Smart soldering

Mike Judd and Keith Brindley of SMART give an insight into all that is new and exciting in the world of soldering.

In SC soldering processes, the component is placed on the printed circuit board at a later stage than solder and flux. At a later stage still, heat is applied. In a nutshell, then, SC soldering processes involve three distinct stages:

• applying solder and flux as a solder plastic;

• placing components;

• applying heat.

This separation of solder application from the application of heat (both go together in CS soldering process - and cannot be split because molten solder in a CS process itself supplies the required heat) effectively means that application of solder, in practice, becomes an assembly process rather than a soldering process. Readers are referred to Chapter 2 for a discussion on methods of solder paste application.

Heat application

There are only three basic ways of transferring heat; conduction - hot liquid, hot belt, heated collet; convection - hot air, hot gas, hot vapour; radiation -infra-red, laser, light beam.

Processes may be a combination of two or all three heat transfer methods but only two of these (infra-red and hot vapour) are used to any great extent.

Both infra-red and hot vapour soldering machines have been developed to such an extent there is really very little to choose between them. Both processes (at least their state-of-the-art variants) are capable of soldering densely-populated printed circuit assemblies to a high level of performance, and with few defects.

Laser soldering is currently in late developmental stages, and shows great promise for specialised SC soldering of surface mounted components. At present it is more costly than infra-red and hot vapour soldering processes, but it may find a niche market for soldering individual components which cannot easily be soldered by other mass means.

Infra-red soldering

Infra-red (IR) soldering processes have developed rapidly in recent years since their first introduction. Main developments of infra-red soldering machine processes with time are:

• focused lamps – near infra-red – fusing tin/lead electroplate;

• diffuse lamps;

• diffuse infra-red lamps and secondary reflector/emitters;

• diffuse vitreous flat panels – far infra-red (black);

• metal-faced flat black panels – far infra-red - natural convection;

• metal-faced panels and interstage recirculated convection;

• metal panels with holes for uniform forced convection;

• metal panels with holes and zoned forced convection;

• metal panels and higher volume zoned forced convection;

• metal panels and higher volume zoned forced convection of nitrogen.

Generally, infra-red soldering machines direct infra-red heat onto the board from above and below. Radiating elements have built-in thermocouples to allow temperature control. Heat transferred onto the assembly and so temperature of the joints to be soldered, however, depends on the materials and shapes of the board and components as well as wavelength of the infra-red radiation. For this reason, in such a basic infra-red machine it is difficult to be sure that all joints reach the same temperature at the same time. The effect of different joint temperatures is known as the shadow effect.

In operating terms, the shorter the wavelength of the infra-red element the more likely are boards and small components to suffer from overheating. On the other hand, more uniform heating of solder paste is obtained with short wavelengths.

Longer wavelength elements have an advantage in their air around them is heated. This hot air provides heat to assemblies by convection means. If hot air is allowed to aid heating of assemblies naturally the process is known as infra­red radiation with natural convection heating. The addition of convection heating to infra-red radiant heat soldering machiners tends to give more uniform heating and reduce temperature differences between joints on assemblies.

This principle of using convection heating to reduce air temperature differentials is extended by forcefully circulating warmed air to provide forced convection heating. It can be extended further by forcing air through perforations in each infra-red panel emitter, such that it is distributed evenly over the assembly surface. In this way, high volumes of airflow can be generated at quite low velocities. In general, the higher the volume of forced airflow the smaller the difference between joint temperatures across an assembly. A further advantage of forced convection is the lower infra­red emitter temperatures required.

By dividing the infra-red soldering machine into distinct and isolated zones, each of which use forced convection, greater control over assembly joint temperatures is available. Typically, air is taken into a zone from the assembly area, recirculated then diffused through heater elements back to the air intake. This effectively segregates each zone from its neighbour and gives a high degree of temperature isolation and controllability. Control of assembly temperature within such a soldering machine is achieved simply by standard computer and electronic closed-loop control principles, allowing high degrees of accuracy and adjustment.

Air is not the only gas which can be used in convection infra-red SC processes. An inert gas, say, nitrogen, can easily be incorporated into the system instead.

The ability of infra-red soldering machines to produce perfectly soldered assemblies depends on temperature uniformity over all the joints to be soldered. As an assembly passes through a soldering machine, the ideal is to maintain a constant temperature at all joints across the assembly; in other words there should be no difference in temperature between any two points on the printed circuit board. This elimination of temperature differential is often known as a zero.

Temperature differentials between joints can be caused by a number of factors, including:

• assembly variations - board size, component densities, component masses and so

on;

• heating elements - some infra-red heaters do not dissipate heat uniformly onto the

assembly surface;

• conveyor systems - which extract heat from an assembly local to the conveyor

supporting arms.

Hot vapour soldering

Hot vapour soldering processes saturated vapour, condensation, or vapour phase (VPS) soldering processes.

Although classed here as a convection form of heating, heat transfer in a hot vapour soldering process takes place when a saturated vapour condenses on the board, and is thus a product of the liquid's latest heat of evaporation. If a liquid is selected with a boiling point of that required to convert the solder paste into molten solder (around 215°C to 230°C), then once that temperature has been reached, no further condensation can take place so no further temperature rise can occur. Upper temperature control in the process is therefore simply not required - a significant advantage over other SC soldering processes, particularly those using infra-red heating elements.

In turn the equipment, in principle at least, is extremely simple. All element heats the liquid to boiling point, while the assembly is positioned in the resultant vapour above the liquid. Liquid used is a perfluorocarbon (this is no environmental threat). Times to reach soldering temperature range from as little as just 5 or 6 for small joints, to around 50 for large joints. The vapour also removes flux and flux residues in a washing action after soldering has taken place, reducing the requirement for post-assembly cleaning.