3D-MID (three dimensional molded interconnected devices) is an innovative idea for the integration of

electrical and mechanical functions into thermoplastic circuit boards of any required shape through the

selective application of metal layers to single-component or two-component molded plastic parts.

The new possibilities in design and function offer a great potential for the rationalization of electrical and

electronic component production. Injection-molded circuit board are generally more compatible with the

environment than conventional circuit boards, but are more likely to supplement them than to replace them.

The main advantages of MID technology

 

Strengths

Cost savings

 

Miniaturization

 

Integration

 

Ease of recycling

 

Free scope for the designer

 

Possibility of three-dimensional circuitry

 

Combination of conductor track and functional components

 

Highly reliable function

 

Reduction in diversity of materials

 

Possibility of new design solutions

 

MIDs (Molded Interconnect Devices) are three-dimensional circuit boards based on thermoplastic materials. The technology was origionally developed in the USA in the 1970s, but did not start attracting

interest until a few years ago, mainly from development engineers and users in the automotive and IT industries. The basic idea behind MID technology is the integration of electrical and mechanical functions

in one metallized thermoplastic injection-molded part. Its electrical functions are those of conductor tracks, plug-in and sliding contacts, or sheelding surfaces, but a MID based part can also include mechanical functions such as fastener elements.

 

Process

MID based parts can be produced by various different processes. The basic distinctions can be made

between:

 

two-component-molding

 

single-component -molding, with the following segmentation:

- hot-embossing

- laser-direct structuring

- photo imaging

 

in-mould decoration combined with metallization.

 

Comparison of two componete molding process

The twin-component injection-molding process opens up the greatest geometrical diversity of all MID production processes, and thus, together with the hot-stamping process, represents the most frequently used process for the production of MIDs. In the twin-component injec-tion-molding process, two types of plastic are combined in one molding, one of these types being suitable for metalizing. The illustration shows three variants of the twin-component injection-molding process, the SKW process (a) and the PCK process (b). In the SKW (or Sankyo Kasei Wiring Board) process, the first shot (pre-molding), made from the metalisable plastic, is taken out of the tool and infected with Pd. The infected pre-molding is then put back in the tool and sprayed over. The disadvantage of this process lies in the additional proc-ess step necessary for the Pd-infection, which causes higher process costs, lower process reli-ability, and thus also higher reject rates.

 

In the case of PCK (Printed Circuit Board Kollmorgen), palladium is added to the plastic components that

are to be metallized so that it can act as a catalyst for the metallization proc-ess, which needs no external

current. Because the palladium is expensive, the raw materials cost for the metallizable plastic omponents are very high.

 

Inherently metallizable plastic combinations such as PA6/PA12 or PA6/PBT are used a third process variant (c). The advantage of the two processes mentioned above over the SKW sys-tem is that the injection-molding process does not have to be interrupted for the wet-chemical infection of the first shot, and this means that automatic tools can be used for producing MIDs. In addition to this, inexpensive technical thermoplastics can be used in this process.

 

ABS

Metallized plastic parts made of ABS or PC/ABS have been in sued since the mid-1960s, mainly in the sanitary and motor industries but also in telecommunications. In MID-technology, ABS is mainly used in applications where the temperature requirements are not very stringent (-40°C to +80°C), such as in the

household products industry.

Polyamide

Where the requirements for dimensional thermal stability are more stringent, as well as for dynamic

load-bearing and flow properties, polyamide is coming to the forefront. Some poly-amide types are

available which are suitable for soldering processes, such as those used for MIDs. When polyamide is

being metallized, a “swell and etch” process generates a micro-roughened surface which has a special

affinity for the Pd-activator applied in the next step, and thus contributes significantly to the adhesive

strength of the metal layer.

REM picture of an injection-molded part made of polyamide: (a) prior to and (b) after chemi-cal pre-treatment.

Particularly good adhesive qualities can be seen with the types of polyamide that are rein-forced with glass fiber and elastomer-modified. The glass fiber reinforcement greatly reduces the co-efficient of thermal expansion of the injection-molded parts, so that a high level of adhesive strength is achieved even during temperature changes.

In addition to other factors, designing a part to be suitable from the start for metalizing is cru-cial to the quality of an MID component. The main points are explained below.

Radii

The transitions between radii are particularly important in tool design, both for the later met-allization and

for the conductor track routes as such. Sharp edges and corners result in thinner layers in the metallization, because the course of the electrical field in the electrolyte is dis-turbed. The mechanical strength of the component is likewise reduced by transitions with sharp edges. The tendency for these to cause cracks in the metal layer when a load is imposed is greatly increased if the radii are too small. Design of edges and corners to ensure suitability for metallization.

Injection points

Injection points must not lie on the conductor track that is to be metallized, as these are the points at

which faults most often occur in the metal layer. They are therefore laid next to the conductor track. One reason for this is the rough areas, and another is that, in the molding system, excessively high shear speeds occur if the cut sections are too narrow and these lead to uncontrolled increases in heat in the molding zone; this, in turn, causes extensive crystalli-zation in these areas, particularly with polyamides. If the carrier material is processed first, and then the conductor track material, attention has to be paid to ensuring that the injection-molding point of the conductor track material in the carrier material is mechanically an-chored, ion order to prevent the second shot from coming loose from the first shot when the mould is opened.

Binding seams

Binding seams prevent the metallization from forming neatly in these areas, so they are laid alongside in

recesses the conductor track. This of course necessitates defining the position of the binding seams by

means of preliminary injection trials, or in filling studies.

Positioning of weld line beside the conductor track

Uninterrupted contacts

When uninterrupted contacts are being designed, the largest possible internal diameter should be

selected. As the diameter increases, so too does the layer thickness of the metallization, and the capacity of the contact is also raised. The minimum diameter for the contacts should be 1.5 to 2 mm for the frame

material (aspect ratio 1:3), but for components that are to be coated in a drum it is perfectly possible to metallize contacts with a diameter of less than 1 mm if the aspect ratio is 1:3.

As has already been mentions, transitions between radii are necessary at all corners and edges, and this

is particularly important in the case of the contacts. The illustration shows one uninterrupted contact that

is “OK” and one that is “not OK”.

 

Design of uninterrupted contact suitable for efficient current flow

MIDs can be coated by means of electrolyte or chemically. The following paragraphs de-scribe the

advantages and disadvantages of these processes.

Electrolytic metal precipitation

Electrolytic metal precipitation is defined as all those galvanizing processes that work with an external

current. The work-piece to be coated is designed as the cathode. The precipitation speed of galvanic

electrolyte is far higher than that of the chemical process, which means that galvanic electrolyte can be

used, particularly for layer thickness of 20-35µm. The disadvan-tage of electrolytic coating is the necessity for contacting. When MIDs with separate three-dimensional conductor tracks are being metallized, electrolytic metallization is only possible if additional design steps are taken to bring the conductor tracks together.

Chemical precipitation (with no external current)

Chemical precipitation procedures are based on a reduction to metal of the metal ions present in the watery process solution. Because this process works without any external current source (current-free precipitation), the layers are not subject to the disadvantage that local differences in current density create different layer thicknesses on the workpiece. The maxi-mum thickness of layers precipitated without current is limited to about 20 µm. Thicker layers are not economical, and because layers precipitated without current are less ductile there is an increasing tendency for cracks to form in the layer system.

Testing of twin-component MID products

As a standard part of the process, the metallizer carries out a visual check and checks for metal adhesion

and layer thickness. For testing adhesion, the switch test (DIN IEC 326 part 3) is in particularly widespread use in the electronics industry. The layer thickness can be ex-amined either by X-ray fluorescence or with a destructive test of the finish.

Typical MID product using twin-component technology and a sprayed-on push-in connection.

Heat-embossing process, foil construction, and uninterrupted contacts

The illustration shows the process steps involved in heat-embossing.

Uninterrupted contacts are made possible by stamping the foil into pre-formed holes in the single-component plastic part. These holes are filled with conductive paste, or else serve as holders for press-in contacts. Heat-embossing is a very quick, clean process for metallizing; the development times for these components are usually very short, because it is possible to do without complex twin-component tools.

For certain applications the disadvantage, in comparison with twin-component techniques, can be the

limited scope this technique gives the design engineer.

ISDN plug, produced using the heat-embossing process

Laser-direct structuring / mask process

Laser structuring of large-area metallized single-component plastic parts permit extremely fine conductor

track structures. This process offers great flexibility in terms of the conductor pattern, because changes only require modifications to the program that controls the laser.

The sequence of events in the mask process is similar to that of laser structuring, but it is cur-rently of

subordinate importance for the manufacture of MIDs.

As a further alternative, the rear-spraying of foils structured with the conductor tracks is at-tracting more and more interest. This includes a process by which the wiring pattern is at-tached to the foil with the aid of a special primer containing catalysts which permit a later metallization of the conductor track structure with a simplified coating process. In this proc-ess, the foil can be used both for decorative and for functional purposes.

Electronic roulette, produced by foil rear-spraying

Durethane, Pocan, Novodur, Bayblend, Makrolon, Makrofol. For partial galvanization, Durethan BKV 115

and 130 are usually used in combination with Pocan. If the heat-resistance requirements are not too stringent (and no soldering process is envisaged), Bay-blend or Novodur are sued in combination with Makrolon.


2017年02月17日

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