What is the process?
Alpha has supported many electronic assemblers in creating more value in their processes as they converted to low temperature SMT processing. We support many OEMs and EMS companies as we investigate the use of Low Melting Point alloys for SMT applications. The SnBiAg alloy system has a melting point of 138°C which enables a peak reflow temperature between 170 - 180°C. ALPHA® CVP-520 solder paste has an SnBiAg alloy system that meets these melting point and reflow parameters. Even though the lead-free alloy in CVP-520 has a lower melting point, it has very good resistance to accelerated thermal cycling tests often used in consumer electronics (0 to 100°C cycles).
With the new Dual Alloy SMT Process, SAC alloy is used on the top side of the assembly. The boards are then flipped, and printed with CVP-520. Large areas of overprint, and or solder preforms will be required to assure proper hole fill with the plated through hole components.
The key to the dual alloy SMT process is the ability to use solder paste to replace the wave soldering step. Overprinting and/or using solder preforms is the essential step in this process. These two techniques maximize the potential for the area that is being worked on. Extreme CVP-520 paste overprinting, followed by board reflowing with the formation of SMT through hole solder joints results in a clear, colorless residue.
When printing enough solder paste alone is not possible due to limited board space, a solder preform can be added to increase the volume of available solder. Placing preforms into the printed paste gives the increased volume of solder needed to produce reliable pin in through hole solder joints.
When is it used?
This low temperature SMT process utilizing a low melting point alloy, CVP-520, is employed to eliminate the wave soldering step in mixed technology assemblies. This can provide the greatest opportunity to create value when converting to a low temperature alloy. In a multi-technology process, there are either one or two SMT steps followed by a wave or selective soldering step. In the Dual Alloy SMT process, the wave solder step is completely eliminated and replaced by a low temperature SMT step.
By implementing an SMT-only process, a more streamlined, higher throughput process can be established. Making the switch to Low Temperature processing requires changing to the SAC305 alloy from the SnBiAg alloy used in standard SMT processing. The elimination of wave soldering is one of the biggest drivers for using a low melting point alloy for SMT.
Also, low temperature soldering can provide improved Secondary Soldering Processing that enables rework of select components. Due to the components and RF shields to be soldered on separately, an efficient rework process enables the component to be tested first. This allows the area of work to be reheated only enough for the removal of the component and not to the higher temperature necessary for the RF shield – a temperature which could damage the surrounding components.
Why are people doing it?
ALPHA’s new Dual Alloy SMT Process creates value by reducing process energy, materials and labor costs. It also eliminates the labor-intensive removal and management of solder dross. Labor savings can also be seen if the wave soldering operation and machine maintenance are eliminated.
This Process allows the electronic assembler to turn off the wave soldering operation. This saves on bar solder costs, wave soldering flux cost, the energy required to preheat wave soldering flux. Savings can also be realized by not having to maintain the temperature of the molten bar solder in the solder pot..
Incentive to convert to a low temperature soldering process includes the potential to use lower cost materials. These might include using a lower amount of silver (Ag) resulting in lower cost solder. One may also save by using temperature sensitive components as well as a lower cost, or thinner, laminates that do not need to withstand multiple high temperature thermal excursions.
The lower temperature SMP process encompasses several areas of value creation. One of these areas being that it is beneficial in that it lowers overall energy consumption by eliminating the wave soldering equipment. It is a means of leaning out processes, creating cost reductions and enabling your company to have a smaller footprint. It streamlines production and requires less maintenance and cost than the present standard SMT / Wave process. An example of these benefits is that the ALPHA® CVP-520 alloy used in our process can be used to elmininate the selective soldering and reduce labor, energy and material costs.
Will it work for me?
Low Temperature SMT processing may be the right choice for many applications, particularly those in the consumer electronics market. It is important to consider the level of fitness for use of low temperature processing in a specific process.. When Low Temperature SMT processing is contemplated, the thermal cycling, drop shock, and mechanical strength required for the end products must be carefully considered. Also, note that it is unwise to mix lead bearing components with tin/silver/bismuth: this is because of the very low temperature PbBi alloy, where the risk of the product not working properly may be too high. In addition, if the board design cannot support sufficient solder paste over printing, and the component design will not allow preform use, then it may not be appropriate to eliminate wave soldering.