Conductive epoxy glue versus soldering for SMT devices
Recently our quality engineers at EPR rejected a PCB after its standard soldering process. The reason: a bad soldered component (TDK Capacitor 150pF CGA3E2C0G1H151J080AD). After analysis it was clear that this particular part should be used for conductive epoxy glue instead of the standard soldering process, because it had AgPd plating on its terminals.
What is conductive epoxy?
Conductive epoxy, also known as electrically conductive adhesive or conductive glue, is a material that consists of an epoxy resin mixed with a conductive filler. These resins require “curing” or a “setting time”, before the connection is secure. This means that the glue hardens out. The conductive material contained in the epoxy is often silver (Ag) or a form of carbon (graphite). An example is Ablebond’s 84-1LMISR4.
Why should we use a conductive epoxy?
Conductive epoxies are mainly used in applications where the risks of mechanical and thermal cracking are very high, as well as when there are concerns about damaging heat sensitive components on the PCB during soldering. Environmental reasons have out phased leaded soldering, but unleaded solders have a higher melting point, and because of this, sensitive electrical components near the solder joint can be damaged during the soldering process. Another issue with lead-free solder is that it is less elastic than leaded solder. This means that lead-free solder joints will be more brittle and more susceptible to developing cracks than with lead-based solder, especially during temperature cycling.
Mounting components with conductive epoxy
The general process for mounting components with conductive epoxy is similar to solder mounting. The following table displays the differences:
Figure 1: Epoxy and solder mounting processes. Source: TDK.
The curing durations for epoxies are significantly longer than the heating and cooling times for solders. So care must be taken to make sure they are maintained at the proper temperature during curing, or the epoxy may not fully cure.
Special care must be taken when placing the conductive epoxy. In the case of solder, excessive solder can be moved back to the landing point when heated because of the surface tension of the solder. However, epoxies cure without moving, and this can create shorts and silver migration issues (which could eventually result in a short). Experience is key in preventing this.
Figure 2: Excessive solder and glue placement. Source: TDK.
As with all engineering, there are trade-offs for every type of epoxy. For example, epoxies with higher conductivity have a weaker bond strength because of the greater filler to resin ratio. At EPR we are currently running a project for a mixed bond interposer construct.
Resources:
- 1. TDK’s Guide to Multilayer Ceramic Capacitors for Use with Conductive Epoxies
- Mr. Andreas Rulek (Quality Controller at EPR)
