Reflow processes

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. 8.16 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...

The thermal-reflow process (or hot embossing) has also been used to reduce significantly sidewall surface roughness in polymer (polystyrene)... 

Photoresist cylinders were then separately built by conventional lithography. In order to achieve large fluidic volume, thick photoresist AZIOOXT was chosen, and its patterned cylinders on the substrate were thermally treated on a hot plate. The reflow occurred at 120 °C in a period of 60 s. Sometimes, the organic solvent vapor surrounding the photoresist pattern can promote the reflow process. Figure 14a shows the SEM image for the photoresist mold, which smoothly joins hemispherical structure 2,200 pm in diameter and 167 pm in depth with the microchannel. 

Protective tapes used during solder reflow processing. 

Pin in paste (PIP) technology, also know as alternate assembly and reflow technology (AART) by Universal Instruments and others, allow the use of THT parts with solder paste and reflow soldering. Paste is deposited onto the board at through-hole locations, the leads of a THT part are placed through the paste, and at the conclusion of the placement process the board and components are reflowed. The two major concerns with the use of the PIP process are the deposition of an adequate volume of solder paste and the use of THT components whose body can withstand reflow soldering temperatures. In the wave solder process, only component leads were raised to soldering temperatures, but in the reflow process the entire part must withstand the process temperatures. 

Subsequently the solder reflow process used in PCB assembly may produce a dramatic reaction, known as the popcorn effect , due to rapid expansion of the moisture within the package which then bnrsts with resulting physical damage. The problem is avoided by pmchasing components in vacuum-sealed moisture-controlled packs which have to be stored in a controlled enviromnent after being opened. The use of the components is then governed by the supplier s instructions as printed on the packaging. 

In selecting suitable PCB material it is necessary to bear in mind that it will usually have to withstand at least three exposures to high temperature during the solder reflow process. One to each side of the board and a third after post test rework. Another potential problem with substandard laminated boards is that the laminations may separate when exposed to high temperatures. 

The benefits of PTE circuitry include lower cost when compared to conventional copper-based circuits on a polyimide substrate. The circuit components are also said to be more reliable because the curing temperature of the adhesives is far lower than the temperatures now being experienced by traditional PCB in solder reflow processes. 

A pulse flexed the channel and forced a droplet through the aperture. This droplet traveled toward a substrate mounted on an XYZ micropositioner. As the molten polymer landed on the substrate, it cooled and solidified into a planoconvex shape, thus forming a microlens. The polymer could undergo a subsequent reflow, as in fhe PR reflow process. The speed of cooling and subsequent reflow were confrolled by a substrate heater. 

The intent in developing this tool was to come up with a simple method for dealing with this multitude of variables in PCB design and assembly. Figure 11.2 shows the basic color-coding selected for this. Figure 11.3 shows an example of the actual chart format. In the horizontal axis it divides PCBs into thickness categories, and in the vertical axis it differentiates them by the number of reflow processes. 

Electrodeposited copper (see Fig. 29.2) has become an effective method of filUng microvias, replacing the alternative via filling techniques, especially as the conductor patterns become finer and the pitch density of the component packaging increases. Resin or condnctive pastes have been used for via filUng, but the reliabihty problems associated with these materials include subsequent pohshing, entrapped air within the blind micro via, and void formation during the reflow process. 

Assembly requirements also place constraints on the circuit board layout. A very high part density requires a large number of apertures in the solder paste stencil, which can cause the stencil to become locally too flimsy to control the solder paste deposit. A surface-mount circuit board with a very wide range of component sizes and package configurations may require multi-thickness stencils to properly control the paste deposit. Solder paste printing quality is a determining factor in solder-joint defects observed after the reflow process. 

A number of assembly-related issues must be addressed with odd-form components. First, it is necessary that correct pad dimensions be designed on the circuit board. Also, the stencil must have the correct aperture size to print an adequate quantity of solder paste. The pick-and-place machine may require custom tooling in order to handle these components. Lastly, odd-form parts are typically larger and heavier. Therefore, it is possible that they wiU not readily self-align while the solder is molten during the reflow process. 

Solder Paste. The dispensing of solder paste is the most widely used means of providing flux and solder metal to the joint for the reflow process. The primary components of the solder paste are the solder metal and the flux. The solder metal is typically 80-90 wt. percent of the paste. Aside from the In- and Zn-containing solders, there is very little sensitivity of the paste properties to the solder metal composition for Sn-based alloys. Important properties of the paste with respect to dispensing are the solder powder particle size and weight percent or metal loading of the paste. 

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