Underfill And Low Temperature Cure Adhesives

A module that combines a sensor, a flex circuit and a mechanical frame has joints that a solder alloy alone cannot protect. The components are small, the substrates expand at different rates, and some of the parts are sensitive to heat. An adhesive performs several jobs at once: it holds the assembly together, it spreads the mechanical load away from the solder joints, and it protects the interface from moisture. Getting it right is a materials and process exercise rather than a mechanical one.

This article looks at two families that appear in this kind of assembly, the low temperature cure adhesive that bonds a module to a frame, and the underfill that reinforces a chip scale package, and at the parameters that decide whether either will work in production.

Why Module Assembly Uses Adhesives

A solder joint is a rigid metallurgical connection, and when the two surfaces it joins expand at different rates it accumulates strain with every temperature cycle. An adhesive placed around the joint shares that strain, because it is compliant and because it bonds over a larger area. The result is a joint that survives more cycles than the solder alone would manage.

The second reason is mechanical. A module is often bonded to a metal frame, and a screw or a clip would concentrate stress at a few points. An adhesive distributes the load, and on a flex circuit it also limits how far the circuit can move when the product is handled. The third reason is environmental: a cured adhesive layer is a barrier that slows the arrival of moisture at an interface that would otherwise corrode.

The Thermal Budget Of A Module

Every adhesive cure is a thermal excursion that the rest of the assembly has to survive. A fingerprint sensor, an image sensor or a microelectromechanical device may have a maximum exposure temperature well below what a standard epoxy requires, and the flex circuit beneath it has its own limit. The cure schedule therefore has to be chosen from the most sensitive part on the board rather than from the adhesive datasheet alone.

This is the reason a low temperature cure system appears in these assemblies. A cure in the range of 70 to 80 degrees Celsius over a period of about fifteen minutes keeps the assembly below the threshold where a sensor or a plastic package is damaged, and it also reduces the residual stress that a hot cure would lock into the joint. The trade is a longer cycle time and a stricter requirement on the curing equipment.

Low Temperature Cure And Process Control

A low temperature cure adhesive is formulated with a catalyst that reacts at a lower temperature, and the reaction is sensitive to both temperature and time. The two parameters define a cure window rather than a single point, and the window has to be characterised for the oven that will be used in production, because a conveyor oven and a batch oven deliver heat differently.

The degree of cure matters more than the schedule on paper. An undercured adhesive has a lower modulus and a higher moisture uptake, and it may continue to cure slowly during the life of the product, which changes the stress state of the joint. A simple check on a production sample, such as a hardness measurement or a differential scanning calorimetry run, confirms that the cure is complete without relying on the oven set point alone.

Underfill For A Chip Scale Package

Underfill is a low viscosity adhesive that is dispensed along one or two edges of a package and drawn under it by capillary action, filling the gap between the chip and the board around the solder bumps. Once cured it couples the chip to the board mechanically, so the strain that a temperature cycle would apply to the bumps is shared with the surrounding material.

The process depends on the gap height, the bump pitch and the surface energy of the board and the chip. A gap that is too small or a surface that repels the adhesive produces an incomplete fill, and an unfilled region leaves the bumps at that corner to carry the full strain. The fill is normally verified by inspecting the fillet along the remaining edges and, on a sample, by sectioning the package to look for voids. The pad and solder conditions that the process starts from are the ones described in pad design standards, since a well formed bump is easier to underfill reliably.

Matching The Coefficient Of Thermal Expansion

The coefficient of thermal expansion of the cured adhesive should sit between the values of the two materials it joins, so that the differential movement is shared rather than concentrated at one interface. A filled adhesive achieves this by carrying a mineral filler that lowers the expansion of the polymer toward that of the board and the metal frame.

The filler loading also raises the modulus and the viscosity, so the three properties move together and cannot be optimised independently. A formulation with a high filler content has a low expansion and a high stiffness, which is good for strain sharing and harder to dispense into a narrow gap. The selection therefore starts from the gap and the dispensing route, and then moves to the expansion that the thermal cycle requires, rather than the other way round.

Viscosity, Dispensing And Voiding

Viscosity is quoted at a temperature and a shear rate, and both matter because the adhesive is pumped through a needle and then has to flow into a gap where the shear rate is very different. A material that dispenses well at a high shear rate can be too thick to flow under a package, which is why the datasheet value alone is a poor predictor of behaviour on the line.

Voids form when air is trapped during dispensing or when the adhesive cures before it has filled the cavity. A controlled dispense pattern, a substrate held at a slightly elevated temperature to reduce viscosity, and a short dwell before the cure starts all help. The needle diameter should be chosen so that the bead is placed where it is wanted rather than relying on the adhesive to flow around a corner, because a large unfilled region is not recoverable after the cure.

Rework And Field Reliability

Rework is the parameter that is usually sacrificed when an adhesive is selected for strength, and it should be considered at the beginning rather than after a failure. A cured thermoset cannot be melted, so a reworkable system is either a different chemistry or a formulation that softens at a temperature above the service range but below the damage threshold of the assembly. Where a reworkable route exists, the process should be defined with a temperature limit and a maximum number of cycles, because each cycle consumes some of the reworkability.

The reliability of the finished joint is then verified by the usual environmental tests, with the addition of a check for adhesion after thermal cycling and after damp heat. Loss of adhesion at the edge of the adhesive fillet is the first sign that the system is absorbing moisture, and it usually appears before an electrical failure. Where the assembly also needs protection at the board level, the considerations in conformal coating of a board and the dispensing practice described for potting and dispensing adhesives apply to the same interfaces.

FAQ

Can a low temperature cure adhesive reach the same strength as a standard epoxy? A modern formulation can approach it, but the cure has to be complete. Confirm the degree of cure on a production sample, because an undercured joint has a lower modulus and a higher moisture uptake.

How is an underfill fill front checked in production? The visible fillet along the edges of the package is the routine check, and sectioning a sample confirms that the fill is complete and free of voids where it cannot be seen.

Should reworkability be specified at the start of a project? Yes. Once the adhesive chemistry is chosen, the rework route is largely fixed. Deciding the requirement after a reliability failure usually forces a change of material and a requalification.