

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