Historically, lead solders have been the conductive bonding materials of choice, mainly due to the reliable performance and availability of the base materials. However, as the electronics industry strives to create more environmentally and worker friendly electronic products, lead-free solders have been developed and ECAs have gained greater acceptance as a solder alternatives.
ECAs come in many shapes and sizes, varying in base chemistry (epoxy, urethane, acrylic, silicone, etc.), as well as choice of conductive filler material (silver, copper, gold, aluminum, nickel, carbon etc.). However, of all the different ECAs on the market, silver filled epoxies provide the most viable alternative to solder.
This post is dedicated to explaining the major practical differences between lead-free solder and silver filled epoxy products in an effort to help engineers understand which product they should choose for their current application.
Lead-free solders were created to reduce the toxicity associated with the normal lead based products. Rather than lead, these solder alloys are generally comprised of such metals as tin, copper, silver, and antimony.
Depending on the particular alloy, the melting point and mechanical properties vary. However, most lead-free solders have a melting point of about 200C. This is higher than standard lead-based solders, which melt at around 180C.
The higher melting point of lead-free solders yields some performance benefits compared to a lead based solder including:
- Improved tensile strength
- Greater thermal fatigue resistance
However, there are also several disadvantages associated with lead-free solders:
- There is no completely lead-free solder
- They require higher assembly temperatures
- They do not behave the same as lead based alloy in flux systems
- They do not wet as easily to form the same intermolecular bonds with substrates
- Modified fluxes may be required to combat the changes in behaviour
Ultimately, in comparison to silver filled epoxies, lead-free solders provide higher thermal and electrical conductivity, and are very similar to traditional lead based systems. However, lead-free solders are often prone to stress cracking on flexible surfaces and are not easily modified. As a result, lead-free solders are increasingly being traded for more versatile polymer alternatives.
Silver Filled Epoxies
Unmodified epoxy adhesives are highly electrically insulating materials, however, these insulating properties can be worked around with the addition of electrically conductive metal powders prior to polymerization. When metals such as silver are added in substantial volume to an epoxy resin, the final cured product will contain electrically conductive pathways (due to consistent metal-metal contact throughout the polymer matrix) allowing an electrical current to pass through the bulk material.
Silver can be added at any concentration, and many different filler particle sizes and shapes are available. As a result, silver filled epoxies can be tuned to provide highly specific conductivity/resistance values, or provide conductivity only in a specific direction.
Depending on the filler particle concentration, shape, and size, silver filled epoxies can fall into one of two categories:
- Isotropic adhesives contain a large amount of silver filler (up to 35% by volume) and exhibit conductivity in 3 dimensions. This is the most common type of conductive adhesive because the metal particles touch (due to high volume ratio) and therefore provide a direct electrical pathway.
- Anisotropic adhesives, by comparison, only contain 1-5% metal filler and only exhibit conductivity across the bond line (and are electrically insulating in the other 2 dimensions). The metal fillers are normally spheres with a diameter the exact thickness of the intended bond line. In this way, when interface materials are pushed together, the bond line is reduced so that the metal particles make contact with both materials, and an electric pathway is created.
It is worth noting that although silver concentration and bulk conductivity are correlated, past a certain filler concentration the polymer product will drastically lose its adhesive capabilities due to the proportionally reduced polymer matrix. As a result, silver filled epoxies are not able to achieve electrical and thermal conductivity values that are directly comparable to solder alternatives – although electrical and thermal values are still sufficient for nearly all applications. Generally, silver filled epoxies are able to provide the following values:
- Thermal conductivities up to 7 W/mk
- electrical resistances as low as 1×10^-4 ohm⋅cm
Conductive epoxy adhesives can be manufactured as one or two-part systems, both of which offer an alternative assembly procedure to that of solder:
- One-part systems such as Acura 80101 require heat (normally 100-150C) in order to cure the final product, and occasionally require refrigeration to insure long shelf lives. These systems, however, provide high temperature performance and don’t require mixing upon application.
- Two-part systems such as Acura 80201 can cure at room temperature, providing a “cold solder” alternative. These systems do require mixing, however, mix cartridges and burst packs can be utilized to avoid the difficulties normally associated with mixing.
The base epoxy resin can also be modified to provide a range of mechanical properties. Unlike solders, epoxies can be formulated to be flexible, increasing adhesion to flexible substrates and reducing stress on bonded components.
Silver filled epoxies also provide the following advantages over lead-free solders
- Environmentally friendly
- Simpler processing
- Lower processing temperature
- Can bond to heat sensitive components (unlike solder)
- Resilient to mechanical and thermal stress
Ultimately, silver filled epoxy adhesives are the only alternative for non-solderable, temperature sensitive, and highly flexible substrates. Silver filled epoxies are also much more versatile than solder alternatives and thus find use in a wider range of applications.