Reflow Soldering

As reflow soldering has increased in popularity the use of surface mount technology has become dominant in electronics assembly. The process essentially involves the use of a screen printed alloy paste to solder components to a printed circuit board. The move to Lead-Free assembly means that many process parameters will need to be examined and possibly adjusted if successful assembly is to be achieved. These factors can include the board design, the choice of materials, the reflow equipment itself, the process parameters used and training of operators. Conversion to Lead-Free reflow can be a relatively straightforward process as long as the issues are understood and taken into account. This section gives an overview of some of the important aspects.

Increased soldering temperatures

The need to achieve higher peak soldering temperatures with many Lead-Free solders will mean modifications to both reflow equipment used and processing parameters. With tin-lead solders the peak reflow temperatures used are typically 30 to 40°C above the melting point of the alloy. Initially there were concerns that using the same incremental temperature increases for Lead-Free solders could cause problems with materials and components For example SAC alloys typically melt between 215 and 220°C and using peak temperatures of 30 to 40°C above this would mean circuit boards and components being exposed to temperatures of 255 to 260°C. Fortunately, it has been found that, by introducing better process control, peak soldering temperatures may only need to be around 15 to 25°C above the melting point of the solder for most applications. This will mean peak reflow temperatures of around 230 to 240°C. Keeping the reflow temperature as low as possible is important for a number of reasons. For example, the higher the maximum temperature the greater the stresses that the board and its components will experience. Stresses caused by the thermal expansion increase significantly above the glass temperature of the circuit board which can lead to problems such as failure of interconnects in the barrels of plated through holes. Using too high a temperature during the first pass or double sided assemblies can cause increased oxidation on the second side, which may cause solderability problems on the subsequent second pass. Avoiding excessive temperatures also helps to reduce intermetallic formation, especially in solder joints exposed to multiple solder cycles as well as reducing the potential for 'popcorning' in components that have been exposed to moisture.

Board design

A narrower process window may well be possible with many boards but thicker more complicated boards with high component densities will be more difficult to accommodate. Careful thermal profiling will be needed for a successful reflow profile especially for more complex board designs. The important factors for optimizing thermal profiles are; assembly size and weight, component density, mix of large and small components and type of solder paste being used.

Choosing solder pastes and fluxes

In most cases the type of solder alloy used for reflow will probably be tin-silver-copper alloys; however there are still a number of other variables that will need to be considered when choosing a solder paste. The Lead-Free solder alloy should offer good mechanical reliability and thermal fatigue resistance, good wetting, a relatively low melting temperature and should be compatible with a wide range of printed circuit board solderable finishes. Flux selection is also an important issue. Solder pastes are often available with a range of fluxes that are suitable for Lead-Free processing so it is important that the paste chosen offers the correct combination of flux activation temperature, activity level and compatibility with the selected Lead-Free alloy. Fluxes also have an impact on subsequent reliability so it is important to consider the impact of flux choice on the surface insulation resistance (SIR) and electromigration properties of the assembled board.

There are likely to be differences in behaviour from the tin-lead pastes used for traditional reflow soldering. Shelf-lives and the storage conditions for Lead-Free pastes may be different to those used for tin-lead eutectic solder pastes. The paste handling recommendations provided by the manufacturer should be strictly adhered to.

Solder paste printing

Once a suitable solder paste has been chosen it is important to understand how it performs during the printing process.

The printing process will need to be adapted in order to ensure that good solderability and yields are achieved during the reflow operation. It is very important to follow manufacturer's guidelines, for printing with Lead-Free solders. In terms of general performance, the Lead-Free solder paste characteristics tend to offer similar results to those of tin-lead based pastes. However, one key factor that must be understood in terms of its implications for stencil design is that Lead-Free pastes generally have higher surface tensions and do not wet or spread on the pad surfaces as readily as tin-lead solder pastes. Stencil apertures with dimensions that would be suitable for tin-lead alloys can lead to the occurrence of exposed solder pad finish material after reflow soldering when a Lead-Free alloy has been used. This may mean the modification of the stencil apertures to increase the level of paste coverage on the pads.

Reflow profiles

To achieve a successful reflow soldering operation, the reflow profile must be optimised for the particular alloy chosen. Details of optimal reflow profiles are given in the JEDEC/IPC standard J-STD-020 revision C. The figure X shows the range of temperature profiles that are compliant to the JEDEC standard and table Y list the reflow parameters and peak temperatures that JEDEC has recommended. There are two common types of profiles used in the reflow soldering process; soak profiles and tent profiles. A soak profile involves subjecting the assembly to a temperature just below the melting point of the solder for a period in order to achieve a uniform assembly temperature. The tent profile uses a continuous temperature ramp from the time the assembly enters the oven until it reaches the desired peak temperature. The type of solder paste and its chemical composition will determine the suitable profile. The paste supplier should be able to help specify the best profile in order to achieve maximum performance. It is important to try and keep peak temperatures below the recommended maximums shown in table Y and also to minimise temperature gradients across the board. This will reduce thermal stresses on both the board and components.

Inerting

Issues related to the narrower process window for Lead-Free can be alleviated by using modern reflow ovens with forced convection, greater numbers of heating zones and tighter process controls. Reflow ovens equipped with a nitrogen atmosphere have been shown to give improved wettability at lower peak temperatures and to reduce temperature gradients across the board especially with double-sided assemblies. The use of nitrogen helps to protect board surfaces through multiple reflow passes, prevents oxidation of pads and leads, allows better wicking of solder, and produces shinier solder joints. However there may be significant costs associated with using a nitrogen atmosphere and ovens can require more maintenance.

Changes to processing equipment

The capabilities of older equipment can be inferior to that of new machines, for example, older infra-red reflow ovens may not have as good temperature control as modern convection reflow ovens. To be certain that the whole of the PCB and all of the components to be soldered attain the required soldering temperature, it may be necessary to have much tighter control of all stages of the reflow process, including the preheat and peak temperatures. This will require equipment with sufficient power, close temperature control and ideally six to eight zones. Many reflow ovens will be perfectly capable of being adapted for Lead-Free assembly however the need to replace older infra-red ovens could be a significant cost burden. Operator safety when running at higher reflow temperatures must also be considered. Higher reflow temperatures could also mean higher energy costs, even though newer equipment does offer better efficiency. Modern ovens can show over a 40% reduction in energy consumption when compared with older, traditional ovens. This increase in thermal efficiency has been achieved by improving heat transfer capabilities and airflows. Existing machines may need to run slower to enable boards to reach the required temperature and that forced cooling could also be needed. This can actually be beneficial, since it has been found that it helps to produce fine grain structure solder alloy joints.

Higher temperatures used with Lead-Free solders may cause some boards to warp during soldering. The laminate type used the board design and the position and type of components used will all have an affect. One way to reduce warping during reflow is to use a centre board support conveyor system. This is becoming more common although on some reflow ovens using a centre board support system can block the air flow and affect the temperature consistency across the board.

Reflow soldering summary

Lead-Free reflow is significantly different from reflow with tin-lead based alloys and the process window is much narrower. If good quality solder joints are to be achieved with the required yields careful attention has to be paid to material, equipment, process optimisation and control. Once a robust process has been established there is no reason why Lead-Free reflow cannot be made to perform well.