AIT TERMS RELATED TO PRODUCTS AND APPLICATIONS
ADHESIVE BOND STRENGTH (Lap Shear)
It is a measurement of “van-der-waal” forces between the adhesives and the adherends (substrate or part).
If the bonding force is higher than the intrinsic strength of the adhesive, the failure mode is said to be cohesive failure. There is typically adhesive on both sides of the adherends.
If the bonding force is lower than the intrinsic strength of the adhesive, the failure mode is said to be adhesive failure. There is at least one side of the adherends that is “clean” or void of adhesive remains.
Lap-Shear bonding is a measurement when adhesive strength is measured by bonding two 3”x1” aluminum coupons with ½” overlap (hence lap-shear). Lap-shear is more useful for structural assembly.
COEFFICIENT OF THERMAL EXPANSION (CTE, Linear)
Measurement of change of linear length, increase or decrease, per unit during temperature change.
Typical unfilled rigid adhesive has CTE of 60 ppm/°C. Filled adhesive has CTE of 20-60 ppm/°C. Flexible adhesive has 2 or 3 times this CTE value.
CORROSIVITY and PH
Some resins and mixtures may be acidic or basic (0-14 scale) depending on the chemical make ups.
Best adhesive should be as neutral as possible.
Some curing agents may emit a corrosive component that causes the metallization of the microelectronic devices to degrade and fail.
AIT products have been engineered to have no corrosive agents and very close to neutral (~7) acidity.
DSC (Differential Scanning Calorimetry)
This is a measurement of heat generated (or absorbed) during curing.
Typically, it is measured using a device called Differential Scanning Calorimeter that measures the amount of heat per unit mass when temperature is “scanning” at 10 or 20°C per minute.
It can measure the time a large change of heat can be generated or absorbed at a specified constant temperature of cure.
When peaks or valleys disappear, the adhesive is considered “cured”.
Shorter cure time or lower temperature of curing are always preferable.
GLASS TRANSITION TEMPERATURE(Tg)
The temperature above which the adhesive changes from a hard solid like glass to soft rubber.
Typical epoxy adhesive has Tg of around 80-150°C.
Tg is a reflection of the molecular mobility or freedom at a particular temperature. Rigid molecule has high Tg.
Tg has no relationship with thermal stability of the adhesive.
Tg can be measured by DSC, DMA, or any method that measures the molecular movement of the adhesive.
HARDNESS and SOFTNESS
Shore D is used to measure hard adhesive. Shore A is used to measure softer rubber-like adhesive.
IONIC IMPURITIES
Epoxies are typically high in ionic impurities such as chloride (Cl-); ammonium; sodium(Na+); potassium(K+) and other ionic.
Free ionics may cause corrosion on metallization and thus are undesirable.
Low ionic contents of less than 20 ppm is desirable even though military specs allow over 100 ppm.
IR (Infrared Spectrum)
A “finger-print” of the resins used in a specific adhesive.
Any major changes will be represented as different peaks in the spectrum.
It measures the molecular structure of the resins.
MODULUS OF ELASTICITY (Flexibility/Rigidity)
Measurement of adhesive rigidity or flexibility.
Defined as the force per unit area (stress), necessary to induce a unit change of length (strain).
Rigid material has high modulus of elasticity.
Typical rigid epoxy has modulus of over 1,000,000 psi.
Dynamic mechanical analysis (DMA) is a measurement of the modulus as a function of temperature of a given strain rate.
OUTGASSING, RESIDUAL GAS ANALYSIS (RGA)
Outgassing is a measurement of weight given-off during an excursion of temperature such as 121°C or 150°C. Most of the weight given off is moisture absorbed within the adhesive.
NASA and ESA have specs that adhesive should lose less than 1.0% weight during 24 hour exposure at 121°C and less than 0.1% condensable.
Residual Gas Analysis is a measurement of weight loss after exposed to 121°C or higher temperature in an enclosed package. Sometimes the gaseous products may cause corrosion of metallization of the device.
POT-LIFE
A measurement of useful life of the adhesive after taken out from freezer or mixed.
Traditional adhesive industry defined pot-life as the time for the viscosity to increase 100% of its starting value.
In electronic industry, because of its delicate dispensing, and requirement for accurate amount of dispensing, AIT defines the pot-life as useful-life as the time for the viscosity to increase 20% in the original container.
Typical pot-life of 2-part epoxy is around 1-8 hours.
Typical pot-life of properly engineered 1-part epoxy should be well over 24 hours.
SHELF-LIFE
A measurement of stable storage life in specified storage condition. For example, ambient condition for film adhesive; <-40°C for single component epoxy; ambient condition for unmixed 2-component epoxy; etc.
Typically, the shelf-life is one year for AIT products. Much of its has to do with the warranty period rather the usefulness of the adhesive.
TGA (Thermal Gravimetric Analysis)
A measurement of thermal stability of a particular adhesive.
It is a measurement of weight loss when the cured adhesive is gradually heated (10-20°C/min).
Typical epoxy adhesive suffers major weight loss at around 250-350°C.
Higher TGA temperature for on-set of major degradation or weight loss suggest longer useful life at 150°C, the normal measurement of thermal stability.
Thermal stability has nothing to do with the flexibility or hardness of the adhesive.
Thermal stability is a measurement of chemical bonding forces between atoms that form the molecular structure.
Most of AIT adhesives have high thermal stability with on-set of degradation around 350-450°C.
VISCOSITY & THIXOTROPIC INDEX
Viscosity:
A measurement of “drag” or resistance to applied shear of a specific liquid or paste.
Thicker paste has higher viscosity.
Typical silver epoxy has viscosity of ~10,000 to 20,000 cps(centipoise).
Typical measurement equipment is cone-and-plate method with specific spindle number for heavier paste.
Typical adhesive is non-Newtonian, i.e. the viscosity is different for different shear rate (speed of spindle). Most paste is thixotropic, i.e. faster spindle speed, or higher shear rate, lower viscosity.
Thixotropic Index(TI):
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