Bond assembly can be done via 1) mechanical attachment, 2) adhesive bonding of which epoxy bonding is one form of adhesive, 3) soldering bonding using lower melting filler metals (< 450˚C), 4) brazing using filler metals melting above 450˚C, and 5) welding such as resistance welding bonding, ultrasonic welding and friction weld bonding that uses locally melted parent metal.
Bonding is done for a variety of technical reasons a) mechanical attachment, b) thermal contact, c) electrical contact d) gas or liquid seal, or e) any or all combinations thereof. The choice of bonding method will then depend on the intrinsic properties of the bonding filler materials ( i.e. hermetic, electrical conductance, thermal conductance, thermal coefficient of expansion, adhesive bond strength related to the intrinsic fillers’ mechanical properties, and their adhesive and cohesive strengths).
Practical Bond Considerations
With all these variables and design considerations how does one choose the appropriate bonding material? The three main guiding principles are:
- Cost of filler and cost of bonding processes
- Performance in service (based on the properties of the bond and bonding materials)
- Compatibility with manufacturing sequence.
To compare epoxy bonds to solder bonds one first has to ask the purpose of the bond. Is this strictly a mechanical bond? Is cost a large factor? If cost drives the choice then many times epoxy is the ideal bonding material. Epoxies are generally low cost thermosetting polymers, mixed chemicals that are thermally or UV cured to achieve hardness and adherence. Epoxy by far is the lower cost material over solder metal fillers and thus if low cost is the driving aim of the bond, then epoxy will be the bonding material selected.
Figures 1-2 illustrate typical epoxy bonded applications.
When bonds have to be thermally conductive or electrically conductive solders are usually the bonding material of choice. Solder are metal fillers melt below 450˚C are normally alloys of Sn, Ag, Pb, In, or Bi with the Pb-free alloys being preferred for environmental reasons. As metals, these materials are intrinsically 10 – 100 time more conductive than epoxy bonds. In recent year epoxy bonds have been filled with aluminum or silver particles to increase the epoxy bond filler conductivity to values of 3 – 5 W/m-K from 0.5 – 2 W/m-K. When compared to solder bond metals with conductivities of 40 – 400 W-m-K, one can see for thermal bonded components that solder bonding would be preferred. S-Bond Technologies makes active solder alloys that bond to metals, ceramics, glass and their combinations without the need for flux or plating and are many times selected over epoxy bonds for their improved thermal characteristics.
Figures 3 – 4 illustrate typical solder bond applications. Figures 5-7 show the solder bond process being used to make a heat exchanger.
Bonding for electrical resistance or conductance will many times determine the choice of epoxy bonds over solder or active solders. If the bond joint has to provide electrical isolation, then epoxy has much higher dielectric strength and resistivity, hence are excellent at isolating electrical components from their base materials. However, if the bond has to be electrically conductive solder bonds are preferred.
Bonding for seals are a mixed choice. In the short term epoxy seals can perform and create a sufficient seal for liquids and many gases. However, in applications for long term use epoxy bonds are permeable to certain gases and moisture and are not used in seals that require high hermetic seal integrity. Metals are impervious to moisture and gases thus solder bonds are the preferred bonding materials for high integrity hermetic seals.
Epoxy bonds are “permanent” and less resistant to thermal cycle and temperatures as well as UV exposures (can degrade with time). Solders on the other hand being metallic can be remelted repeatedly to renew or rework the bond. Additionally, as metals, solders are resistant to cracking being ductile and tough and are not susceptible to UV degradation.
Finally, the issue of compatibility with manufacturing sequences and the choice of solder bond vs. epoxy one has to select the bonding materials that will suit not only cost but the sequence of manufacturing operations. The bond has to have the properties that will take the exposure to all the assemblies operations. Bonding is many times completed after machining and fabrication but before plating or coating. If an electrical package the bonding has to be done in a compatible sequence with the electrical soldering operations. For example if a printed circuit board needs to bonded to a heat sink solder bonding the circuit board has to be done a temperatures below the solder reflow temperature on the circuit, for example below 200˚C or one has to epoxy bond the circuit board with a thermally filled epoxy. Compared to solder bonding, epoxy bonds can be less expensive in a manufacturing operation with no need for heating and reflowing solders. On the other hands solders “cure” as soon as the heating is off, while epoxy bonds need “setting time” to cure, which in some high volume applications provide some problems. When electroplating a full assembly, the bonded parts need to be bonded electrically hence solder bonding is the choice, while if powder coating, the epoxy bond may be the bond of choice.
This blog discussed how the choice of epoxy bond vs. solder bond is determined by a host of factors that need to be considered. We hope the discussion has been useful. If you need help in making the choice of epoxy bond vs. solder please Contact Us, we can offer the proper counsel for making the right choice and we also offer alternative active solder technology to bond without flux and pre-plating.