Picking the right soldering paste can bring forth more questions. Variations in wetting characteristics, void control, flux residue, alloy strength, alloy flexibility, and other performance measures can all play significant roles. For example:
Quality can be maximized by choosing a paste that has the best performance with the materials, geometry, and heating processes used to manufacture a product.
Throughput can be maximized by picking a solder product that accommodates the optimal deposition and heating methods.
Cost Goals is dependent on the quality and throughput, and can be maximized through consideration of material, direct labor, inspection, rework, and scrap values.
It can be overwhelming.
In the end it all comes down to your priorities. However, to help narrow the field, there are a few important considerations when it comes to choosing the right soldering paste.
Lead vs. Lead-Free Soldering Paste
The main practical difference between lead and lead-free soldering paste is the melting temperature. Lead-free soldering paste has a higher melting point, so it is a bit harder to work with.
When it comes to deciding which to use, the most important factor in deciding between lead and lead-free soldering paste is the target market. As noted in our recent blog focused on RoHS, it can potentially be costly to ignore RoHS directives. If you are creating a product that will be sold to consumers and/or to an international market, it is in your best interest to be RoHS compliant and avoid lead-based soldering pastes. Even though they are typically more expensive, it is an investment that can save you more in the long run.
Water-Soluble vs. No-clean
This decision often comes down to cosmetics. Water-soluble fluxes contain high molecular weight compounds, such as polymers, which are not as effective as rosins/resins at preventing re- oxidation. Water-soluble pastes give boards a cleaner look after the board passes through the reflow stage, the flux residue is burned off and easily washed off in a board washer. If you opt for no-clean, the flux’s function is the same as the water wash paste but the residue remains on the board. No-clean chemistries are generally rosin/resin-based materials. Rosins/resins make excellent oxide barriers and protect the “cleaned” surfaces during reflow from re-oxidation. In addition to detracting from the aesthetics, experts have mixed opinions about the consequences of the remaining residue. Some claim the leftover flux residue becomes inert, but others contend it can have negative impact on the product later in the lifecycle.
Selecting the Right Alloy Ratio
When choosing a solder alloy, there are four key considerations: lead content, melting temperature, alloy powder particle size, and tensile strength.
The solder paste alloy ratio is a decision best left to your manufacturing engineer, as they are the experts in regard to the reflow ovens as well as which alloy ratios work best for the product. However, this decision can affect your costs. You can take steps to be more informed about the selection process by:
Determining what strength and other required properties will work best with the assembly.
Knowing the preferred soldering and the operating temperatures your product requires.
Familiarizing yourself with which materials are being soldered and which ones are the most compatible.
Considering the malleability of various alloys and metals.
Understanding the operating environment for assembly – for example, will it operate under extreme temperatures, vibration, or pressure?
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