Metallization of polymer surface plays an important role in conductive interconnect, electromagnetic shielding, thermal management, decoration and protection of electronic products. Compared with vacuum sputtering and other methods, electroless plating has advantages of uniform coating, low cost and easy large-scale production. In recent years, polymer used for electronic applications such as epoxy resin, polyimide (PI), liquid crystal polymer (LCP), polytetrafluoroethylene (PTFE), polydimethylsiloxane (PDMS), polyethylene terephthalate (PET), polyurethane (PU), polystyrene (PS) have been actively studied. Coarsening treatment has obvious influence on the binding force of coating and the degree of coating on the substrate. At the same time, the activation method affects the speed and thickness of electroless plating. In the surface pretreatment of polymer electroless plating, coarsening and activation methods have a great influence on the coating properties, especially for LCP and other polymers that will be used in 5G equipment. In polymer electroless plating pretreatment, coarsening processes such as chemical etching, plasma treatment and graft treatment, as well as activation processes such as ion adsorption reduction, catalyst direct adsorption and ink printing also have many new developments. In this paper, the latest development of coarsening and activation pretreatment of various polymer substrates in electronic applications is summarized, in order to provide reference for the development of electroless plating technology for electronic circuits.
Contents
1 Introduction
2 Application of polymer electroless plating in electronic field
2.1 Mechanism of electroless plating
2.2 Conductive interconnect
2.3 Electromagnetic shielding
2.4 Heat management
3 Polymer surface coarsening method
3.1 Chemical etching
3.2 Plasma treatment
3.3 Grafting treatment
4 Activation method before polymer electroless plating
4.1 Catalyst metal ion adsorption and reduction
4.2 Direct catalyst adsorption
4.3 Direct ink writing and graphic printing
5 Polyimide(PI)pretreatment method for electroless plating
6 Polyethylene terephthalate(PET)pretreatment method for electroless plating
7 Polyurethane(PU)pretreatment method for electroless plating
8 Polydimethylaminosiloxane(PDMS)pretreatment method for electroless plating
9 Polystyrene(PS)pretreatment method for electroless plating
10 Polypropylene(PP)pretreatment method for electroless plating
11 Polytetrafluoroethylene(PTFE)pretreatment method for electroless plating
12 Liquid crystal polymer(LCP)pretreatment method for electroless plating
With the increasingly wide application of electronic devices in today's society, electronic devices have become ubiquitous. In the face of faster and faster update iteration of electronic products, the requirements for their performance are also increasing. On the other hand, electronic devices are gradually developing towards miniaturization, intelligence and wearability. Polymers are widely used in electronic products because of their light weight, good stability and flexibility, good insulation performance, low cost and easy processing. At present, the polymers commonly used in the electronic field are epoxy resin, polyimide (PI), ABS plastic, liquid crystal polymer (LCP), polypropylene (PP), polytetrafluoroethylene (PTFE), polydimethylsiloxane (PDMS), polyethylene terephthalate (PET), polyurethane (PU) and polystyrene (PS)[1⇓~3][4⇓⇓~7][8,9][10⇓⇓~13][14][15⇓⇓~18][19,20][21⇓⇓~24][25⇓~27].
In electronic products, besides polymers, metal materials play a very important role in conductive interconnection, heat conduction and heat dissipation, electromagnetic shielding and so on. Electroless Plating is an important method to realize the metallization of polymer surface. For example, in the manufacture of commonly used multilayer PCBs, the metallization interconnection of electroless plated holes on polymer substrates is the most critical link[28,29]. Compared with vacuum sputtering and other metallization methods, electroless plating has the advantages of not being restricted by the material and shape of the substrate, uniform metal coating, low cost, easy large-scale production, and no need for expensive equipment. The process flow of polymer electroless plating is simple, which can be generally divided into four steps: cleaning, roughening, activation and electroless plating (as shown in Figure 1). The purpose of cleaning is to remove grease and other dirt on the surface of the substrate material. Generally, surfactant is used together with ultrasonic treatment, and the cleaning step is simple and easy to implement. The purpose of roughening is to make the polymer surface rough, so as to improve the bonding force between the electroless metal layer and the substrate surface. During electroless plating, the metal ions in the plating solution need the catalysis of the catalyst to be reduced to metal, and the activation treatment is to make the catalyst particles better attached to the surface of the substrate and fully exposed to provide active sites for catalyzing the electroless plating reaction. Roughening and activation are the most important steps affecting the performance of electroless plating on polymer surface, which are very critical for the quality of electroless plating metal layer and the adhesion of coating.
As more and more polymer substrate materials can be used in the electronic field, it becomes more important to select different roughening and activation methods for different materials as the pretreatment method of electroless plating. According to the organic group of the polymer, a series of surface treatment designs can be carried out before electroless plating. For example, in the roughening treatment, the groups on the surface of the polymer can be opened by hydrolytic etching[11,30]; The surface group can also be grafted and modified[31⇓⇓⇓⇓~36]; New groups can also be introduced onto the polymer surface by plasma treatment[15,19]. With the continuous improvement of coarsening treatment methods, the way of activation treatment has also been greatly changed. For hydrolytically etched or grafted polymer surfaces, metal ions can be introduced by ion exchange and then reduced[14,33⇓ ~35]; Metal nanoparticles coated by polar groups can also be directly soaked and adsorbed[37⇓⇓⇓~41]; It is also possible to write or print with an ink mixed with a catalyst and a binder (Figure 2)[1,42,43].
With the development of electronic field, the application of polymer electroless plating is more and more extensive, and the surface treatment technology before polymer electroless plating is becoming more and more diversified. For different polymer materials, choosing a suitable roughening and activation method as the surface treatment before electroless plating can often get twice the result with half the effort. This paper will focus on the roughening and activation surface treatment technology in the electroless plating process of polymers commonly used in the field of electronics.The characteristics of different polymer matrices and the related roughening and activation surface treatment technologies were summarized, which provided the basis and reference for the application and surface treatment of polymer electroless plating in the field of electronics in the future.
2 Principle of Polymer Electroless Plating and Its Application in Electronic Field
2.1 Principle of electroless plating
Electroless polymer plating needs to load metal particles with catalytic activity on the surface of the polymer, and the metal ions are reduced under the catalytic action to achieve the complete coating of the substrate. The overall reaction formula of electroless plating is shown in formula (1):[44]
+Redsol → M+Ox sol
In the above formula, Ox is the oxidation product of the reducing agent Red. This reaction can be split into cathodic and anodic reactions:
+me- → M
Redsol → Oxsol+ne-
In the formulas (2) and (3), m and n represent the number of electrons. The above two reactions occur simultaneously at the same electrode and the same solution-metal interface, so the equilibrium potential of the reducing agent must be lower than that of the metal electrode for the reaction to be sustainable. Taking formaldehyde as a common reducing agent for electroless plating as an example, the total reaction in electroless copper plating is as shown in formula (4):
Cu2++2HCHO+4OH- → Cu0+2HCOO-+2H2O+H2
The above formula can be divided into an anode reaction formula (5) and a cathode reaction formula (6):
HCHO+3OH- → HCOO-+2H2O+2e-
Cu2++2e- → Cu0
The above reaction can be subdivided into several steps: formaldehyde dissociates after hydration (formula (7), formula (8)) and reacts with hydroxide ions to form methylene glycol anions (formula (9)). The methylene glycol anion then continue to react on that catalyst metal surface (formula (10), formula (11)).
From the above reaction mechanism, it can be seen that the metal particles with catalytic activity are very important for the whole electroless plating process, and the catalyst is needed in the (11) ~ (13) steps of the above reaction. In the pretreatment of electroless plating, the purpose of coarsening and activation is to make the polymer substrate uniformly and compactly loaded with these catalytic active metals. Therefore, in the field of electronics, the application of electroless plating technology will have different pretreatment methods according to the substrate materials corresponding to different performance requirements.
2.2 Conductive interconnection
The integration of electronic devices depends on the multilayer stacking of PCB circuits, and the conduction between layers requires longitudinal hole metallization (as shown in Figure 3A)[44,45]. Electroless plating has become the main process of PCB manufacturing in industrial production because it can be widely used for irregular devices, whether it is planar conductive lines or vertical interlayer connections, which can achieve conductive interconnection well[46,47].
2.3 Electromagnetic shielding
After the electronic equipment is highly integrated, it faces the interference problem of high frequency signal transmission. It is a very wise strategy to use electroless plating to metallize the lighter polymer surface to replace the metal shell for electromagnetic shielding. Zhang et al. And Xing et al. Prepared lightweight electromagnetic shielding materials by electroless plating using fibrous and porous PU as substrates, respectively, and the electromagnetic shielding efficiency was tested to be up to 37.5% and 20%[49,50].
2.4 Heat dissipation and conduction
The high integration of electronic devices also faces the problem of thermal management. Electronic circuits with high integration and lightweight are difficult to make room for traditional metal fin-type cooling panels. Recently, Gebreal et al. proposed a method of covering the surface of components with a layer of organic polymer as an insulating layer, then electroless copper plating, and using the copper coating for heat dissipation (as shown in Figure 3C)[51]. Lee et al. Perforated the organic substrate and performed electroless copper plating, also using copper plating for heat management[39].
3 Polymer surface roughening method
3.1 Chemical etching
Chemical etching roughening before polymer electroless plating is to enhance the surface roughness and hydrophilicity of the polymer substrate by chemical reagents such as oxidation and hydrolysis. At present, the commonly used chemical etching agents are mainly strong acids, strong bases and strong oxidizing substances, such as nitric acid, sulfuric acid, sodium hydroxide, dichromic acid, potassium permanganate and so on. The degraded part of the polymer surface in the chemical etching reaction often forms tiny pits to enhance the surface roughness, and hydrophilic polar groups such as carboxyl, hydroxyl and amino groups are introduced into the polymer surface in the oxidation and hydrolysis etching reactions. Lin et al. Used NaOH to chemically etch the polyimide film to convert the polyimide groups on the surface into polyamic acid groups[52]. However, the conditions of chemical etching treatment also need to be controlled. Chen et al. explored the factors affecting the time conditions in the chemical etching treatment of LCP substrate, and found that the etching time has an optimal condition for the performance of copper coating. If the etching time is too long, the glass fiber agglomeration on the surface of LCP will weaken the adhesion of the coating[8]. Therefore, the selection of chemical etching conditions is also one of the key points of the roughening process. The polar group after chemical etching is beneficial to the attachment of the catalyst in the subsequent activation and adsorption, and improves the binding force between the electroless metal layer and the polymer matrix.
3.2 Plasma treatment
Plasma treatment is to act on inert gases such as helium, argon or some oxygen-containing gases by discharge, laser, high temperature and other methods to ionize these gases and obtain plasma. When these plasmas contact with the surface of polymer matrix, a series of complex chemical reactions will occur, which will cause oxidation and denaturation of the surface of polymer matrix, so as to achieve the purpose of roughening the surface of the matrix. Moreover, during the plasma treatment, active functional groups are often generated on the surface of the substrate, which can improve the hydrophilicity of the polymer surface, facilitate the adsorption of the electroless plating catalyst in the subsequent activation process, and improve the bonding strength between the electroless plating metal layer and the polymer.
Oshikiri et al. Used high power ultraviolet light to excite N2 to generate plasma for plasma treatment of liquid crystal polymer, which only needed 75 s of irradiation, and the sample was observed to be wrinkled and the roughness was increased under atomic force microscopy[20]. Lin et al. Used O2 plasma to treat polyimide fabric to roughen its surface, while introducing aldehyde and ketone groups with reducibility, and then directly sprayed with silver ink to obtain Ag-PIF composites[52].
3.3 Graft modification
Graft modification of polymer surface is to introduce some active functional groups on the surface of polymer matrix. These active functional groups generally have adsorption effect on electroless plating activator (catalyst), which improves the deposition rate of the coating and the bonding strength between the coating and the substrate. The common grafting methods are blend grafting, irradiation grafting and chemical grafting. The blending grafting is realized through physical bond combination, the chemical grafting is realized through chemical bond combination, and the irradiation grafting is realized by the irradiation of high-energy light. Kang et al. Used irradiation grafting method to irradiate PET soaked in azide propyltriethoxysilane (PTES) with ultraviolet light, which broke the C-H bond on the surface of PET and combined it into C-N bond. Then NTES was used for molecular self-assembly, and Si-Si bonds were formed by hydrolytic condensation to complete the grafting[34]. Liu et al. Used a chemical grafting method to chemically graft [2- (methacryloyloxy) ethyl] trimethylammonium chloride (METAC) and 3- (trimethoxysilyl) propyl methacrylate (MPTS) onto PI (Figure 4)[67].
4 Activation method before polymer chemical plate
4.1 Catalyst metal ion adsorption reduction
The premise of catalyst metal ion adsorption and reduction is to introduce some special groups on the surface of the polymer matrix in the roughening step. Chemical roughening, plasma and grafting can obtain the matrix with hydroxyl, carboxyl and other groups on the surface of the polymer. These groups can be adsorbed with metal ions, and then reduced to realize the loading of catalyst metal particles for electroless plating.
Post-grafting treatment is generally the isotopic conversion of cations, which converts the cations or ionic groups in the surface groups that have been successfully grafted into metal cations, such as palladium, platinum ions, etc. After isotopism and reduction, the grafted group adsorbs metal particles with catalytic activity, and because the grafted polymer group is well connected with the substrate, the subsequent electroless plating layer can be well combined with the substrate (Figure 5)[53]. Zhou et al. Used 6- (3-vinylsilyloxypropylamine) -1,3,5-triazine-2,4-azide TTA and TTD to graft PET, then used Sn2+ to replace Na+, and then reduced the metal ions in situ to metal particles with catalytic activity by reducing agent. The adhesion between the coating obtained by this method and the substrate has been greatly improved[33].
4.2 Catalyst direct adsorption
The direct adsorption of the catalyst is to directly load the catalyst particles with electroless plating catalytic activity on the surface of the polymer matrix. Because precious metals such as palladium, platinum, silver and the like have catalytic activity and can provide catalytic active sites for promoting the reduction of metal coating ions such as copper and the like in electroless plating, metal coating atoms such as copper and the like can be deposited on the surface of a substrate, and a coating with good performance can be obtained by using the precious metal catalyst. However, the reserves of precious metals are scarce, and the use of precious metals such as palladium as catalysts in the activation process of electroless plating will increase the cost of electroless plating. To this end, researchers have, on the one hand, designed noble metal catalysts with higher catalytic activity and, on the other hand, used other inexpensive metals as partial or total replacements. After research, bimetallic catalysts often have synergistic effect, and their catalytic performance can even surpass that of monometallic catalysts. Commonly used second metals are copper, tin and transition metals nickel, cobalt and manganese, the addition of the second metal can greatly reduce the use of precious metals, and can also be used for structural design.The common ones are core-shell structure, hollow structure and monatomic dispersion, which fully expose the noble metal atoms and catalytic active centers, greatly improve the performance of the catalyst and reduce the cost. Liu et al. Designed the catalyst by referring to the process of palladium/tin catalyst, prepared silver/tin catalyst, and explored the influence of different acidic groups in the activation process. By comparing the catalytic time and the bonding strength of copper coating, it was found that the catalytic activity of silver/tin was Ag/Sn P2O7>Ag/Sn SO4>Ag/Sn C2. Through continuous exploration, the preparation conditions of direct adsorption catalyst will be gradually optimized (Fig. 6).
4.3 Direct ink writing and pattern printing
The activation can also be carried out by preparing the catalyst into the form of ink or paste/glue, directly writing or printing the required pattern on the polymer matrix, and directly completing the graphical activation step through the steps of sintering or light curing. Sato et al. Used ink prepared by (p-chloromethyl) phenyltrimethoxysilane and tetraethoxysilane to write on PI film, and used atom transfer radical polymerization to achieve electroless copper plating[1]. Xie et al. Prepared silver catalyst ink with a ratio of silver nanocatalyst: epoxy resin: curing agent of 1:1:1, and then printed it on the PI film to obtain a patterned surface with catalytic activity, and then carried out electroless copper plating to obtain a copper coating with fine patterns[1]. At the same time, the ink/paste/glue often contains substances chemically bonded with the substrate, so the method can directly catalyze the required pattern through inkjet printing, and the subsequent electroless plating can directly deposit on the activated substrate, so the patterning accuracy is high.
5 Pretreatment Technology of Polyimide (PI) Electroless Plating
Polyimides can be made into flexible fiber or film materials, which are therefore commonly used in the fields of conductive fabrics and wearable electronic devices[56]. Through the design of pretreatment, the copper coating on the polyimide film can be used not only as a conductive circuit, but also as a capacitor[57]. In a recent study, when copper is electrolessly plated on polyimide as a copper conductor, the source of impurities in the copper conductor can be traced by time-of-flight secondary ion mass spectrometry (TOF-SIMS) technology, which provides a reference for improving the conductivity of the conductor[54]. When using electroless plating to manufacture electronic devices, the design of 3D electronic circuits can be realized through multi-layer coverage technology. Although there is still a big gap between the accuracy and the mature lithography technology, the advantages of simplicity and cheapness of polyimide electroless plating still attract the attention of a large number of researchers (Figure 7)[52,55].
The roughening and activation of polyimide electroless plating is usually based on the process design of hydrolyzing the imide ring in the main chain of polyimide into imide acid. The most direct way to hydrolyze the imide ring is to soak it in an alkali solution. Wang et al., Jones et al., and Liu et al. All used KOH or NaOH hydrolysis to coarsen the polyimide matrix. For thin films or fibrous polyimide materials, the difference in the coarsening process lies in the time of alkali treatment[2,56,57]. Wang et al. Injected KOH solution into a ballpoint pen to achieve surface selective coarsening of polyimide by writing[52]. The activation is carried out by the hydrolyzed polyamic acid radical, and the activation can be completed by ion exchange with different metal ions, such as Ag, Fe, Ni, etc. In addition to the method of hydrolysis roughening and ion adsorption activation, plasma treatment, grafting and other methods can also be used for pretreatment to improve the adhesion between the coating and polyimide. Hussain et al. First treated PI fiber with air plasma to produce functional hydroxyl groups, then copolymerized it with vinyltrimethoxysilane (VTMS) and [2- (methacryloyloxy) ethyl] trimethylammonium chloride (METAC), and grafted it to adsorb Pd+ for activation[58].
Sato et al. Used a sol-gel solution synthesized by p-chloromethylphenyltrimethoxysilane and 1,4-tetraethoxysilane as an ink to carry out inkjet printing to graft the imide ring with p-chloromethylphenyltrimethoxysilane, and then carried out electroless copper plating to obtain a coating with good performance[1]. In addition to direct grafting, the combination of hydrolysis and grafting is also one of the ideas for polyimide coarsening. Chen et al. Used the pre-reaction product of glutaraldehyde (GA) and polyethyleneimine (PEI) to graft the PI film after alkali treatment, and then carried out electroless copper plating, and also obtained a good copper coating[11].
6 Pretreatment Technology of Electroless Polyethylene Terephthalate Plating
The application of polyethylene terephthalate in the field of electronics is not only the common flexible conductive circuit of polymer, but also the catalytic electrode prepared by electroless plating with catalytic properties, and the electrode of nano-tribogenerator prepared by covering PET with Cu/Ag coating which can generate electricity[53,59,60][61][21].
Because the main chain of PET contains benzene rings, PET can maintain good mechanical properties in a wide temperature window, and has good electrical insulation, fatigue resistance and friction resistance. Because the main chain of PET also contains ester groups, the coarsening of electroless plating pretreatment is often designed around the hydrolysis of ester groups (Figure 8).
The roughening and activation method of PET is mostly through the preparation of ink for writing or printing to complete roughening and activation. Ink often contains metal ions such as Ag+, and different inks depend on the choice of complexing agent between curing agent and Ag+[62,63]. Wang et al. Printed ink on the surface of PET film in the form of pattern by inkjet printing, and then converted the Ag+ into metal Ag particles by plasma bombardment, so that the active particles were loaded on the surface of PET film, and the coarsening and activation process was completed in one step[64]. The use of thiourea to form a complex with AgNO3 to formulate ink has also been reported.
In addition to direct ink printing, the ester group of the PET backbone is also the object of hydrolysis coarsening by acid treatment. Huang et al. Used HCl to coarsen PET, and then used a mixed ethanol solution of KH550, AgNO3, and PVA as ink for writing activation[63]. In addition to hydrolysis, the ester group on the main chain of PET can also be grafted. Liu et al. Used plasma to treat the surface of PET to generate hydroxyl, carbonyl, carboxyl, amino and other functional groups on the surface of PET.Then poly [2- (methacryloyloxy) ethyl-trimethylammonium chloride] (PMETAC) was used for grafting, which changed the surface of PET from hydrophobic to hydrophilic, and the chemical groups acted as anchors, greatly improving the adhesion of the coating[67]. Liang et al. Used VTMS solution to modify PET, so that silane reacted with the hydroxyl groups on the surface of PET, and then used METAC to carry out copolymerization reaction. Grafting coarsening was carried out through a series of reactions. After grafting, the hydrophilic groups on the surface of PET could be converted by ions, so that Pd2+, Ag+ and H+ on the groups could be replaced to complete the adsorption of catalyst ions[53,67].
7 Pretreatment Technology of Polyurethane Electroless Plating
Can be made into foam, fiber, or rubber, and polyurethane as a substrate can provide a variety of options depending on the properties required of the electronic device[68]. At the same time, polyurethane has good chemical stability and resilience. In the field of electronics, it can be used in flexible and telescopic electronic devices or electromagnetic shielding foam by electroless copper plating.
In the roughening and activation steps of polyurethane, acid is often used for surface treatment to roughen the surface of the material, and the ester bond on the surface is hydrolyzed to form carboxyl groups. The subsequent activation steps can be ion exchange or classical colloidal activation. In the recent literature, this coarsening activation method has been very popular, Zhu et al. Used Tannic acid[69]; Li et al. Used a mixed solution of KMnO4 and H2SO4[70]; Fathi Dehkharghani et al. Used chromic acid and sulfuric acid[71]; Han et al. Used Na2S2O8 solution as the solution for coarsening[72]. After obtaining the polyurethane surface with hydroxyl groups, they used (NH4)2PdCl4 solution or mixed solution of PdSO4 and H2SO4 for ion exchange, and also used Pd/Sn and Ag/Sn colloids to complete the activation (Fig. 9).
In addition to the acid treatment roughening method, there is also a commonly used grafting method. Lu and Li et al. Used dopamine and polyurethane to roughen the surface of polyurethane through in-situ polymerization[23]. Also using the copolymerization grafting method, Yu et al. Used [2- (methacryloyloxy) ethyl] trimethylammonium chloride (METAC) and 3- (trimethoxysilyl) propyl methacrylate (MPTS) for graft coarsening, and Xia et al. Used chitosan for graft modification[73]. After grafting, similar to the acid hydrolysis method, the use of ion conversion and colloidal adsorption activation is a common method[22].
In a recent report, a pretreatment method using a roughening activation treatment performed simultaneously with the fabrication of a polyurethane matrix has also appeared. Ryspayeva et al. Used 3D printing to prepare PU parts with special shapes. Before printing, the AgNO3 was mixed with polyurethane precursor. After the parts were formed, the mask was used to reduce the Ag+ by illumination, and the pretreatment of electroless plating was directly completed[74]. Similar methods were used by Yan and Piotr, who also mixed activated copper powder or copper complex with polyurethane precursors and recast them[22].
8 Pretreatment of Polydimethylaminosiloxane (PDMS) for Electroless Plating
The backbone of PDMS is composed of Si — O, which has good chemical stability, flexibility and elasticity, as well as good biocompatibility, so it has great potential in the field of bioelectronics. At present, a variety of electroless plating methods have been developed to prepare stretchable conductive lines on the surface of PDMS, but the problem of stretchable matching between conductive lines and PDMS is still to be solved.
Acid hydrolysis treatment, grafting, plasma treatment, ultraviolet light treatment and the like are commonly used for roughening of PDMS; The subsequent activation steps often include ion replacement and colloid activation. The roughening method of dopamine copolymerization grafting is still a feasible method for roughening on PDMS. Zhou et al. Used this method for grafting, and then used AgNO3 solution for ion replacement activation[76,77]. In addition to dopamine grafting, Wu et al. Used APTMS and APTES to copolymerize and graft PDMS films, respectively, and finally used Ag(NH3)2NO3 solution for activation (Fig. 10)[78]. Selective activation has always been an important part in the application of electronic devices, and the selective activation of matrix materials by different masking methods has always been a hot research topic.
In a recent report, Ryspayeva et al. Used plasma treatment as a roughening method, followed by the activation of conductive patterns using silver perchlorate and photoresist dissolved in 2-propyl 1-methoxyacetate (MPA), coated and mask selectively irradiated[79]. Polywka et al. Irradiated the PDMS film under a mask using an ultraviolet lamp, and then activated it using AgNO3[16]. Mirmohammadi et al. Etched an aluminum plate to form a micron-nanostructure as a template of the PDMS material, cast the template to obtain the surface of the PDMS material, then used a mixed solution of phosphoric acid and nitric acid to preliminarily etch the aluminum template, and then used a copper plating solution to carry out a replacement reaction to cover the surface of the FDMS material with a copper coating[17]. Coulm et al. Used plasma to pattern-transfer the surface of PDMS spin-coated with mica, then roughened with alkanethiol, and finally activated with PdNO3[80].
9 Pretreatment of Polystyrene (PS) Electroless Plating
The main chain of polystyrene is a saturated carbon chain, and the side chain has a benzene ring. Due to the presence of benzene ring, it has a high glass transition temperature and is a rigid polymer at room temperature. In the application of the polymer in the electronic field, hollow microsphere PS material is often used as a matrix, and electroless copper plating is carried out first, and then hot pressing is carried out to make the hollow microspheres contact with each other to form a three-dimensional conductive copper mesh structure. On the one hand, the three-dimensional conductive copper mesh structure can reduce the heat loss generated in the conductive process, on the other hand, it can also reduce the resistance, and it can also be used as an electromagnetic shielding material.
For the application of PS materials in electronic devices, microsphere PS is often used for electroless plating, and then the microsphere is pressed into the required device shape by means of hot pressing and other means. At this time, the coating on the surface of the microsphere is interconnected to form a three-dimensional conduction structure, which greatly improves the electrical and thermal conductivity of the material. For the pretreatment of electroless plating of PS microspheres, the commonly used method is acid and alkali immersion etching coarsening, and then using Pd/Sn or Ag/Sn colloid for activation[81]. Lee et al. Used NaOH, Cheng et al. Used a mixed solution of CrO3 and H2SO4, and Xie et al. used HCl for coarsening, and then all used Pd/Sn or Ag/Sn colloids for subsequent activation, respectively (Fig. 11)[82][25][91].
In addition to the acid and alkali etching coarsening of the microspheres, the PS microspheres can also be pretreated and coarsened by using the precursor styrene of PS and AIBN initiator to polymerize to generate a new polystyrene layer, or by using pyrrole-1-propionic acid monomer and pyrrole monomer to graft acetic acid groups on the surface of the PS microspheres, and then activated by Ag/Sn colloid[83].
10 Pretreatment Technology of Polypropylene (PP) Electroless Plating
Polypropylene (PP) is one of the cheapest polymers and has certain mechanical strength, chemical and thermal stability. Polypropylene film is lighter than other polymers, and polypropylene is very valuable as a substrate material for electronic circuits for thin and light electronic devices. At present, the conductive polypropylene film after electroless plating has shown its potential application in the field of electromagnetic shielding of electronic devices[13]. The pretreatment method for electroless plating of PP is similar to that of most other polymer electroless plating. The roughening methods include dopamine grafting modification, hydrolytic etching with mixed solution of K2Cr2O7 and H2SO4, and printing with AgNO3 ink. The activation also requires Pd/Sn colloid or ion replacement activation of Pd2+ and Ag+ (Fig. 12)[11][13].
11 Pretreatment of PTFE Electroless Plating
PTFE is a hydrocarbon substituted by fluorine atoms in the side chain, which has good thermal stability, chemical stability and aging resistance. Thanks to the strong electronegativity of fluorine atoms, PTFE also has excellent dielectric properties[87]. In the era of 5G high frequency development of electronic equipment, the stability of PTFE material is a reliable base material for 5G high frequency signal transmission line. However, due to its extremely stable characteristics, the surface energy of PTFE is very low, and its wettability with solution is very poor, so the current application of PTFE in the field of electronic electroless plating focuses on the development of pretreatment technology.
Dopamine polycondensation modification grafting is one of the widely used methods in PTFE materials, which uses polydopamine methylpropionamide groups on the surface to obtain metallized patterns with strong adhesion by Pd/Sn or Ag/Sn colloid or ion replacement in the subsequent activation process. A roughening method selective to ultraviolet irradiation can also be selected. Qiang et al. Used DMA and PTFE to polymerize under ultraviolet irradiation, and controlled the roughening pattern through a mask[15].
12 Pretreatment of Liquid Crystal Polymer (LCP) Electroless Plating
LCP material is a kind of intermediate polymer between solid phase and liquid phase. This special property makes LCP film have good flexibility, which is a common substrate material for flexible electronic devices. It also has good thermal and chemical stability, high mechanical strength and low dielectric constant. At the same time, it has good fluidity and dimensional stability during injection molding, which is also conducive to its large-scale industrial production. At present, LCP is a very popular substrate material in the field of 5G and flexible electronics, and its liquid crystal state enables it to obtain electronic circuits with high adhesion and good compatibility with copper coating after appropriate pretreatment[8].
In the pretreatment of LCP, the grafting method uses polyphenylene sulfide (PPS). Because of the special crystalline state of liquid crystal polymer, the chemical potential of copper coating reduction after grafting modification is lower[10]. In addition to grafting, plasma treatment is a coarsening method. After the injection of external energy, the C-N and N = N bonds on the surface of LCP are broken and hydrophilic groups containing N and O are introduced, which improves the surface free energy of LCP and facilitates the adsorption of active particles in the subsequent coarsening process[15].
13 Conclusion and prospect
With the miniaturization of electronic devices and the requirement of higher integration, the vertical conduction of multilayer PCB circuit, the electromagnetic shielding of electronic components, heat dissipation and heat conduction can be realized by polymer electroless plating. In this paper, the roughening and activation methods of polymer matrix materials before electroless plating, which are commonly used in the field of electronics recently, are summarized. In the pretreatment of polymer electroless plating, the roughening and activation processes have a great influence on the quality of the electroless plating coating. The first idea of the roughening process is to improve the physical roughness of the substrate to increase the probability of contact and adsorption with catalytic particles and the adhesion with the metal coating during the activation process. The second idea is to improve the chemical roughness of the substrate, that is, through plasma treatment, illumination, hydrolysis, etching and other methods to produce chemical groups on the surface of the substrate, through the adsorption of catalytic particles by the groups, to achieve activation and improve the adhesion between the polymer substrate and the metal coating.
In the activation process, ion exchange and coarsening complement each other. For the design of substrate materials, the groups introduced by chemical etching, hydrolysis, grafting or other means can form a complete electroless plating process design with the activation process. The design ideas of ink activation and catalyst direct adsorption are both around the effective attachment of particles with catalytic activity, at this time, the utilization rate and attachment rate of catalyst are more considered.
With the increasing demand for metallization in the field of electronic devices, electroless plating has become a common method of metallization because of its advantages of simplicity, cheapness and industrial production. The application of electroless plating is more and more extensive. Although the treatment process of electroless plating for different materials is different, some common methods of pretreatment for electroless plating can still be used as a reference for the design of polymer electroless plating process.
RyspayevaA, DavidArthur Jones T, KhanS R, EsfahaniM N, ShuttleworthM P, HarrisR A, KayR W, DesmulliezM P Y, Marques-HuesoJ. IEEE Access, 2019, 7: 104947.
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