Conductive Adhesive Instead Of Reflow: When It Works

Most of the board is assembled in a reflow oven, and the oven is efficient, well understood and cheap to run. Some assemblies, however, contain parts that cannot go through it. A camera module with a plastic lens barrel, a focus coil wound on a moulded former, a reinforcement bonded to a flexible circuit, an antenna formed on a temperature sensitive substrate: each of these will deform, drift or degrade if it is heated to a lead free reflow profile.

The alternative for those parts is a conductive adhesive that cures at or near room temperature, connecting them electrically and holding them mechanically without a furnace. This article looks at what such an adhesive can and cannot do.

The technology is not new, but it has become more attractive as more assemblies have come to include parts that cannot be soldered at all.

Why Some Assemblies Cannot Take Reflow

A lead free reflow profile reaches two hundred and forty five degrees or more at the joint. Anything in the assembly that softens, expands or decomposes at that temperature is at risk, and the risk is not only thermal. A plastic substrate with a low softening point will distort, and a wound coil can shift on its former, changing the position of the part that has to be aligned to an optical axis.

The parts at risk are usually the ones with the tightest mechanical tolerances, which is what makes the problem awkward. A camera module has to hold its alignment to a fraction of a degree, and a coil has to sit concentric with its magnet. Where those parts are attached by soldering, the thermal excursion of the reflow pass works against the alignment that the assembly needs.

The Cure Window And What It Costs

A two part epoxy system cures at room temperature, but the schedule is not instantaneous. The material becomes tack free in tens of minutes, and it reaches a useful strength over a period measured in hours rather than seconds. Where the production rate demands faster turnover, a short bake at a moderate temperature can shorten the cure, but the point of the material is to avoid heat, so the bake is a compromise rather than the normal route.

The consequence for the line is that the reflow oven is replaced by waiting time and by space. Parts have to cure somewhere, and that space has to be held at a controlled temperature for the duration. The trade is straightforward: energy and furnace capacity on one side, floor space and work in progress on the other. A line that is short of space will find that trade less attractive than a line that is short of oven capacity.

Electrical Performance And Its Limits

A conductive adhesive conducts because it is filled with metal particles, usually silver, which form a network through the cured resin. Its volume resistivity is measured in the region of ten to the minus four ohm centimetres for a typical silver filled system, and a heavily loaded formulation can reach a lower decade. That is far worse than copper or solder, and several orders of magnitude away from a soldered joint.

The consequence is that the adhesive is suitable for grounding, for a jumper between two points and for electromagnetic sealing, and not suitable for a high current path. A connection carrying an amp through a joint of a few square millimetres will have a measurable resistance and will heat, and the heating is what limits the application. The material is a connection, not a conductor, and the design has to be arranged so that the current is small or the joint is large.

Mechanical And Environmental Behaviour

The mechanical figures look adequate on paper. A silver filled epoxy bonded to aluminium will show a shear strength in the range of ten to eighteen megapascals, which is more than enough to hold a small component. The complication is where the joint fails. With a low surface energy plastic, the failure often occurs in the plastic rather than in the adhesive, which means the measured strength of the joint is limited by the substrate rather than by the material.

Environmental behaviour has to be qualified rather than assumed. A damp heat exposure at eighty five degrees and eighty five percent relative humidity for a thousand hours will reveal a drift in volume resistivity and a loss of bond strength, and different formulations differ by a factor of two. A thermal cycling test of five hundred to a thousand cycles between minus forty and plus one hundred and twenty five degrees is the baseline, and automotive applications add vibration on top of it. Those tests are what distinguish a material that works from one that works in a datasheet.

Where It Wins

The applications that benefit most are those where a heat sensitive part has to be attached and where the current through the joint is small. A focus coil in a camera module is the classic case: the part must not shift, the substrate cannot take the heat, and the current is modest. Bonding a stiffener to a flexible circuit is another, since it replaces two operations with one and removes a reflow step from the sequence.

Electromagnetic sealing is a third. Where a conductive gasket or an adhesive bead is used to close a seam, the requirement is a low resistance path across a large area rather than a low resistance joint at a point, and the adhesive does that job well. The materials used for dispensing and bonding on an assembly are chosen together, and a conductive adhesive is often specified alongside a conventional one on the same board.

What To Control In Production

Because the material is cured rather than soldered, the process control is different. The pot life, the working window after mixing, is measured in tens of minutes to about an hour, so the quantity mixed has to match what can be dispensed. The silver particle size distribution and the uniformity of the dispersion decide both the conductivity and the consistency of the dispensed volume, and they vary between batches.

It follows that the material should be qualified on a sample build and then held to the qualified recipe, with the mixing ratio, the dispensing pattern and the cure schedule recorded. The joint should be inspected by measuring its resistance on a test structure rather than by looking at it, because a conductive joint that looks correct can still be an open circuit. Where the assembly is coated afterwards, the protection applied over the flexible or heat sensitive part has to be compatible with the adhesive, and the layout of the board should leave room for the cure. gopcb works with customers on assemblies that mix reflow and room temperature cured connections, and will advise on which joints belong in each category.

FAQ

Can conductive adhesive replace solder for a power connection? Generally not. Its resistivity is orders of magnitude higher, so a joint carrying significant current will heat. It is used for signal, ground and shielding connections.

How long does the cure take? A tack free surface in tens of minutes, and a useful strength in hours at room temperature, or an hour at a moderate temperature where the part can tolerate it.

What is the most common cause of a failed joint? Insufficient or inconsistent dispensing, often from a batch whose viscosity or particle distribution has drifted. The pot life of the mixed material is the other frequent cause.