BASIC PRINCIPLES OF ADHESIVE BONDING

 
1. Adhesive Bonding is Dipolar Attraction Force: Wetting & Cleanliness are essential

While traditional soldering forms intermetallic joints, almost all  “non-soldering” bondings are non-chemical (non-atomic and non-molecular interaction) and are primarily Van Der Waals in nature.  That is, they are attracted together with dipolar forces.

Like soldering joining process, adhesive must be liquidous at some point of the bonding process to allow molecular intimacy for Van Der Waals attraction forces to take hold. While the adhesive is in its liquidous state, it must be able to wet the parts it is trying to bond together.  This wetting phenomenon assures proper molecular intimacy.  Wetting by its very nature is a measurement of the compatibility of the adhesive with the adherends.  If adhesive in its liquidous state shows inadequate wetting (“balls up” instead of spreading out), the joint will most likely be weak and potentially unsuccessful.



Adhesive materials must be compatible in the sense that the surface tension must be similar or lower than that of the adherend surfaces.  A typical adhesive such as epoxy has relatively high surface tension in the range of 80 dyne/cm, the electronic parts that it tries to bond together must also have similar high surface tension.  Luckily, most metal and ceramics that we used in electronic devices when they are clean of organic contaminants are relatively high in surface tension and successful bonding is common. Surface cleanliness may be tested with a simple drop of de-ionized water.  It the water wets the surface, the surface is generally good for bonding.  If the water droplet is unable to wet (water droplet forms a ball-like shape), the surfaces may be cleaned or modified by the following methods:

• Plasma etching:  one of the most effective methods in removing the organic surface contamination.
• Chemical etching:  is one of the oldest methods of cleaning. Different chemical etchants will be required to clean or modify different metals, plastics, and inorganic surfaces.
• Corona oxidation:  is used for modified surfaces of  polyolefins and other difficult to bond plastics.

2. Proper Internal Stress Management

As discussed by many technical papers and in the first section of this catalog, internal stress of the bonded parts must be minimized to achieve reliable adhesive joints.  In fact, the bond strength measured in typical lap-shear and die-shear testing is basically the inherent bulk strength of adhesive material minus the internal stress build-up by the adhesive bonding.  The following factors are important contributors to internal stress:

• Coefficients of thermal expansion mismatch between the adhesive and adherends; in fact, if adhesive and both adherends match in CTEs, it will be very reliable joint.  This is like the use of steel to reinforce concrete, the matching of their CTEs assures long-term reliability. Mismatches in any of the three materials will induce internal stresses in the final assembly.
• Modulus of elasticity of adhesive;  a CTE mismatches of  adherend and adhesive in the order of 30 ppm/°C. If adhesive is rigid (modulus of 1,000,000 psi) , internal stress in the level of over 5,000 psi are not uncommon.  However, if adhesive is compliant (modulus 10,000 psi), the internal stress may be as low as 500 psi.
• Flexibility for the range of temperature of interests; most of electronic devices are required to survive under repeated exposures from -65 to 150°C. It will be desirable that adhesives remain flexible with toughness in this complete temperature range.

3. Processing Considerations

Electronic devices typically undergo at least three levels of packaging related processing. Some of the following processing posts difficult problems for adhesive are:

• Wire-bonding temperature; performed from 125-250°C with ultrasonic energy. Flexible adhesive may absorb too much energy or move excessively for small die.
• Molding temperature and forces; performed at typically 175°C, the movement of molten epoxy molding compound may destroy the die-attach joint.
• Soldering; performed at 220-280°C, adhesive must be able to maintain its thermal stability and position during this processing step of device assembly
• Chemical cleaning for flux removal; some of these solvents may attack and weaken or extract undesirable materials from the adhesive (for example silicone is not acceptable because its ability to prevent soldering and subsequent bonding).

4. Usage Environment Considerations

Careful consideration must be exercised for a long-term reliable adhesive joint and finished assembly of electronic devices. Besides temperature extremes, it could entail a set of conditions that may contradict requirements listed above.