Before replacing solders with alternative materials, such as conductive adhesives, the following must be considered:
- Adhesives form a chemical and/or mechanical, not metallurgical, joint which may have a different joint strength.
- Adhesives and solders have different rheological characteristics, necessitating different deposition requirements and processes. They have the benefits of having a relatively lower processing temperature (eg solders typically 240-270oC, epoxy adhesives 150oC) and not needing metallised joint surfaces.
- Solders allow some component misplacement tolerance (at least 20%) whereas component placement with adhesives needs to be very closely controlled.
- The metallic content of a silver-filled adhesive is considerably lower than that of solder. When an adhesive joint is made, electrical contact is formed by a pathway of physical contacts between the silver particles and the substrate/component lead. As a result, the electrical and thermal resistance of this joint will be higher than with a solder joint. Accelerated ageing of such joints can result in an increase in the electrical contact resistance.
One other issue to consider is that moisture and oxygen permeate the adhesive and interacts with the underlying component metallisation, causing tin oxide to form. Production, and/or increase in thickness, of this oxide layer will result in increased electrical resistance. Additionally, the oxide bond is relatively weak and could result in an adhesive joint failing at the oxide/metal interface. Newer silver-filled adhesives are available that are claimed to counteract this adverse effect.
In summary, there are applications in which solder can be replaced with an adhesive. However, careful consideration must be given to the potential impact of such a change.
FAQ: Why replace solders with epoxies?