Design Guildelines: Metallization
The next critical parameter the designer faces is the selection of metallization. Solderability, solder types, solder hierarchy, circuit attach, skin depth, conductivity and thermal extremes of the design are all factors that must be considered in determining the proper metallization. Metallization is the single largest factor in determining lead time and compatibility of processes. Many organizations use the same metallization on every product because "that's how it's always been done." Consultation with the sales engineer or factory regarding this parameter will always result in the optimum choice for your specific application. Standard metallizations are TiW/Au, TiW/Ni/Au, TiW/Pd/Au and TiW/Cu/Ni/Au with TiW forming the main adhesion layer, Ni or Pd (optional) functioning as a barrier layer for solderable devices and Au or Cu as the main conductor. Au can be fully sputtered or plated to various values of thickness to enhance skin depth and conductivity. Resistor layers are deposited before adhesion layers due to post processing requirements. TaN
2 is the most common resistor material offering well established self-passivating characteristics. NiCr resistor films are available for those applications where minimum resistor change over temperature extremes is critical. Various sheet resistance values are available depending on the resistor range for the overall design.
|
Resistor
|
TaN, Tantalum Nitride |
8 - 150 ohms/square (1) |
| NiCr, Nickel Chromium Alloy |
25 - 300 ohms/square (1) |
|
Adhesion
|
TaN, Tantalum Nitride |
250 - 1000 Angstroms |
| NiCr, Nickel Chromium Alloy |
250 - 1000 Angstroms |
| Cr, Chromium (4) |
250 - 750 Angstroms |
| Ti, Titanium |
500 - 1000 Angstroms |
| TiW, Titanium Tungsten Alloy |
250 -750 Angstroms |
|
Barrier
|
Cr, Chromium (4) |
250 - 750 Angstroms |
| TiW, Titanium Tungsten Alloy |
250 -750 Angstroms |
| Ni, Nickel (sputtered)(2)(4) |
18 - 22 microinches |
| Ni, Nickel (plated)(4) |
50 - 300 microinches |
| Pd, Palladium (3)(4) |
18 - 22 microinches |
|
Conductor
|
Aluminum (sputtered) |
50 - 200 microinches |
| Copper |
20 - 200 microinches |
| Gold |
20 - 200 microinches |
|
High Power
Conductors
|
Copper/Nickel/Au (3)(4) |
500 - 3000 microinches |
| Gold (3) |
400 - 800 microinches |
1 - Post heat treatment
2 - Pure Nickel
3 - Limited minimum feature size
4 - Temperature limited due to metal diffusion
Plated Metals
Some designs need thicker conductors for skin depth requirements or to meet incorporated company standards. Sputtering these thicknesses can have a negative impact on cost, depositing precious metals and then etching the circuit features. Thick sputtered structures, that are subsequently chemically etched, undercut the patterned resist due to the isotropic nature of the process. This results in a cross-sectional profile that is more trapezoidal than square. Structures thicker than 100µ" benefit from selective plating as a means of creating the circuit. This processing method eliminates the negative attributes of an etch-back process. Knowles Precision Devices offers in-house plating capability where thicker materials and sidewall control is critical.
Standard Metal Systems (RoHS Compliant) 
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