Paper Key Facts

• Written in the context of the research conducted for my phD thesis in “Advanced Materials” at National Technical University of Athens.
• The goal of our team is to develop a new innovative and more environment-friendly method for the PCB through-hole plating stage with the use of Polypyrrle conductive polymer.
• Based on a paper posted at 1st WSEAS International Conference on Materials Science, Bucharest, 2008 under the title “Polypyrrole-based metallization of insulators – An application to Printed Circuit Boards (PCB)” with C.D. Yfantis and D.K. Yfantis as co-authors. Additional material is being added continuously.
• The author Spiros Kakos is a member of Professor’s D.K. Yfantis team at the Chemical Engineering school, Materials’ Sector of NTUA (Athens, Greece).

1  Introduction

Conducting polymers (CPs) were first produced in the mid-1970s [1] as a novel generation of organic materials that have both electrical and optical properties similar to those of metals and inorganic semiconductors, but which also exhibit the attractive properties associated with conventional polymers, such as ease of synthesis and flexibility in processing.

Polypyrrole (PPy) is one of the most widely used conductive polymers, due to its high chemical and physical stability, low toxicity of the monomer and its easy chemical or electrochemical synthesis, even in aqueous solutions [2] [3] [4]. PPy may be prepared by chemical or electrochemical oxidation. It has been proven that by inserting doping anions (Figure 1) like PTS (para-toluene-sulfonic acid) the conductivity of PPy can be increased up to 10² S/cm. [5] [6] [7].

Figure 1 – Doping mechanism of PPy

Applications of PPy were essentially extended during last years and include now such different fields of science and technology as corrosion protection of metals, development of individual electronic devices e.g. diodes, metallization, electromagnetic interference shielding, biosensors, tissue engineering scaffolds, neural probes, drug-delivery devices, and bio-actuators [8] [9] [10].

2  Problem Formulation

In microelectronics, printed circuit boards (PCBs), are used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive blank fiberglass substrate (“the board”). There are many types PCB substrate material, but by far the most common is a standard woven epoxy glass material known as FR4.

The complexity of PCBs, vary from single-sided boards, where circuitry is found on only one side, to double-sided boards, to boards comprising several layers of circuitry. Connections between the two sides of a board and layer-to-layer connections are made with copper-plated through-holes (PTHs) [8].

Figure 2 – PCB through-hole copper plating with PPy used as a precoat (pretreatment)

The through-holes are drilled into the laminated substrate and are then copper-plated (Figure 2).

The introduction of double-sided, followed by multilayer boards, was achieved by metallization of the drilled holes with electroless copper. Autocatalytic electroless copper plating has been successfully used for over 30 years. However, this process has certain disadvantages [11].

Firstly, electroless deposition of copper requires a reducing agent. Formaldehyde, the most commonly used reducing agent in electroless baths is toxic and poses environmental concerns. Secondly, electroless baths are generally unstable and require close monitoring. Control becomes a problem and cyanide and chelating agents have to be added, which in turn are difficult to remove from waste streams [11].

In addition to the environmental concerns about electroless copper metallization, circuit board manufacture using this process requires the use of expensive noble-metal salts, such as PdCl₂ and as many as 8- 10 steps (including rinses).The inherent disadvantages of the electroless copper metallization fostered interest in direct metallization of copper. There are processes in which the activation of the non-conducting wall of the PCB holes is effected by deposition of carbon particles or conductive polymers [11] [12].

3  Problem Solution

The aim of the present study is to investigate the application of polypyrrole (PPy) as a conductive in situ precoat for the metallization of PCB holes.

Several variables of the process at a laboratory level were studied.

In particular, the effect of the substrate pre-treatment (H₂SO₄)and the polymerization bath synthesis (addition of ΚΟΗ) on the peel strength of copper on the FR4/polypyrrole electrode was evaluated. An assessment of the peel strength of the copper plating was performed. Furthermore, the interface and the thickness of the copper electrodeposited was studied by SEM/EDS analysis.

3.1 Materials and Methods

Firstly, FR4 specimens (2,5 x 2,5 cm)  were etched by immersion for 30 min in a firmly agitated aqueous solution containing 8% pv (NH₄)₂S₂O₈. Ammonium persulfate acts as an oxidant in the polymerization of pyrrole. PPy is polymerized on FR4 surface, from an appropriate optimized solution that contains 3.6% pv pyrrole and 4.4% pv. PTS that acts as a dopant [10]. Thus, a new electrode is formed that can be electroplated.

The FR4/PPy electrode has been electroplated using the electrolytic cell that contained a copper ion bath. The composition of the copper deposition bath was:150 g/L CuSO₄*5H₂O, 50 g/L H₂SO₄, 50 g/L CH₃CH₂ΟΗ (EtOH). Copper was deposited onto FR4/PPy surface resulting in the formation of a FR4/PPy/Cu sample. After being covered with PPy film, FR4 specimens were immersed in AgNO₃ 0,1 N solution for 10 min before metallization started.

During the copper electrodeposition of FR4 substrates the current density and the metallization time were recorded.

The peel strength of copper coating (Cu) on the polypyrrole-modified FR4, was measured according to ASTM-B-193-02 specification. SEM/EDS analysis of the copper coated FR4 specimens was performed.

3.2 Results – Discussion

3.2.1 Through hole plating with polypyrrole

The results of our experiments show that the use of polypyrrole can lead to sufficiently conductive holes to allow their metallization at a later stage. We have used spraying as well as immersion techniques for the in situ polymerization of pyrrole into polypyrrole at the hole walls.

PCB copper plated holes with the use of polypyrrole [Immersion technique] {Z1A Experiment}

From the experiments conducted it seems that the use of spraying technique does not allow the full metallization of holes, especially in cases of PCB with more than 10 inner layers. Immersion technique looks like the only viable solution for such metallizations.