Where surface mount and traditional through-hole components are combined, it is necessary to use an adhesive to attach SM devices to the underside of the mixed technology board. Leaded components, having been inserted through holes in the board, are held in place mechanically while the board passes over a molten solder bath; unsecured SM devices under the board would just float off into the solder wave!
Adhesive placed between solder pads also secures the device to the board, and has the spin-off benefit that it will also provide a degree of strain relief for thermal mismatch between components and PCB.
Although solid preforms of adhesive are available, liquid glues are generally used. These can be applied to the board by screen/stencil printing, pin transfer, or syringe dispensing. After the device is placed, the wet adhesive must have sufficient wet, or ‘green’, strength to hold the device in position until cured. The cured adhesive must then have sufficient strength to hold the device to the board during the solder wave process.
Once the PCB has been soldered effectively, the hold-down function of the adhesive is complete as the soldered joints will be much stronger than the adhesive bond. However, the adhesive will still be present and so must not adversely affect the performance of the circuit in any way. In particular, the adhesive must maintain sufficient insulation properties to avoid electrical shorts, and it must not create long-term corrosion problems.
Correct selection and use of the adhesive can reduce assembly defects, particularly with the latest high-speed dispensing equipment. Recent developments have enabled a faster process and higher yields. However, this has only been possible by understanding and controlling the process surrounding the adhesive.
Glues are commonly applied by three methods:
An adhesive which is suitable for one of the above methods may not be suitable for the others. Ideally a chip-bonding adhesive should satisfy the following requirements:
There are two main adhesive families in general use, epoxies and acrylics, although experiments have been carried out with both hot melt resin systems and hybrid urethanes.
Epoxies were the commonest of the first surface mount adhesives, developed from resin systems already in use in hybrid circuit assembly. Their positive and negative features are:
- Excellent electrical and physical properties
- Outstanding adhesion to a wide range of substrates
- Excellent tolerance to wave soldering temperatures
- Soften sufficiently to allow components to be removed easily for repair
- Solvent free systems eliminate solvent bleed problems and ensure controlled rheological properties
- Cure times can be too long for high volume production, although the temperature required is lower than for acrylics, and most can be cured at temperatures as low as 100°C
- Most single component epoxies are heat sensitive, and have to be stored under refrigerated conditions (5°C) to ensure maximum shelf life
- Two component epoxies are inconvenient to mix before use and once mixed may have a short pot life and change their rheology over long production runs, possibly blocking application equipment.
Acrylic adhesives are however strong competitors:
Hybrid urethane adhesives are tough, non-corrosive and adaptable. They can be chemically engineered to be suitable for pin transfer, syringe dispensing or printing, and can be cured by heat or by relatively safe soft radiation. Highly thixotropic, these adhesives have excellent dot definition and the ability to give higher dots to improve contact on components with a greater stand-off height.
Contact-pressure adhesives, which are active long after application, could be applied to the board whenever convenient before component assembly, even by the PCB manufacturer.
Hot melt systems, in which an applicator heats and melts adhesive granules, are not yet established in the electronics industry, but have the potential to provide highly raised discrete dots with minimal slumping.
Work is being carried out on other thermoplastic adhesives, which are already being used successfully to attach substrates to packages, housings and main boards, even where there is a substantial CTE mismatch. Since there is no chemical reaction during or after bonding, these adhesives can be used in hermetically sealed units.
Thermoplastic performance is generally comparable to the thermosets, but the feature which sets them apart is that they soften on reheating, which allows easy rework. This feature is especially important with die-bonding on complex multi-chip modules (MCMs). Solvent-removable thermoplastics can also be used to bond bare die temporarily for test and burn-in, with the bond ‘unglued’ with a safe solvent which leaves no residue.
The ability to control dispensing determines whether or not a material is considered to be a satisfactory surface mount adhesive. The dot profile produced by a particular adhesive will be defined by its thixotropic recovery rate, viscosity at zero shear rate and surface tension. But it will also depend on non-adhesive parameters such as the wetting characteristic of the PCB surfaces.
Other factors that influence good dispensability and dot shape control are:
In practice, machine operation settings have to be established by process trials. Unfortunately, such settings are not universal for every adhesive grade because of different rheological characteristics. Simple substitution of one adhesive grade for another is not always possible without some tuning of the dispense machine settings.
Experience has shown that dispense nozzle cleanliness is perhaps the most common source of dispensing problems. Dot size inconsistency can often be traced back to partial clogging of the nozzle. Adhesive build-up can occur inside the nozzle and restrict flow. Partial curing of adhesives in the nozzle may occur if it is left for long periods in a warm environment. Changes to different adhesive grades can cause cross-contamination and nozzle blockage. For these reasons, it is essential to clean nozzles regularly and thoroughly.