Page 440 - Handbook of Adhesives and Sealants

380 Chapter Ten

than epoxy resins. As a result, polyimide adhesives can operate at
higher temperatures than epoxies or phenolics. However, they are ex-
pensive and require high curing temperatures. Two types of polyim-
ides (PI) are currently in use: (1) those that cure by condensation re-
action and (2) those that cure by addition reaction mechanisms.
Condensation PI resins were the first type commercially introduced.
Polymeric reactions that cure via condensation produce moisture as a
by-product of the reaction. This moisture can result in voids in the
bond-line and general weakening of the bond strength. Condensation
PIs are based on the reaction product of aromatic diamines and aro-
matic dianhydrides and are capable of withstanding prolonged expo-
sure to temperatures of 500–600 F and short term exposure to tem-
peratures up to 1000 F. Because of their condensation reaction and
necessary solvents to make the adhesive pliable and capable of flow,
these polymers are often processed under vacuum conditions. Conden-
sation PIs can be cured under pressure at 350 F. After about two hours
at 350 F they develop sufficient strength to permit post curing at 500–
600 F without pressure. The resulting polyimide is insoluble and in-
fusible.
Addition reaction PIs are also available. These are low molecular
weight polyimides with acetylene end-groups, which cure on heating
by addition polymerization. Polymers that cure by addition reactions
do not liberate moisture during cure. However, they still require high
temperature solvents to make the adhesive film pliable. Typical proc-
essing conditions involve a stepped heating from room temperature to
400 F under light contact pressure, and then a final cure at 550 F. The
oxidative stability of addition reaction PIs is not quite equal to the
condensation type; however they are somewhat easier to apply and
cure. For prolonged exposure, the temperature limit is about 500 F,
and it is 600 F for short term exposure.
Thermoplastic polyimide adhesives have also been developed. 20
These adhesives have less stringent high temperature cure require-
ments, but they are not as resistant to high temperatures as the con-
ventional polyimides. They have been used in microelectronics for at-
tachment of components, lid sealing and wire bonding.

10.3.7.2 Bismaleimide. Bismaleimide (BMI) resins are used in elec-
tronic circuit board manufacture and in applications in which higher
temperature resistance is required than can be provided by standard
epoxy systems, but where the extreme high temperature resistance of
a polyimide are not required. BMI adhesives are suitable for long term
exposure to temperatures up to 400 F and short term exposure up to
450 F. Systems currently available are quite rigid and for this reason
have low peel properties.