Lead-Free Interconnect Technique by Using Variable Frequency Microwave

Journal of Electronic Materials, Jul 2005 by Moon, Kyoung-Sik, Li, Yi, Xu, Jianwen, Wong, C P

The solder paste surface could not be a direct target of the IR light. If the sensor reads the temperature of the solder paste surface during heating, the readings could be inaccurate. Such noncontact IR sensors read an amount of emitted IR energy at a specific object, which indicates the temperature of the object. Emissivity is the measure of an object's ability to emit infrared energy. Emissivity can have a value from O (shiny mirror) to 1.0 (blackbody).

In order to read an accurate temperature of the target materials by the IR sensor, the emissivity in the sensor module should be set to a certain value corresponding to that of the target material. The organic flux vehicle in the solder paste that mainly absorbs the microwave energy starts to evaporate at 150-160�C, and then the material composition of the solder paste surface begins to change from organic materials to the Sn alloy metal with the increase of the temperature. A difference in the emissivity between the organic compound and the alloy metal causes differentiation between a reading temperature and a true temperature on the target material. It is difficult to manually adjust the emissivity value in the sensor module at a certain temperature. Therefore, the IR sensor was faced to the FR-4 board rather to the solder paste surface. Instead, the IR spot was placed as close to the solder pastes as possible (-2-3 mm), as shown in Fig. 1. The emissivity was set to 0.95, which is commonly used for most organic, painted, or oxidized surfaces.

Thermal profiles of the solder pastes used were obtained by using a differential scanning calorimeter (DSC, TA Instruments Inc., New Castle, DE).

For metallographic observation, specimens were ground down to 4,000 grit on a silicon carbide paper under water cooling. Polishing was performed using 1-�m and 0.1-�m Al^sub 2^O^sub 3^ suspension. The specimens were then etched in a 10% HNO^sub 3^ solution for about 10 sec. This process provides a contrast between the intermetallic compound and the solder matrix boundary.

Optical microscopy and scanning electron microscopy (SEM, Hitachi S-800, Chiyoda-ku, Tokyo) were used to study the morphology of IMCs. Energy-dispersive x-ray (EDX) was also used to characterize the composition of the IMCs.

RESULTS AND DISCUSSION

Observation of Lead-Free Solder Shapes Formed in Reflow Machine via Optical Microscopy

Figure 2 shows the thermal profile of the reflow machine used in this study, in which the preheating temperature and time were kept constant and only the peak temperatures were varied by 250�C, 260�C, and 270�C. The peak temperatures were much higher than the melting point of the solders used because there is a temperature difference between the test boards on the convey belt inside the reflow oven and the specific zone in the reflow oven controller. The DSC measurement for the solder pastes used is shown in Fig. 3. The onset points for the melting point of the SnAgCu and SnAg solder paste were 217�C and 221�C, respectively.