Bonding is the process of establishing a firm chemical bond using an adhesive material for securing mechanical and electronic parts to a mounting surface. Bonding provides a cost effective method for reducing total structural weight while reinforcing strength.
Bonding Surface Preparations
The substrate surfaces to be bonded must be abraded to impart sufficient roughness to the area. This is a critical step in effecting reliable adhesion and maximum strength to the bond.
The cleaning solvents are acetone and alcohol.
Acetone is used to remove oils, waxes, and greases from the substrate surface, while alcohol is employed for the final cleaning. The surfaces are usually cleaned before and always after the abrasion process. The solvents are then gently baked out by heating the surfaces in a convection oven. Heat guns may be used in cases where oven use is not feasible.
Black anodized aluminum is generally not abraded for bonding applications.
G-10 epoxy fiberglass (heat resistant) blanket stand-off buttons and Kapton blankets (multilayer insulation) are substrates that require abrasion prior to bonding.
Z306 paint and iridite coating must be removed from the aluminum surface, followed by abrasion before bonding can be effected.
Cleaning the abraded surface with an approved clean solvent is mandatory.
Aluminum surface preparation:
Use Kapton or Mylar tape (1/2 inch) to frame the area for abrasion.
First clean the surface with a precleaned cotton fabric and ethyl alcohol (200 proof).
Then abrade the surface with aluminum oxide sandpaper or with a wire-wheel brush (using a drill or dremel tool).
Give the abraded surface a deep cleaning using a lint-free cloth with acetone as the solvent.
Then clean the surface with a lint-free cloth using 200 proof ethyl alcohol as the solvent.
Dry the abraded surface using an oven or heat gun.
Allow the aluminum to cool before polymeric applications are performed.
If heat drying is inappropriate, sufficient time must be allowed for it to dry out at room temperature.
Thermally nonconducting epoxy adhesive is applied to bond parts that function only as structural enhancements. Cable tie-down metal clips, blanket buttons, velcro, and various kinds of fasteners fit the category of structural aids.
Thermal sensing parts are structurally bonded with a thermally conductive epoxy adhesive. PRT's, thermostats, thermistors, and heaters are bonded in this way. Normally, foil heaters are bonded with an approved acrylic pressure sensitive adhesive. Special high temperature applications are usually bonded with high temperature epoxy adhesive.
General Bonding Acceptance
Bonding shall be tack-free and fully cured. The adhesive must have good adhesion between the part to be bonded and substrate surface, with no visible lifting or peeling. The bonding material shall be free of air bubbles. All bonding shall be free of contamination and foreign material.
A mixing record and witness sample shall be completed for each batch of polymeric material. Excess material shall be removed from edges of the bonded area. Any smears of adhesive on the parts shall be removed. Clips, PRT's, thermostats, thermistors, blanket buttons, and velcro shall show evidence of 100% squeeze out.
There shall be no bonding material on terminals. Specific acceptance criteria may apply for fasteners, buttons, velcro, clips, etc.