Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:email@example.com
PLATING OF CERAMIC MICROELECTRONIC CIRCUITS
Electroless nickel-boron (Ni-B) alloys are best suited for this type of circuit s. There are several types of NI-B processes, differing slightly in deposit properties. For best soldering and wire bonding, where long shelf live before soldering is not required, low boron content alloys. 0.3-1.0% B work well. Low boron deposits have the lowest resistance (7_8 micro-ohm cm). For longer shelf life solderability and wire bonding, higher boron content processes are used (2-3%). Resistance ranges from 20-60 micro-ohm cm. For best overall solder ability, low boron poly-alloys are suggested. All of these provide diffusion and migration barriers. All are suitable for die attachment by gold silica eutectic, or epoxy adhesive bonding. In some cases nickel phosphorus deposits can be used for these purposes. Los phosphorus deposits (4%or less) can be soldered and wire bonded. Shelf life is limited therefore soldering should take place soon after plating, or an over coating of gold can be applied. Nickel phosphorus deposits above 4% P tend to crack under brazing conditions making it difficult to achieve hermetic seals. Brazing to nickel-boron results in good hermetic seals as tested by mass spectrometer methods.
ELECTROLESS NICKEL PLATING PROCESS CYCLES FOR METALLIZED CERAMIC DEVICES
Plating onto molybdenum/manganese (moly-mag)
Plating onto tungsten metallized ceramic.
Electroless nickel plating onto bare ceramic (except AlN)
It is possible to plate directly onto a ceramic surface without first "metallizing" the surface with a "painted on" or screened metal frit. Preparation for plating directly on a ceramic surface takes many forms. The basic requirements are: proper cleaning, a means of developing micro-porosity in the surface to produce maximum adhesion, a means of making the surface catalytic to an elect\roles plating solution, and a suitable electroless plating process.
Ideally, a slightly porous surface (where interlocking of plating in the pores with the surface deposit can occur) will produce maximum adhesion. The adhesion is comparable with the bond strength achieved by fired-on metallization coatings. The bond strength diminishes to bonds of 1 to 5 pounds pull (1 inch wide strip), as the porosity becomes less than ideal. Resin and organic and organic coatings as preparation materials have produced bond strength of from 2- 15 pounds pull. Porosity in the ceramic surface can be controlled somewhat by the conditions under which the ceramic device is produced. The composition of the ceramic also plays a role in whether or not porosity of a suitable nature can be produced. Where there is a little or no porosity in the ceramic surface, it can sometimes be developed by etching in mixtures containing fluorides. Ninety seven percent alumina is an example of a ceramic that can be etched I fluorides to develop micro-porosity. Alumina cannot be etched effectively to develop micro-porosity suitable for plating. An alternate procedure is to immerse the ceramic in a 10% solution of KOH followed, by heat treating at 450C for 10 minutes.
PLATING ONTO ALUMINA CERAMIC
PLATING BARIUM TITANATE CERAMIC
Note: other ceramic materials have been plated using these processes and modification thereof. Examples or other ceramics plated are: Yttria, stabilized zirconia, lead zirconate, (acetic acid is added to the etching solutions), garnet ceramic, zirconium oxide, lithium noibate and ferrites.
PLATING ONTO BARIUM TITANATE
1. Alkaline clean (see above method)
2. Rinse ( ultrasonic)
3. Etch (see above method)
4. Rinse in DI water (ultrasonic)
5. Sensitize in stannous chloride (see above)
6. Rinse in DI water.
7. Catalyze (see above method)
8. Rinse in DI water.
9. Electroless nickel plate
11. Electroplate (optional) Use copper, gold, silver or rhodium or as required
12. Rinse and dry.
PLATING ONTO SILVER-FIRED FRIT
1. Mild alkaline clean.
2. Rinse in DI water
3. Nitric acid dip (10% by vol.) for 15 seconds (Alternate 30 g/L NaCN0
4. Rinse in DI water
5. Electroless nickel-boron plate. Note: if Nickel phosphorus EN is to be used, a nickel-boron strike may be
required to initiate deposition.
6. Rinse in DI water
7. Electroplate (optional)
8. Rinse and dry
PLATING ONTO METALLIZED AlN (aluminum Nitride)
Note: AlN is sensitive to alkaline solutions; therefore an all acid treatment cycle is required.
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