In the field of integrated circuit or LED packaging, wire bonding as a mature metal interconnection technology occupies an important market position. This is a kind of use of fine metal wire as a bonding wire, using heat, pressure and ultrasonic energy to make the bonding wire and the pad are tightly welded to realize the electrical interconnection between the chip and the substrate and the signal transmission between the chips. With the development of integration and high performance of electronic packaging, the wire diameter, pad size and lead spacing of the bonding wire are gradually reduced, which leads to a sharp increase in the thermal stress, mechanical stress and electrical stress at the bonding solder joints, resulting in a series of service reliability problems. In a variety of wire bonding failure modes, it is more common to break the bonding interface due to cyclic tension or shear force caused by thermal mismatch. Therefore, improving the bonding force between the bonding wire and the pad is the key to improve the reliability of wire bonding service.

It is found that in addition to the influence of bonding wire materials and bonding process parameters on the bonding interface bonding force, the cleanliness of the pad surface during bonding also plays a vital role. During the welding process, the bonding wire melts to form a free air ball ( FAB ) and contacts the surface of the pad to form an inter metallic compound ( IMC ), thereby forming an effective and reliable bonding point. If the surface of the pad is oxidized or contaminated particles are present, it will hinder the atomic diffusion between the solder ball and the pad, thereby reducing the bonding strength and even forming a virtual welding and de-welding spot.

In actual production, in order to obtain a clean surface during wire bonding, the pad is generally cleaned. Plasma cleaning, as one of the dry cleaning methods, has gradually been widely used in industry due to its advantages of environmental protection, high efficiency and no damage.

Effect of plasma cleaning on bonding strength of pad surface and wire

In order to study the effect of plasma cleaning on the effect of wire bonding, the samples were divided into 5 groups : A ~ E.Among them, group A was directly bonded without plasma treatment, and the other 4 groups were treated with different plasma cleaning power and treatment time, the specific parameters are shown in Fig.1.Before and after plasma cleaning, the contact angle tester was used to test the contact angle of water droplets to observe the hydrophilicity of the surface of the pad, and the laser confocal microscope was used to detect the size and quantity of the contaminated particles on the surface of the pad. The number of samples in each group was 8, and the shear force test was carried out on the bonding solder joint after the wire bonding was completed.

The bonding solder joint samples without plasma cleaning were selected for three times of reflow test and high temperature storage ( 24h, 125 °C ) test. After the test, the contact resistance of the bonding interface was detected by four probes to identify the failure samples and analyze them. Firstly, the failed samples were metallographically prepared, and then the focused ion beam ( FIB ) was used to perform sheet processing at the interface between the bonding wire and the pad. Finally, the transmission electron microscope ( TEM ) was used to characterize the high-resolution microstructure of the interface.

The contact angles of water droplets on the surface of uncleaned and cleaned pads were calculated to be 96 ° and 12.6 °, respectively, as shown in Fig.2.After plasma treatment, the contact angle decreased by 86.9 %. The results show that plasma cleaning can effectively improve the hydrophilicity of the bonding surface and reduce the surface tension of the pad, which is beneficial to the spreading of FAB and the formation of a good bonding interface. Studies have shown that high-energy particles can effectively remove the oxide layer on the surface of the sample under the action of physical bombardment or chemical reaction during the cleaning process, thereby exposing the clean surface of the aluminum atom.

Fig.2 Contact angle before and after plasma cleaning

The contaminated particles on the surface of the pad before and after plasma cleaning were observed, as shown in Fig.3.The statistics of contaminated particles and shear force before and after plasma cleaning are shown in Fig.4. It can be seen from Fig. 3 ( a ) that a large number of white granular pollutants were distributed on the surface of the pad without plasma cleaning, and the average particle size was about 0.5 μm. After plasma cleaning, the number of granular pollutants on the surface of the pad was significantly reduced, as shown in Fig. 3 ( b ). In order to obtain accurate distribution data of the number of granular pollutants, all samples in each group were tested and statistically analyzed, as shown in Fig. 4 ( a ). The results showed that the average number of contaminated particles on the surface of each pad before and after treatment was 18.7 ± 9.9 and 3.7 ± 1.6, respectively, that is, the number of contaminated particles decreased by 80 % after treatment. This shows that plasma cleaning can effectively reduce the number of contaminated particles on the surface of the pad. The particle contamination on the surface of the solder pad may come from the colloid residue introduced during the preparation or plastic packaging of the solder pad, or from the volatile gas deposition generated during the baking process. In addition, the dust floating in the air of the production line may also be the source of pollutants. During the plasma treatment process, high-energy Ar + can decompose these solid particles into gases such as CO2, CO and H2O under the action of sputtering, and then be vacuum pumped off the surface of the pad.

Fig. 3 Contaminated particles on the surface of the pad before and after plasma cleaning