Three-Dimensional Circuit Interconnections with Thermoplastic

Chapter 38

Three-Dimensional Circuit Interconnections with Thermoplastic Performance Polymers David C. Frisch and John F. Rowe

Downloaded by IOWA STATE UNIV on April 15, 2017 | http://pubs.acs.org Publication Date: September 5, 1989 | doi: 10.1021/bk-1989-0407.ch038

Pathtek, 250 Metro Park, Rochester, NY 14623

This chapter presents an overview of performance plastic polymers i n commercial planar and 3-dimensional circuit board products, and describes in detail one approach (two-shot molding) developed as an integrated 3-D circuit manufacturing technology. The distinctions between conventional planar (2-dimensional) circuitry, based on thermoset laminates and "subtractive" etching processes, and the enhanced design f l e x i b i l i t y afforded by expanded interconnection capacity in three axes are discussed. Specific examples of 3-dimensional interconnect protoypes and products are described and pictured.

P l a s t i c materials appear i n a l l aspects of everyday l i f e , providing a vast range of protective, insulating and packaging r o l e s . Within the p l a s t i c s f i e l d , the technology of i n j e c t i o n molding probably provides the greatest opportunity to c a p i t a l i z e upon the benefits o f p l a s t i c materials. By " f i l l i n g " space with a molded material, i t i s possible to create very complex and sophisticated components which exploit the benefits o f the material properties i n a 3-dimensional way. Most p l a s t i c moldings provide a kind of " s t r u c t u r a l " or " s p a t i a l " customization. They allow a product designer to inclose three dimensional space with a component which either acts as a container or enclosure, or which fixes the r e l a t i v e positions of other components i n r e l a t i o n to each other. (1) Printed c i r c u i t s are generally planar objects. Usually square or rectangular, they are e s s e n t i a l l y f l a t platforms supporting interconnect networks on which are mounted a mixture of electronics or e l e c t r i c a l components. These components may be leaded devices (either r a d i a l or axial) passing through the board, or mounted components on the board surface. The c i r c u i t board acts as a "customizing" device, l i n k i n g standard or special components to create c i r c u i t f u n c t i o n a l i t y . (2) 0097-6156/89/0407-0456$06.00/0 ο 1989 American Chemical Society

Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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Three-Dimensional Circuit Interconnections

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Molded p r i n t e d wiring boards create revolutionary new dimensions for designing and manufacturing e l e c t r i c a l and e l e c t r o n i c interconnection products. The commercial value f o r these new substrates resides not s o l e l y i n their p o t e n t i a l to increase density, but more so i n t h e i r unique a b i l i t y to consolidate mechanical and/or electro-mechanical features and s t r u c t u r a l components into a "one piece" single unit package. The printed c i r c u i t board becomes transformed into a multipurpose device supplying both e l e c t r i c a l as well as mechanical elements. Using s e l e c t i v e additive p l a t i n g techniques and i n j e c t i o n molded thermoplastic r e s i n polymers, c i r c u i t boards do not need to be l i m i t e d to planar or 2-dimensional forms. Instead, by incorporating molded-in 3-D s t r u c t u r a l features such as; bosses, r i b s , stand-offs, and recessed c a v i t i e s , boards can be transformed into multi-functional devices possessing customized forms and shapes. " C i r c u i t s can be: - Designed to provide mounting points f o r components - Shaped to f i t into the i n t e r i o r of equipment - Provided with locating points to "snap into" other components - Designed to provide a dual function, doubling as a s t r u c t u r a l component - Instrumental i n saving "down stream" assembly costs"(2) In addition, molded holes can be custom designed to a i d i n i n s e r t i o n of leaded components (entry r a d i i or countersink) or to provide recessed mounting of hardware (counterbore). Furthermore, holes can be oblong, rectangular or p a r a l l e l to the plane of the molded substrate. Possible hole geometries are i l l u s t r a t e d i n Figure 1. Molded 3-Dimensional c i r c u i t boards, as shown i n Figure 2, can be formed into almost unlimited shapes, sizes and geometries. Conductors can be routed around corners, into deep recesses, along ramps and up nearly 90 degree walls. In addition, depending upon material selection, non-plated areas may be transparent, translucent, or colored to s a t i s f y s p e c i f i c performance objectives. Multiplanar c i r c u i t r y and through-hole interconnections enable product or component designers to produce customized "Application s p e c i f i c " multifuctional devices. As shown i n Figure 3, c i r c u i t r y may traverse a h o r i z o n t a l plane and then gradually t r a n s i t a raised hemisphere molded feature. Conversely, as shown i n Figure 4, c i r c u i t r y may be predominantly 2- dimensional with non-conductive molded-in s t r u c t u r a l features simplifying assembly of down-stream assembled components. The development of a molded interconnect technology has spanned more than a decade of materials and chemistry development and process refinement. What began as a concept i n the minds of several entrepreneurial companies and research technologists, has f i n a l l y achieved true market commercialization. Molded boards and 3- dimensional interconnect devices comprise a new manufacturing industry as opposed to being merely another niche market segment of



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Figure 1.

Figure 2.

Molded through-holes o f f e r custom geometries.

3-D Molded Interconnects feature unique

shapes/forms.



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Figure 3.

Figure 4.


Molded substrate combines c i r c u i t r y and Mechanical structure.

Recessed "Molded-in" features simplify component assembly.


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the conventional c i r c u i t board industry. While o f f e r i n g benefits of both economy and performance over standard epoxy-glass laminate 2D boards, a molded board achieves f a r greater value through the 3-dimensional feature-forming c a p a b i l i t i e s which integrate mechanical support, structure, and custom shapes with highly conducting, r e a d i l y solderable c i r c u i t r y patterns. Molded c i r c u i t s w i l l take interconnection into new areas as they l i n k together the printed c i r c u i t industry with the molded component industry. "Additional "value added" w i l l be achieved by augmenting the c i r c u i t with extra features. This w i l l cause the molded c i r c u i t business to grow outside the hitherto t r a d i t i o n a l market areas, currently f i l l e d by PCBs, into new f i e l d s which embrace the s t r u c t u r a l components market." (3) As a manufacturing technology, s e l e c t i v e m e t a l l i z a t i o n of a molded p l a s t i c substrate has wide u t i l i t y i n e l e c t r i c a l and e l e c t r o n i c componentry, connector and receptacle devices, customized IC c a r r i e r s , electro-mechanical and e l e c t r o - o p t i c a l products. "The major benefits which come from a molded c i r c u i t board may be grouped under four headings including; structural/mechanical, material, cost and design/aesthetics. 1.

Structural/Mechanical Create 3-dimensional configurations Save space and weight Provide an enclosure or r i g i d structure Reduce the number of secondary assembled components Minimize assembly complexities Integrate f u n c t i o n a l i t y of components Accuracy and r e p r o d u c i b i l i t y of products within desired tolerances.

2.

Material Provide e l e c t r i c a l i n s u l a t i o n Lower weight per unit volume Enhanced d i e l e c t r i c properties:

Volume and surfaceresistivities d i e l e c t r i c constant d i s s i p a t i o n factor arc tracking Options for color matching and transparency Resistance to corrosion and h o s t i l e environmental exposure Permits l i q u i d retention or exclusion 3.

Cost Reduce component costs Provide a "systems l e v e l cost reduction" Reduce a product's "net cost to assemble" Minimize assembly and/or component t o o l i n g costs Lessen parts inventory and control costs


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Design/Aesthetics Create customized styles Textured f i n i s h options" (4).


MANUFACTURE OF MOLDED INTERCONNECTION DEVICES MATERIALS. Selection of a base polymer thermoplastic r e s i n from which a molded substrate i s produced i s influenced by factors of price and performance. Secondary considerations include supplier preference. Given the uniqueness of each product a p p l i c a t i o n , standardization of generic polymers i s u n l i k e l y . In fact, the selection p o s s i b i l i t i e s are l i k e l y to grow with continued d i v e r s i f i c a t i o n of a p p l i c a t i o n requirements/specifications. Materials with high temperature properties are desired where flow soldering assembly operations are involved. In these applications, material s e l e c t i o n includes high-temperature, engineering grade, thermoplastic polymers characterized by high heat d e f l e c t i o n and/or high glass t r a n s i t i o n temperatures. Conversely where less stringent thermal demands are appropriate, choices are broadened to encompass materials more commonly referred to as "commodity resins". Ultimate end product operational environments w i l l impact the material s e l e c t i o n process. Where in-service temperatures range from room ambient to moderate thermal extremes, lesser thermally tolerant resins may be considered. Conversely, for h o s t i l e environments such as those encountered i n "under the hood" automotive, down-hole (geothermal), t r o p i c a l , or corrosive climatic axtremes, enhanced material properties w i l l be required. P l a t e a b i l i t y or m e t a l l i z a t i o n compatibility cannot be overlooked, as not a l l r e s i n materials are readily capable of being plated using currently available commercialized techniques. Typical examples of materials now being used are given i n Table I followed by a b r i e f properties description.

Materials For Molding (5) -Polyarylate -Polysulfone -Polyarysulfone -Polyethersulfone -Polyetherimide -Polyphenylene Sulfide


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POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION TABLE I


Polyarylate-

Aromatic polyesters having an amorphous molecular structure. Compared with other amorphous engineering p l a s t i c s i n terms of heat resistance, polyarylates are generally positioned between polycarbonate on the low side and sulfone and polyether polymers on the high side. Compared with c r y s t a l l i n e and semi-crystalline engineering p l a s t i c s , polyarylate resins o f f e r better resistance to warping, and generally comparable mechanical properties. The HDT of commercial polyarylate-based compounds range from 310F to 345F at 264psi. Polyarylates have inherently good flame retardancy, and generate l i t t l e smoke during burning. Grades meeting requirements for UL 94V-0 down to 0.062" are available.

Polyetherimide-

Amorphous thermoplastic introduced i n 1982. The material i s characterized by high strength and r i g i d i t y at elevated temperatures, long-term heat resistance, and highly stable dimensional and e l e c t r i c a l properties. Polyetherimide has a chemical structure based on repeating aromatic imide and ether units. High performance strength c h a r a c t e r i s i t i c s at elevated temperatures are provided by r i g i d imide units, while the ether linkages confer the chain f l e x i b i l i t y necessary f o r good melt processing and flow. Polyetherimide resins are rated for continuous-use temperatures by U.L., UL94-V0 flame l i s t e d down to 0.010" upon grade). The HDT of commercial based compounds ranges from 387F to 264psi.

Polysulfone-

338F and 358F and are (depending polyetherimide 420F at

Aromatic amorphous thermoplastic introduced i n the mid-1960s. Polysulfone i s a transparent, heat resistant, u l t r a - s t a b l e engineering polymer. I t possess good e l e c t r i c a l properties that remain r e l a t i v e l y unchanged up to temperatures near i t s glass t r a n s i t i o n temperature (Tg) of 374F. Polysulfone i s UL l i s t e d f o r continuous service at 320F, although i t w i l l withstand higher temperatures intermittently. I t offers a good combination of e l e c t r i c a l properties: d i e l e c t r i c strength and volume r e s i s t i v i t i e s are high, while d i e l e c t r i c constant and d i s s i p a t i o n factor are low.


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Polyethersulfone-


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Aromatic amorphous engineering thermoplastic. L i s t e d by the UL for continuous service at 356F, operation at higher temperatures up to 400F i s feasible.


Inherent low flammability meeting UL94-V0 requirements at thicknesses of 0.017" and above. Possesses good resistance to most inorganic chemicals, o i l s , greases a l i p h a t i c hydrocarbons and gasoline at ambient and elevated temperatures. Most proprietary cleaning solvents do not attack i t unless the components are heavily stressed; known exceptions are esters, ketones, methylene chloride and polar aromatic solvents. Polyary1sulfone-

Typical c h a r a c t e r i s t i c s of aromatic, amorphous sulfones. Able to withstand high temperatures i n long-term usage as characterized by heat d e f l e c t i o n temperature (HDT) values of 399F at 264psi, and glass t r a n s i t i o n (Tg) ratings of 428F. Good d u c t i l i t y and toughness are retained from -148F to 392F.

Polyphenylene Sulfide -

High performance c r y s t a l l i n e aromatic polymer. Exhibits outstanding high-temperature s t a b i l i t y , inherent flame resistance, and good chemical resistance. Its structure also promotes a high degree of c r y s t a l l i n i t y . UL l i s t e d f o r continuous service from 392F to 464F depending upon compound, thickness and end use. Compounds can withstand higher temperature exposure due to heat d e f l e c t i o n temperature ratings of over 500F. Nearly chemically inert, i t i s highly r e s i s t a n t to attack by solvents. In fact, no chemical has been found to dissolve i t r e a d i l y below 400F.

Reinforced PET-

Thermoplastic polyesters based on polyethylene terephthalate. Closely related i n terms of chemistry, properties, and areas of a p p l i c a t i o n to reinforced polybutylene terephthalate (PBT) compounds. Key d i s t i n g u i s h i n g features are higher strength properties and higher use temperatures.

The materials shown i n Table I are available i n varying molecular weights and f i l l e r compositions. For molded c i r c u i t applications, the most commonly used f i l l e r s comprise chopped or m i l l e d glass f i b e r s and/or mineral t a l c s . In addition to these f i l l e r s , additional components may be incorporated to impart flammability resistance or to promote e l e c t r o l e s s p l a t i n g . The l a t t e r i s achieved by the addition of a proprietary mineral f i l l e r which renders the polymer " c a t a l y t i c " to p l a t i n g .


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This additive does not adversely a f f e c t the d i e l e c t r i c properties of the base polymer, but merely ensures i n i t i a t i o n of the e l e c t r o l e s s p l a t i n g process upon exposure of the molded p l a s t i c substrate to the p l a t i n g solutions. The unique attributes of "catalyzed" resins are that they eliminate the need for secondary (after molding) pre-plate a c t i v a t i o n or "seeding" operations common to conventional plating-on-plastics (POP) processes. For molded c i r c u i t board manufacture, c a t a l y t i c resins are used i n two-shot (two-component) molding processes which form highly complex 3D p l a s t i c structures which are capable of being s e l e c t i v e l y plated without the need f o r p l a t i n g masks or r e s i s t s . Polymers currently available i n a " c a t a l y t i c " composition include only amorphous sulfone and imide based systems. MOLDING/METALLIZATION. Molded thermoplastic c i r c u i t board substrates may be rendered s e l e c t i v e l y conductive by several additive process techniques including conductive polymeric thick f i l m inks (PTF), and semi and f u l l y additive e l e c t r o l e s s / e l e c t r o l y t i c platings. Of the various chemical process methods developed to produce c i r c u i t r y on a molded p l a s t i c substrate, one method practiced by Pathtek, A Kodak Company, combines both " c a t a l y t i c " and "non-catalytic" resins i n a highly automated commercialized two-shot molding/selective m e t a l l i z a t i o n process. The process starts by i n j e c t i o n molding a p l a s t i c substrate (or " f i r s t shot"). Using a " c a t a l y t i c " polymer r e s i n , the f i r s t shot mold t o o l cavity produces a p l a s t i c substrate containing only those features and component elements which are designed to accept subsequent e l e c t r o l e s s deposition. Those features formed into the " f i r s t shot" p l a s t i c molding w i l l produce the metallized through-holes, c i r c u i t traces, and other conductive s t r u c t u r a l elements. As i l l u s t r a t e d i n Figure 5, molded-in features are formed as raised tracks or traces which allow for subsequent encapsulation with "non-catalytic" r e s i n during the "second shot" molding cycle The " c a t a l y t i c " molded p l a s t i c part i s then inserted, either manually or automatically, into either; (a) a second mold (tool) base or (b) a second cavity within the same mold (tool) base. The second molding cycle injects a non-catalytic (or "non-plateable") r e s i n which overmolds (encapsulates) the non-raised background areas of the f i r s t shot p l a s t i c part. This second shot cycle completes the molding process. The f i n i s h e d (ejected) p l a s t i c substrate, as shown i n Figure 6, contains predefined patterned areas of exposed " f i r s t shot c a t a l y t i c " r e s i n and areas of "second shot-non c a t a l y t i c " r e s i n . Another way of looking at the two-shot process i s to consider the technique to be a derivation of metal insert molding except that the f i r s t shot i s p l a s t i c rather than a stamped or formed metal. The unique attribute of t h i s process i s that upon e j e c t i o n from the molding machine, two-shot ( f u l l y formed) p l a s t i c parts are ready for s e l e c t i v e e l e c t r o l e s s p l a t i n g without the need f o r imaging, masking or r e s i s t application/developing processes. Highly complex 3-D shaped molded substrates containing recessed c a v i t i e s and multi-axis c i r c u i t r y may be produced using the "in-mold" imaging techniques of two-shot molding. The f i n a l process step i s a chemical p l a t i n g of " c a t a l y t i c " r e s i n s i t e s with e l e c t r o l e s s copper/nickel/tin etc.



FRISCH & ROWE

Figure 5.

Figure 6 .


Two-Shot Process:

F i r s t shot molded i n s e r t .

Two-Shot Process: Second shot composite molding.


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Deposition i n i t i a t e s only on exposed (non-overmolded) " f i r s t shot" catalyzed r e s i n areas. Second shot (non-plateable) material i s "non receptive" to e l e c t r o l e s s p l a t i n g . The r e s u l t i n g s e l e c t i v e m e t a l l i z a t i o n i s shown i n Figure 7.


MOLDED CIRCUIT INTERCONNECT APPLICATION An example of a multi-functional electro-mechanical interconnect device produced using the two-shot process i s shown i n Figure 8. This device i s used i n an new Eastman Kodak Desktop Microfilmer performing functions of document/paper sensing. E l e c t r o l e s s copper and n i c k e l platings are deposited on highly i r r e g u l a r surfaces with high consistency and uniformity. Furthermore, through-hole m e t a l l i z a t i o n i s achieved with p l a t i n g aspect r a t i o s approaching nearly 8:1; i . e . , 0.030" diameter i n a 0.230" part thickness. The 3D molded interconnect i s uniquely shaped to provide s p e c i f i c mechanical features. As shown i n Figure 8, three i n d i v i d u a l c i r c u i t traces (plated pads) function as three separate switches whose p i t c h i s such that for documents containing holes, even i f one or two traces coincided with a hole, at least one c i r c u i t trace would s t i l l detect a document presence. Trying to package three o p t i c a l sensors that close together would be d i f f i c u l t or nearly impossible.( 6) The p r o f i l e of the surface containing the three c i r c u i t traces, as shown i n Figure 9, i s sloped (angled). This slope, prevents edges or corners of a document from getting caught or snagged. FUTURE OF PERFORMANCE POLYMERS and 3-D MOLDED SUBSTRATES The d r i v i n g force behind this emerging technology i s the engineering design freedom and c r e a t i v i t y developed through three-dimensional component packaging. "The strongest a t t r a c t i o n for "moving into molding" occurs when the benefits of s t r u c t u r a l customization can be synergized with the c i r c u i t customization. The molded board industry w i l l grow on the following bases : (7) Some penetration into conventional double-sided, plated through-hole (pth) printed c i r c u i t board areas on the basis of pure cost savings i n the board cost. 11

A degree of penetration i n t r a d i t i o n a l areas on the basis of improved d i e l e c t r i c performance and/or thermal s t a b i l i t y under h o s t i l e environmental exposure. The r e a l i z a t i o n of t o t a l system cost savings by designing boards with assembly and other component cost-saving features b u i l t - i n . The creation of molded c i r c u i t boards which reduce "down stream" assembly costs. The bringing together of 3-dimensional molding s k i l l s and interconnection systems to create components which do two jobs i n one, acting as a case, cover or some kind of housing, at the same time doubling as a molded c i r c u i t boards." £ 8 ^



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Figure 7.


Molded document sensor integrates s e l e c t i v e p l a t i n g and Mechanical/Structural features.

Figure 8.

Two-Shot Process: Selective p l a t i n g completes c i r c u i t i z a t i o n process.


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Figure 9.

Molded document-sensor side-view: C i r c u i t r y on 5 degree tapered surface assures r e l i a b i l i t y and product functionality.


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LITERATURE CITED 1. BPA (Technology & Management, Ltd.) Technology Update 8705, Molded Circuit Boards: Surrey, U.K., 1988, p 1. 2. Reference 1, p 3. 3. Reference 1, Preface. 4.

Reference 1, pp 1,2.


5. MODERN PLASTICS ENCYCLOPEDIA; McGraw-Hill, Inc.: New York, 1988; Vol. 65, No. 11, pp 34,46-49, 50,84,108-110. 6.

Hamlin, R.J.; Romansky, JA. Proc. Technical Program, NEPCON EAST Boston, MA, 1988.

7. Reference 1, p 23. 8. Reference 1, p 26. RECEIVED April 7, 1989


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