The performance and long-term reliability of conformal coated PWAs depend not only on the type of coating selected and its curing conditions, but also on the nature of the surfaces to which it is applied and the cleanliness and dryness of those surfaces. Circuit boards consisting of epoxy-glass and polyimide-glass laminates can absorb large amounts of moisture especially during aqueous cleaning. Flux residues are generally hygroscopic and augment the absorption and retention of moisture. If not cleaned and dried well, a subsequently applied conformal coating such as polyurethane will blister and lose adhesion. Adhesion of coatings to gold-plated surfaces or to Teflon-insulated wire is poor and, unless these surfaces are primed, physically roughened, or chemically etched, the coating will peel off.
Specifications for Conformal Coatings
MIL-I-46058C, Insulating Compound, Electrical (For Coating Printed Circuit Assemblies) and IPC-CC-830A, Qualification and Performance of Electrical Insulating Compound for Printed Board Assemblies describe five polymer types that are widely used as printed wiring board conformal coatings. These types and their designations are:
Type AR Acrylic resin
Type ER Epoxy resin
Type SR Silicone resin
Type UR Polyurethane resin
Type XY Paraxylylene
There are also coatings that combine two of these types such as ARUR, a combination of acrylic and polyurethane resins.
IPC-CC-830A is an industry specification closely paralleling the military specification. It has become the dominant specification since MIL-I-46058 has been canceled for new designs. Other applicable specifications are MIL-P-55110E, Printed Wiring Assemblies and UL746C, Polymeric Materials-Use in Electrical Equipment Evaluations, which specifies Underwriters Laboratories’ tests for flammability. Coatings for military and high-reliability applications should meet the requirements of these specifications. In addition, for space applications, further requirements are superimposed, such as outgassing in a thermal-vacuum environment and vulnerability to radiation and atomic oxygen. A listing of organic coatings along with other polymeric materials that have been tested for outgassing and their results are given in NASA Reference Publication 1124, Revision 4, June 1997.
Qualification of Conformal Coatings
Although many coatings have been qualified to MIL-I-46058C, the current standard for qualification is IPC-CC-830A which follows MIL-I-46058C very closely, but uses many of the test methods of IPC-TM-650. Basically, qualification is performed using a test coupon: the IPC-B-25A Multipurpose Test Board that consists of several comb patterns whereby insulation resistance measurements can be made before and after subjecting the coated board to humidity/temperature cycling and other physical and electrical tests. These tests include: fungus resistance, shelf life, fluorescence, appearance, thickness, flammability, dielectric withstanding voltage, moisture and insulation resistance, thermal shock, temperature/humidity cycling, and hydrolytic stability. The conditions for the humidity/temperature cycling test are slightly different from those specified in MIL-I-46058C or MIL-STD-202. Two test conditions are described: Class T in which the coated test board is aged at 65°C ± 2°C and 90% ± 3% RH for 24 hours without bias and Class H in which the coated board is cycled from 25° to 65°C ± 2°C and 90% -5/+3% RH with 50 Vdc applied for a total of 160 hours. Insulation resistance measurements are made at 100 Vdc. Insulation resistance must be 1 x 108 ohms minimum for most electronic applications and 5 × 108 ohms for the high-end applications.
The IPC-B-25A test board is also used for the fungus test and other qualification tests. A simpler Y pattern test board is more practical for use in production for in-process control.
Qualification of PWB conformal coatings for space, automotive, and other specific applications will require meeting additional application-specific tests such as outgassing and radiation resistance for space and high-temperature resistance for automotive.
COATINGS FOR SEMICONDUCTOR SINGLE CHIP AND MULTICHIP MODULES
The use of high-performance polymer coatings in the semiconductor industry has grown considerably during the past decade. High-purity, temperature-stable polyimides and benzocyclobutenes are now widely used to complement and even replace the standard glassivation and passivation layers of semiconductor device surfaces. Further, high-purity silicones and polyimides are effective in relieving stresses on semiconductor junctions and multichip assembled devices particularly when followed by plastic encapsulation. Finally, photoimageable polyimides and BCB coatings have been developed and are being widely used to redistribute bonding pads on semiconductor devices in preparation for solder bumping.