Reflow

In the early 1970s, the first companies to apply low cost, mass production techniques to photovoltaics, a technology that had previously been considered an exotic aerospace technology, emerged. These techniques included the use of electroplated and screen printed metal paste electrical conductors, reflow soldered ribbon interconnects, and by 1977, low cost, automobile windshield-style, laminated module constmction. Such processes benefitted from a substantial existing industrial infrastmcture, and have become virtually ubiquitous in the present PV industry. 

PCT may be used for the production of electronic and automotive components such as circuit board components, connectors, switches and relays, and alternator armatures and pressure sensors. The main application has been in the fabrication of surface-mount connectors that can withstand infrared reflow soldering operations. 

Protective tapes used during solder reflow proeessing. 

Copper/silver-plated copper Solder-dipped copp>er/tin-plated aluminium Copper/tin-plated copper Copper/solder-dipped copper Copper/reflowed tinned copper Silver-plated copper/tin-plated copper Silver-plated copper/solder-dipped copper Gold-plated copper/tin-plated copper Aluminium/tin-plated aluminium (zincate process)... 

If a motor car has to be refinished after repair, commonsense suggests that the original finishing system would be ideal for maintenance of protection and durability. However, with tyres, upholstery, fabric and plastic trim fitted and petrol in the tank, the use of such high stoving temperatures is not practical. The practical upper temperature limit is 80°C. This means that none of the original materials is suitable, not even the acrylic lacquer, since this is designed to be sanded and the scratches reflowed at 155 C. 

Electrolytic Tinplate. Much of the tin mill product is made into electrolytic tinplate (ETP). A schematic of an ETP cross section is given in Figure 1. The steel strip is cleaned electrolytically in an alkaline bath to remove rolling lubricants and dirt, pickled in dilute mineral acid, usually with electric current applied to remove oxides, and plated with tin. It is then passed through a melting tower to melt and reflow the tin coating to form the shiny tin surface and the tin-iron alloy layer, chemically treated to stabilize the surface to prevent growth of tin oxide, and lubricated with a thin layer of synthetic oil. 

Koo, A., Komatsu, H., Tao, G., Inoue, M., Guth, P.H. and Kaplowitz, (1991). Contribution of no-reflow phenomenon to hepatic injury after ischaemia-reperfusion evidence for a role for superoxide anion. Hepatolt, 15, 507-514. 

Figure 11. The effect of reflow nearby the glass transition temperature for a PMMA-DR1 waveguide (a) device structure, (b) SEM picture of waveguide before reflow, (c) idem after reflow.

Leinse A., Diemeer M.B.J. and Driessen A., Scattering loss reduction in polymer waveguides by reflowing, Electron. Letters 2004 40 992-993. 

Figure 12.4. Left—Scanning electron microscope image of the input/output pads of a silicon IC bumped with eutectic lead/tin solder after solder reflow. Right—Photograph of printed wiring board interconnect pads that have been printed with lead/tin solder prior to reflow.

During the thermal-reflow process, waveguides are reshaped from a rectangular shape to a mushroom one. Slight deformation does not affect the microring optical performance, but only shift its resonance position. However, too high reflow temperature and/or too long process time can dramatically deform the cross section... 

Fig. 8.14 Scanning electron micrographs shows sidewall roughness and waveguide cross section of polystyrene microring resonators (a, d) without thermal reflow process, after reflowing at (b, e) 85°C for 120 s, and (c, f) 95°C for 60 s, respectively. Reprinted from Ref. 48 with...

Fig. 8.15 Gap reduction has linear relationship with the reflow time at the reflow temperature of 90°C. The inset SEM shows a polystyrene microring with a coupling gap of 200 nm, reduced from the initial width of 450 nm. Reprinted from Ref. 48 with permission. 2008 American Institute of Physics...

Fig. 8.16 Measured and curve fitted (solid) spectra of a polystyrene microring resonator at the through port are compared before (left) and after (right) thermal reflow process at 95°C for 60 s. Reprinted from Ref. 44 with permission. 2008 Institute of Electrical and Electronics Engineers...

In fabrication, a gap is intentionally made wider than the desired. Then it can be reduced to the desired separation together with smoothed sidewalls during the thermal-reflow. Such a process can relieve the challenge of fabricating a very small coupling gap. 

Chao, C. Y. Guo, L. J., Reduction of surface scattering loss in polymer microrings using thermal reflow technique, IEEE Photon. Technol. Lett. 2004, 16, 1498 1500... 

Fig. 18.5 Scanning electron microscope images showing cross sections of ARROWS with (a) trapezoidal geometry formed from an aluminum core, (b) rectangular geometry formed from an SU8 core, and (c) arched geometry formed from a reflowed photoresist core...

PCX withstands SMT (surface mount technology) reflow soldering. 

---