Authored By:
Leslie (Kwangsuk) Kim, Wei Yan, James Martin, Graham Lee
MacDermid Alpha Electronics Solutions
CT, USA
Summary
Since Multi-Layer Printed Circuit Boards (MLB) with holes appeared in the market, the processes making holes conductive (MHC) have played critical roles to achieve MLB’s function. Current trend of technology requires much lighter, thinner, smaller parts and this trend brings the market lots of technical challenges which we need to overcome. The reliability of Laser drilled Micro Via Hole (MVH) is getting more critical.
Electroless Copper process is a Copper plating by chemical reaction (oxidation-reduction reaction) to provide conductive layer between layers of the PCB. It is composed of 6 steps such as conditioning, micro etching, Pre-dip, Palladium activation, acceleration, Copper plating and generates Palladium/Copper layer on the overall surface. Because Electroless Copper plating is also occurred on base Copper area, there still can be interfacial defects like D-Sep and ICD for Though Hole and Nano/Micro-Void, Separation at the target pad of MVH. The factors causing these defects are internal stress of Electroless plated Copper, H2 gas generated during reaction in the Copper bath and excessive activation, etc. Specially for MVH, while hole size goes smaller, these defects come more.
Direct Metallization (DM) is another MHC process which is an alternative process and mainly represented by colloidal conductive carbon-base process (Carbon black or Graphite). DM is composed of simple 3 steps including conditioning, conductive colloid adsorption and micro-etch removing carbon particles from the Copper area including target pad. Because DM provides direct bonding between Base Copper and Electro plated Copper without additional layer, it can dramatically reduce interfacial defects commented above. DM enables a simpler MVH structure with only one interface while removing the chance for Electroless Copper’s signature nano/micro voiding to promote more reliable electronics even with smaller MVH size. Additionally, non-dynamic conductive colloid bath brings us stable performance and quality of product without side effect like cannizzaro reaction of Electroless Copper process.
Conclusions
The microvia has been the primary enabler of high-density interconnect since its inception but concerns over weak via target pad interfaces have so far limited the usage of this design feature in devices that require very high reliability. As HDI designs are becoming more widely utilized in mission-critical and safety-oriented applications, the reliability of the microvia hole is getting more critical.
There are many factors related to the microvia reliability such as excessive Pd, oxidation of the target pad, control of hydrogen gas during plating of copper, electroless Cu grain refiner concentration, SPGR of electroless Cu bath solution, chemical etching treatments, rinsing, solution movement, the condition of the copper at the target pad before plating begins including the presence of recast copper, surface roughness, etc., but many of these factors can be removed with direct metallization and it provides strong Cu(base Cu)-to-Cu(electro-plated Cu) direct bonding with no additional weak layer between them and it allows to move forward to apply more HDI technology to the wider market.
Additionally, direct metallization is a green technology with no use of precious metal like Pd and harmful components like cyanide and formaldehyde. Water usage, power usage, waste treatment also much lower than electroless Cu and these contribute to its sustainability.
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