The three main preheater types each have specific strengths.

Preheating is a critical step in wave soldering. Its main purpose is to evaporate the carrier solvents in the flux (i.e., alcohol or water) and to activate the flux. When done, the flux is able to remove oxides, protect surfaces from oxidation during assembly, and promote wetting. To evaporate the solvent, the topside board temperature during preheating must be higher than the boiling temperature of the solvent. The temperature should be above 82˚C for alcohol (i.e., isopropyl) fluxes and above 100˚C for water fluxes. Special care is needed so as not to burn off the flux from the bottom of the boards (i.e., where flux is deposited) in order to achieve the appropriate topside temperatures. Thicker boards or palletized assemblies require more heat to reach boiling temperature than do thinner boards and in the process flux can burn off. In this case, it is also important to monitor bottom-side temperature. Complete evaporation of the solvent is required to reduce soldering defects such as openings, voiding and solder balling. Depending on the type of flux, a higher preheat temperature may be required. The appropriate preheat temperature usually is specified by the flux supplier.

Preheating is also used to reduce component thermal shock and to promote better through-hole penetration, especially with multilayer boards.

Wave soldering machines offer different preheating systems such as forced convection, a combination of middle wave IR radiation with natural convection (i.e., calrod), and a system with short-wave IR lamps. The proper combination of these preheat systems will ensure good solder joint formation and lower defect rates.

Preheating Systems

Forced air convection offers an advantage in that the maximum PCB preheat temperature is limited by the system’s temperature setting. The forced air promotes quick evaporation of the flux solvent and provides the most efficient energy transfer. It is recommended this system be positioned as a second preheat unit when a wet flux layer is deposited on the board. Reason: Airflow from the heater may displace the flux from its position. Note: This system can prematurely burn off the flux due to its high efficiency. To overcome this, advanced wave soldering systems offer recipe-driven fan speeds to control heat transfer.

Calrod systems evaporate some of the solvent, making the flux less fluid and more viscous without moving the flux. In the case of wet flux, it is advisable to position this system as a first preheating unit. The middle-wave IR transfers relatively more heat into the PCB material (through direct radiation) than does the forced convection preheater. The main energy source is the radiation emitted by the heating elements. As a result, the air in the process area is heated and creates a natural convective flow that assists solvent evaporation Other applications where this technology should be used is when the assemblies have bottom- and topside surface mount components. Components located on the bottom have thermal limits with respect to heating rates and ΔT of roughly 100˚ and 110˚C from preheat exit to solder contact. Components located on the topside are subject to possible unintentional reflow, especially when traditional SnPb and 0.062" boards are used. The placement of this preheat type in the last heating zone can maintain a rate of rise of temperature but minimizes thermal penetration to the topside.

IR lamp is recommended when a mixture of boards that require different preheating temperatures are processed, or when one needs a preheater that can act quickly in response to different demands. Because of its fast reaction time and immediate radiation output in accordance with its settings, this system can boost the board preheat temperature. Most of the energy is emitted to the board material and not so much to the transparent flux layer. In this manner, the flux is not burned off and can still protect solderable surfaces from oxidation during soldering.

Preheater selection is also related to board complexity, conveyor speed and flux type. For example, for alcohol fluxes, the specific heat is 0.65 cal/g˚C, while for the water fluxes it is 1 cal/g˚C. The specific heat is the heat energy required to raise the temperature in one unit of the solvent. This means the preheaters need to supply more energy in order to increase the temperature 1˚C for each gram of solvent when a water flux is used.

Figure 1 shows the effects of the three different preheating systems with their respective settings. (All were used in the same double-wave machine.) VOC-free water-based flux and SAC alloy were used in the assembly. As can be seen, the calrod system provides a passive heating that ensures the flux dries out and remains in place. The forced convection in the second zone brings the solvent to its boiling point, and the IR lamps ensure the solvent evaporates and the component temperature does not increase.

The best indication of a proper preheating configuration is the soldering quality.  More than two heating units are recommended for Pb-free applications. It is also important to pre-dry the flux before the board enters the wave and to avoid excessive preheating that may expose solderable surfaces to oxidation.

Ursula Marquez de Tino is a process and research engineer at Vitronics Soltec, based in the Unovis SMT Lab (vitronics-soltec.com); umarquez@vsww.com. Her column appears monthly.

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