Cleaning and Process Monitoring
BY ALBERT HEILMANN, Umicore, AND STEFAN STRIXNER, Zestron
Increasing power levels and power density requirements for multiple end products means that high-power semiconductor modules and components are often assembled using a clip-bonding technology.
Solder paste leaves flux residues on solder joints and must be cleaned from high-power modules to meet high reliability requirements, and for satisfactory clip bonding, wire bonding, and molding.
Figure 1. 1 clip and wire bonding connection.
In contrast to the traditional method of die attach by adhesive bonding or wire soldering, high-power packages and discrete devices, such as metal oxide semiconductor field effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs), and switched output differential structure (SODs) use solder paste to connect the die to the basic substrate and/or leads. Clip-bonding technology partially replaces the standard wire-bond connection between die and lead by a solid copper bridge, which is also soldered by solder paste (Figure 1). This allows for unique package resistance, better thermal transfer, and ultra-fast switching performance due to the small package.
It is important to avoid remelting the alloy used for die-attach applications during soldering steps in production. Therefore, high-lead alloys are used, with melting points above 260°C. Due to a lack of cost-effective lead-free alternatives, those alloys are exempt from RoHS.
As the melting point range of high-lead alloys lies between 280° and 310°C, the peak temperature during the reflow process is 320°-400°C. These high temperature conditions lead to decomposition of some vehicle components. This fact, as well as the high flux content, requires more tailored cleaning processes and agents to guarantee the quality of subsequent mounting processes, such as molding or wire bonding.
Improper Cleaning Issues
An effective cleaning process must remove all critical flux residues, such as rosin/resin and activator residues, as well as solder balls. At the same time, no cleaning agent residues should be left on the surface and some kind of surface activation for further process steps such as wire bonding and molding is desired.
Lacking or inappropriate cleaning often results in poor wire-bonding quality, as well as problems with encapsulation and corrosion. As a result, residues of resin-/rosin-based contaminants might lead to improper bonding connections, resulting in lift-offs and heel cracks (Figure 2). It may also have a negative influence on molding materials adhesion, causing micro-cracks, delamination and/or voids (Figure 3). Alternatively, residues of flux activators might increase the long-term risk of corrosion.
Suitable Cleaning Solutions
To remove flux residues after clip bonding effectively and reliably, the cleaning agent has to chemically match the properties of solder paste residues. Thus, selection of both solder paste and cleaning agent should occur at the same time and in correlation to each other.
A recent study dealt with lead-based solder pastes, which are mainly used for soldering high-power packages. During this study, various solvent-based, semi-aqueous, and water-based processes were tested using ultrasonic, spray-under-immersion, spray-in-air, and centrifugal cleaning equipment.
Adjusting solder paste and cleaning agent is important. The study demonstrated that special die-attach paste types and cleaning agents based on a micro-phase cleaning technology are well matched for proper cleaning results. The relevance of mechanical agitation during the cleaning process was of minor importance.
Figure 2. Poor wire bonding quality: lift-off (left) and heel crack (right).
According to the product screenings of the study on one side and based on field experience on the other side, micro-phase cleaning products demonstrate significant technological benefits and establish improved cleaning efficiency, compared to previous generations of surfactant, terpene-based, or hydrocarbon cleaning agents.
Less ultrasonic power/time is required to create a reliable bond when using proper cleaning agents for wire substrates. Due to lower bonding energy, less deformation of the bonding wire is observed. Thus the danger of heel cracks is minimized (Figure 4).
Figure 3. Crack in component started by delamination.
Additionally, total pull-shear forces of the wire bonds can be increased and the reliability and reproducibility of the bonding process improved. Pull-shear forces of cleaned substrates show a lower standard deviation than unclean or improperly cleaned substrates. The surface energy, especially on copper-based lead-frames, is increased; improving adhesion of molding material on the component.
To ensure reliable cleanliness, suitable qualification methods are required. Currently, the level of cleanliness is verified indirectly by standardized specification methods, such as pull/shear tests for wire bonding, or IPC tests such as moisture-sensitivity, temperature-cycle, and pressure-cooker test, respectively. These tests are destructive, and are usually conducted after the final manufacturing step. In addition, quick analytical tests, which are non-destructive and can be applied directly on-site during production, are offered for cleanliness control. These methods enable real-time monitoring of the manufacturing process (Table 1).
Investigating the surfaces using interference contrast provides information on whether the metallization of the bond pad is intact or not. The test reveals when oxide layers or structural errors exist; these can make wire bonding more difficult.
The resin test visually identifies the distribution of resin- or rosin-based flux residues via color reaction. These residues negatively impact surface bondability and can also impair the adhesion of embedding materials. The evaluation is based on J-STD 001D.
Other organic impurities on the surface, which could influence the bonding and embedding processes, can be determined by measuring surface tension or energy. The organic layer test is able to analyze the reactivity of the metallic copper/nickel surfaces of the leadframe material. A highly active surface is important for the subsequent processing of high-power modules bonded to copper ceramic substrates, as well as leadframe-based high-power components.
The ion equivalent value detected by a standardized IPC TM 650 also represents an important factor. A high-ion equivalent reading indicates the existence of a large amount of hygroscopic and conductive impurities. These impurities lead – In the long run – to moldings delamination, and hence to failures. Also, local corrosion of bonding wires and/or die surface might be observed.
Figure 4. Required bonding energy depending on surface condition. High power = risk of heel cracks.
Other organic impurities, such as flux activator residue, can decisively influence the quality of the molding and trigger failure mechanisms under it. For demonstrating such organic impurities, there are quick and easy-to-use discoloration methods (such as the flux test, which can serve as an alternative to existing expensive testing methods like infrared spectroscopy). By means of a color reaction, organic acids used as activators in fluxes are selectively identified.
These complementary tests provide the operator with a clear understanding of the surface condition and enable immediate interaction with the process prior to wire bonding and molding.
To ensure a high-reliability process when manufacturing high-power modules with clip bonding technology, a proper assignment of single but correlated process steps is essential.
Cooperation between solder paste manufacturer and the cleaning agents supplier makes for well-defined processes where solder paste and cleaner go hand-in-hand. Cleaning results optimize the rest of the production processes and increase the reliability of high-power modules.
ALBERT HEILMANN, M. S. Ch. E., applied technology manager, may be contacted at Umicore, Hanau-Wolfgang, Germany, +49/6181-59-5312; E-mail: email@example.com. STEFAN STRIXNER, M. S. Ch. E., senior process engineer, may be contacted at ZESTRON Europe, Ingolstadt, Germany, +49/841-635-90; E-mail: S.Strixner@zestron.com.