RTM Process

Room Temperature Metallizing For Alumina

Room-Temperature nickel metallizing process for alumina ceramics and other dielectric materials – strongly bonded nickel can be plated, soldered, brazed or welded.

UNIQUE FEATURES

  • Low cost metallization process.
  • Excellent bond strength – 100 psi typical.
  • Withstands temperature extremes: – 65 °C. to + 850 °C.
  • Passes Mil specifications for temperature cycling and shock.
  • Hermeticity – leak rate < 10—8cc helium/second.
  • Permits registration of metallization patterns with good resolution using photolithographic techniques and mechanical masking.
  • Process applicable to beryllia, barium titanate, ferrites, epoxies, mylar and other dielectrics.

RTM PROCESS
DESCRIPTION

The RTM process offers a reliable yet simple and economical method to metallize dielectric (non-conductor) materials at room temperature. This metallizing process with nickel is specially suited for commercial alumina type ceramics. The process sequence in 5 basic operations is illustrated.

RTM PROCESS

SURFACE
PRETREATMENT
BUFFERED
ETCH
SENSITIZER
AND
CONDITIONER
FIXING ELECTROLESS NICKEL DEPOSITION
POLIMET SOLUTION A SOLUTION B SOLUTION C SOLUTION D

Pretreatment of the ceramic material with Polimet lapping compound may be omitted if the ceramic material already shows surface roughness of at least 20 microinches, average. Surface lapping with Polimet compound will result in a uniform surface with a roughness of 25 microinches average.

The composition and formulation of A-B-C-D solutions used in RTM permit excellent control of the metallization process. The nickel deposit is found to be unstressed, highly conductive, and extremely adherent with strong bond strength exhibited from –60 °C. to + 850 °C. It has a hardness of at least 500 (vickers) as deposited. The adherent nickel deposit may be plated, soldered, brazed or welded.

APPLICATIONS

The RTM process is applicable to metallizing alumina substrates in hybrid and microcircuits, insulated heat sinks, ceramic tubes, windows and terminals. Strong bond structures are produced. Structures with hermetic seals are formed as well.

The RTM process is also useful for metallizing barium titanate and ferrites. In addition, many polymeric materials, notably polyesters, epoxies, cellulosics, mylar and acrylonitrile-butadiene-styrene (ABS terpolymer) may be metallized following suitable modifications of the etch (Solution A).

RTM lends itself productively to the application of photolithographic technology using photoresist materials to register metallization patterns. In this manner, conductive pads may be formed for thin film and hybrid circuits. Base plates for building capacitor structures are also quite feasible.

INSTRUCTIONS

Detailed instructions are provided with each of the solutions used in the RTM process for alumina ceramics. For many other applicable materials, suitable etchants or process modifications are recommended.

PROCEDURE FOR RTM PROCESS

ROOM-TEMPERAURE METALLIZATION FOR ALUMINA

  • STEP I. Surface Pretreatment (Polimet)
    The ceramic surface should exhibit surface roughness of at least 20 microinches, average, as shown by measurement with Brush Surf- indicator, Model MS-lOOO. If necessary, lap surface with Polimet to obtain adequate surface roughness of 25 microinches. Other means to obtain adequate surface roughness are applicable, E.G. sand- blasting. Polimet is used by converting it to a thin paste with water on a glass plate. The specimen is worked on the glass plate until the entire ceramic surface appears uniformly lapped. Rinse well in water to remove lapping compound.
  • STEP II. Etch (Solution A)Place the lapped ceramics in the buffered etch (Solution A), contained in a polyethylene beaker, for 1/2 hour. CAUTION! Solution A contains HF. Finally remove ceramic pieces from the etch solution and rinse thoroughly in water.
  • STEP III. Surface Sensitizing (Solution B)Expose etched ceramics to Solution B for 4 minutes. Do not stir or agitate the solution. Avoid contamination from metal objects. Use glassware and plastic coated tweezers or transfer racks for handling. Remove surface-sensitized ceramics, and without rinsing transfer the ceramics to the next solution.
  • STEP IV. Fixing (Solution C)The ceramics should be allowed to stand in Solution C for 4 minutes. Again, do not agitate or stir. Finally remove ceramics and carefully rinse in distilled or deionized water by limiting the water rinse to a maximum of 30 seconds. Transfer to next step immediately.
  • STEP V. Electroless Nickel Deposition (Solution D)Special Electroless Nickel (Solution D) must be held at optimum operating temperature of 90°C ready for use during the RTM Process. Addition of 12 gm/gal dimethylamineborane prior to using Solution D is an effective means of lowering the plating temperature to 65°C. The ceramic pieces are placed in this special electroless nickel solution without agitation for first few minutes. Plate for 20 minutes to obtain a nickel deposit of 0.1 mil. Finally remove ceramics and rinse in water.

GENERAL INSTRUCTIONS FOR THE RTM PROCESS for CERAMIC and POLYMER SUBSTRATES

SUBSTRATES PRETREATMENT ETCH SENSITIZER & CONDITIONER FIXING NICKEL DEPOSITION OPER.TEMP.
Alumina Polimet or Equivalent Solution A Solution B Solution C Solution D
Beryllia HF (1 1/2 min) 65 oC
Magnesium Oxide Chromic Acid
(2 min.)
50 oC
Steatite 50 oC
Barium Titanate Solution A
(2 min.)
65 oC
Ferrites HCl 20%
(1 min.)
65 oC
Epoxies Chromic Acid
(3 min.)
45 OC
Mylar Chromic Acid
(3min.)followed with 25% KOH dip and rinse
45 oC
Polyethylene 45 oC
Polystyrene 45 oC
Lucite 25% 45 oC
Cellulose
Acetate
Chromic Acid 45 oC

INSTRUCTIONS FOR THE RTM PROCESS

Applied to Lead Zirconate-Lead Titanate Pretreatment

  1. Enough surface is required to obtain surface finish of 25-30 microinches average.
  2. Soak in chromic-sulfuric HF solution 1 or 2 minutes, then rinse.
  3. Soak in Solution A for 1 minute, then rinse.
  4. Soak in potassium hydroxide 20% for l~ minutes. Rinse.
  5. Soak in stannous chloride solution for 4 minutes. Follow by 1 mm. rinse in distilled or deionized water.
  6. Continue RTM stops with steps III, IV, and V, using the solution B, C and D as indicated. Plate for 15 minutes.

Chromic-Sulfuric-HF- Solution

Chromium Trioxide 5%
Sulfuric Acid 10% by volume
Hydrofluoric Acid (4896) 19% by volume

Stannous Chloride Solution

Stannous chloride 2% with hydrochloricacid added to obtain a clear solution.

CARE OF RTM SOLUTION

Special Electroless Nickel (Solution D) has a pH of 8.0. This pH should not be allowed to drop below 7.0; although the solution is well buffered, the addition of ammonium hydroxide from time to time may be required. The use of pH test paper is satisfactory for control.

The electroless nickel solution should be filtered to extend the usefulness of the solution whenever particles occur. The glass vessel used for RTM may sometimes develop a deposit of nickel, due to overheating. The glass vessel should then be cleaned as well.

Sufficient quantities of RTM solutions should be used in the RTM Process. These used solutions, however, should not be returned to the original container to guard against contamination.

COMPOSITION OF ETCH SOLUTIONS

Buffered Etch (Solution A) – a buffered solution hydrofluoric acid.

Chromic Acid

Chromium Trioxide 7.5 grams
Sulfuric Acid 25 mL
Water to 1 liter

Metal Etch

Nitric Acid 50% by volume
Glacial Acetic Acid 40% by volume
Water 10% by volume

NOTE: Estimated Thickness = Plating Rate x Immersion Time