FYS4260/FYS9260: Microsystems and Electronics Packaging and Interconnect Printed Circuit Boards

Transcription

1 FYS4260/FYS9260: Microsystems and Electronics Packaging and Interconnect Printed Circuit Boards Interior detail from an Apple iphone 5 printed circuit board

2 Learning objectives Understand how printed wiring/circuit boards are constructed Types of printed circuit boards High density interconnect technology Background literature: Malestroem: The printed circuit handbook 6 th ed.

3 Printed Circuit Board A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. PCBs can be single sided (one copper layer), double sided (two copper layers) or multi-layer. Conductors on different layers are connected with plated-through holes called vias. Advanced PCBs may contain components - capacitors, resistors or active devices - embedded in the substrate

4 Printed Wiring Board vs. Printed Circuit Board In principle/by definition, an electrical circuit is not a circuit before it is closed, that is, components have been attached. When the board has only copper connections and no embedded components, the term printed wiring board (PWB) has been recommended. Although more accurate, the term printed wiring board has fallen into disuse, but is used to clarify when we specifically discuss issues related to the substrate and wiring.

5 What do you think are important requirements when you must select substrate technology? Discuss with the one next to you!

6 Some requirements on substrate technology Function Requirement Electrical property High conductivity in conductive wires High resistance between wires Mechanical properties Stiff and rigid Low weight Low coefficient of thermal expansion (why?) Chemical resistance Must withstand solder fluxes and other chemicals during manufacturing Must be stable in usage environment Fire resistance An overheated component may not cause the entire circuit board to catch fire. (FR = flame retardant) Processability Compliant with component assembly process Possible to shape to fit the application Adhesion Many components are glue attached to the substrate Metallization must stick on the substrate Moisture absorption Organic polymers tend to absorb water from the environment. Price and availibility Cost and availability of processing equipment/facilities are always concerns

7 Classification of circuit boards 1. Base material 2. How wires are defined 3. Physical nature 4. The way wires are formed 5. Number of layers (single/double/multi layer boards) 6. Plated through holes 7. Production process Malestrom: Printed Circuit Board Handbook, 6 th ed

8 RoHS directive: Impact of lead-free soldering requirement changes FR-4 composition Malestrom: Printed Circuit Board Handbook, 6 th ed

9 Another way of classifying laminates Reinforcement Flame Resin Cotton Woven Mat Glass retard- Grade Epoxy Polyester Phenolic paper glass glass veil ant XXXPC * * FR-2 * * * FR-3 * * * FR-4 * * * FR-5 * * * FR-6 * * * G-10 * * CEM-1 * * * * CEM-2 * * * CEM-3 * * * * CEM-4 * * * CEM-5 * * * * CEM-6 * * * CEM-7 * * * * CEM-8 * * * Table 5.1: Conventional laminates for printed wiring boards. (The designations are according to National Electrical Manufacturers Association, NEMA, USA.)

10 Laminate material classification (alternative to Fig 5.1 in H&O) Malestrom: Printed Circuit Board Handbook, 6 th ed

11 Physical properties of some common base materials Malestrom: Printed Circuit Board Handbook, 6 th ed

12 FR4 laminate structure FR4 is the most widely used starting point for PCBs FR4 consists of a woven glass fibre mesh soaked in organic polymer (epoxy) with copper layers laminated possibly with filler material (similar to steelenforced concrete) FR means Flame Retardant Organic substrates: Substrates where organic polymers are binders Prepreg: The laminate before copper is added and before final curing of laminate Fig. 5.1: Woven glass fibre for printed wiring board reinforcement

13 FR4 build-up (two-layer structure) HDI Handbook p 188

14 Conventional manufacturing process for PCB labminates Printed Circuits Handbook 6 th Edition

15 Printed Wiring Board technology examples Fig. 5.2: Printed wiring board structures with varying complexity: a) Single sided and double sided. b) Double sided through hole plated with bare Cu or Sn/Pb surface. c) Four layer board. d) Six layer board with two Cu/Invar/Cu cores.

16 And multilayer structures become increasingly dense Multilayer boards and high density interconnects are increasingly more common and available Illustration of multilayer Printed Wiring Board FYS4260/FYS9260 Frode Strisland 17

17 Multi-layer PCB Cross section of 14-layer multilayer printed wiring board, showing a typical inner layer and prepreg material relationship. In this case, to reduce z-axis expansion, the innerlayers are polyimide, while the prepreg material is semicured polyimide.typical signal, power, and ground layers are also indicated, as well as the thickness of the copper foil for each layer. Printed Circuits Handbook 6 th Edition FYS4260/FYS9260 Frode Strisland 18

18 Single sided boards processing steps 1. Drilling / punching of holes 2. Panel cleaning 3. Printing of etch resist 4. Etching 5. Stripping 6. Printing solder resist 7. Curing of solder resist 8. Cleaning of solder areas 9. Deposition of solder coating 10. Punching of holes and edge contour (or drilling/milling) Illustration of steps 3-5 This is a subtractive process (wires defined by removal of Cu) Alternative: Additive processes (based on plating Cu)

19 Printed Wiring Boards: Manufacturing Process Two main classes of wire definition approaches: Graphical the image of the master circuit pattern is formed photographically on a photosensitive material, such as treated glass plate or plastic film. The image is then transferred to the circuit board by screening or photoprinting the artwork generated from the master. Discrete wire interconnection boards Discrete-wire boards do not involve an imaging process for the formation of signal conductors. Rather, conductors are formed directly onto the wiring board with insulated copper wire. Wire-wrap and Multiwire are the best known discrete-wire interconnection technologies. Because of the allowance of wire crossings, a single layer of wiring can match multiple conductor layers in the graphically produced boards, thus offering very high wiring density. However, the wiring process is sequential in nature and the productivity of discrete-wiring technology is not suitable for mass production.

20 Discrete wire interconnection board (example) Printed Circuits Handbook 6 th Edition

21 PCB production virtual tour How to make a 4-layer PCB Nice 32 min tour of how small volume 4-layer PCBs are produced. Follow tour and check against procudure outlined in lectures Link: FYS4260/FYS9260 Frode Strisland 23

22 Steps described in the EuroCircuits video FYS4260/FYS9260 Frode Strisland 24

23 Steps in making a 4-layer PCB 1. Preparations Front end tool data preparation Prepare the CAD files to produce Gerber production files Preparing the phototools Photoplotter creates films to be used during photoprocesses Add registration holes to align the different films and substrate layers FYS4260/FYS9260 Frode Strisland 25

24 Steps in making a 4-layer PCB 2. Print inner layers 1. Clean the copper laminted prepreg 2. Coating of photoresist (both sides) 3. Expose the laminated prepreg to UV light through photomask 4. Spray with alkaline material to remove unhardend resist. Afterwards a pressure wash and drying of panels 5. End result: Copper we want is covered by resist FYS4260/FYS9260 Frode Strisland 26

25 Steps in making a 4-layer PCB 3. Etch inner layers 1. Put panels in a powerful alkaline solution to etch away the exposed copper 2. Strip of the hardened photoresist 3. End result: Copper pattern we want is established on inner layer panels FYS4260/FYS9260 Frode Strisland 27

26 Steps in making a 4-layer PCB 4. Punching of registration holes and optical inspection 1. Punch registration holes in panel corners to allow alignment with other panels 2. Inspect the panel to ensure that it is according to objectives (this is the last chance to fix failures before the panel is sandwiched with other panels) FYS4260/FYS9260 Frode Strisland 28

27 Steps in making a 4-layer PCB 5. Lay-up and bonding of inner and outer layers 1. Outer layer consist of sheets glass cloth impregnated with uncured epoxy (prepreg) and a thin copper foil 2. Stack foils in press 3. Heat and pressure join the sheets and hardens the prepreg to form a lasting 4-layer PCB with copper on outer layers Heat & pressure Heat & pressure Comment: More complex boards have more layers involved in the lay-up and bond process End result FYS4260/FYS9260 Frode Strisland 29

28 Steps in making a 4-layer PCB 6. Drilling the PCB 1. Drill holes for thru-hole components and for holes linking the different conductor layers togheter. 2. Drill registration holes (use x-ray imaging to align with inner layers) 3. High speed drilling, but drilling is a slow process (one hole at a time). Minimum hole size 100 µm 4. Excess material squeezed to the outer parts of the panel are cut away. The panel is now ready for plating. FYS4260/FYS9260 Frode Strisland 30

29 Steps in making a 4-layer PCB 7. Electroless Copper deposition (plating) Need to deposition ~ 25µm thickness copper inside holes. Start by electroless process to make the hole conductive 1. Electroless copper depostion process i. Pretreat the panel ii. iii. iv. Seed with palladium particles Rinse Electroless copper deposition (approximately 1 µm) 31

30 Steps in making a 4-layer PCB 8. Image the outer layers 1. Starting point: Plated 4-layer panels with holes 2. Deposit photosensitive film (hot-roll process) 3. Expose photosensitive film to UV light through photomask 4. Electroplating the board with copper 1. Place in electrolyte that contains Cu 2+ ions, for example CuSO 4 dissolved in H 2 SO 4 2. Add bias to panels to drive Cu 2+ ions to the panels 3. Deposit electroplated copper to reach 25 µm 4. Rinse 5. End result: 25 µm copper thicknessin holes Comment: The process also adds copper on the 5 exposed panel planes 32

31 Steps in making a 4-layer PCB 9. Etch the outer layer 1. Cover with tin as a etch resist. 2. Dissolve and wash away the photosensitive film 3. Etch exposed copper (not covered by tin) in a alkaline solution 4. Strip off the thin tin coating End result: 25 µm copper thicknessin holes Comment: Exposed copper tracks and vias, unwanted copper removed 4

32 Steps in making a 4-layer PCB 10. Apply solder mask Most PCBs have an epoxy solder ink mask applied to prevent the solder circuit and to prevent shortcircuiting during assembly 1. Panels are cleaned and brushed 2. A photo-sensitive epoxy solder mask ink applied 3. Dry the solder mask ink sufficiently to allow it to be printed (photo processed) 4. Coated panels are imaged. UV lamps harden the solder mask where the mask is clear. This is where the solder mask is needed. 5. Unhardened solder mask is dissolved and washed away 6. The solder masks are hardened further in a conveyerized oven

33 Steps in making a 4-layer PCB 11. Electroless gold over nickel coating A surface coating are added on the exposed copper surface to ensure solderability and durability of the metal pads 1. Electroless nickel deposition (5 µm) on exposed copper surfaces 2. Electroless gold depostion (0.1 µm) on nickel/copper surfaces 3. Deposition of tin: Panels are dipped in molten tin. When removed, hot air flow is used to blow off excess tin. A thin layer of tin (~2 µm) remains on all surfaces that are not covered with solder mask. Cu (> 25 µm) Ni (5 µm) Au (0.1 µm) Sn (2µm)

34 Steps in making a 4-layer PCB 12. Hard electroplated gold for edge connectors Edge connectors that will be inserted and removed multiple times need hard gold protection of pads. These are electroplated with only the edge of the PCB lowered into the plating bath. Approximately 2 µm of gold is applied. Other parts are protected by a tape during the process. Tape is peeled off after the process is completed

35 Steps in making a 4-layer PCB 13. Silk screen The silk screen is used to define legends to the different components. This is done using ink jet printing. Often, the ink jet printing is cured together with the final curing of the solder mask

36 Steps in making a 4-layer PCB 14. Final electrical testing, profiling and 1. Logical testing that the different conductive paths are connected where required and nonconductive elsewhere. 2. Following the successful electrical test, the panels are cut into the target size PCBs using a milling machine or a V-cutter. 3. Final optical inspection 4. Wraping and boxing of the finished boards ready to be shipped to customers

37 Processing of Double Sided Thru-Hole Plated Boards Comment: Process description based on the production of eutectic tin-lead solder. This is not according to industry standard, but illustrates the principles and the way the project prototypes are manufactured FYS4260/FYS9260 Frode Strisland 39

38 Graphical illustration of the thru-hole plated boards process steps Fig. 5.5: Through hole plated PWB, process steps: a) Panel plating. b) Pattern plating. c) Hot air levelling. FYS4260/FYS9260 Frode Strisland 40

39 FYS4260/FYS9260 Frode Strisland 41

40 Via Holes in Multilayer Printed Wiring Boards Fig. 5.8: Types of via holes: a) Through hole. b) Buried hole. c) Blind hole. Figure d) shows a microscope section of a drilled blind via. (Contrave s "Denstrate" process).

41 Etch factor Etch control: Under etch/etch factor Accuracies in subtractive processes limited by underetching. Additive process avoids this problem, but requires cleanroom processing and collimated light Fig. 5.9 b): Underetch and etch factor.

42 Metal Core Printed Wiring Boards Better heat conduction TCE matching with ceramic packages Most common: Cu/Invar/Cu Fig. 5.2.d) Six layer board with two Cu/Invar/Cu cores.

43 Metal Core Boards, continued Fig a): Cross section of metal core board with one Cu/Invar/Cu core (Texas Instruments). Invar Cu Fig b): Thermal coefficient of expansion of Cu/Invar/Cu, as function of the composition (Texas Instruments).

44 Special Boards Flexible printed wiring boards Dynamic or static bending. Uses: Movable parts and odd shaped, cramped places

45 Flexible Printed Wiring Boards, continued Fig. 5.19: Flexible printed wiring boards: Most of the electronics in Minoltas camera Maxxum 9000 is on two flexible printed circuit boards.

46 Rigid-flex: Combination of rigid and flexible components on a sigle PCB Rigid-flexible printed circuit boards directly combine the advantages of flexible and rigid printed circuit boards. This combination of technologies brings the user a variety of advantages especially in terms of signal transmission, overall size, 3 dimensional assembly flexibility and stability. FYS4260/FYS9260 Frode Strisland 48

47 Table 5.4: Properties for materials used for flexible printed wiring boards. Flexible Printed Wiring Boards, continued Typical values Unit Glass Epoxy Polyester base laminate Polyimide base laminate Solderability C/s 260/10 230/1 260/10 Max. continuous operating temperature C Tensile strength kp cm Peel strength to copper kp 4,5 1,8 1,3 Moisture absorption % 0,5 0,8 2,5 Coefficient of linear expansion C -1 1, , , Etch shrinkage: Machine direction % 0,2-0,8 1,0-0,55 0,45-0,25 /transverse direction Dielectric constant (60 Hz) 3,4 3,25 3,5 Dissipation factor (1 khz) 0,037 0,006 0,003 Resistivity ohm cm 1, Cost ratio (laminate only) 1,4/2 1 2/3 Comments Not suitable for continuous folding use. Max. peel strength to copper and minimum elongation. Sensitive to solder heat. Lowest cost. Good physical and electrical properties Non-flammable. Outstanding physical and electrical properties.

48 Membrane Switch Panels Purpose: Switches and informative instrument fronts. Fig a): Membrane switch panel, schematically. Top: Structure Bottom: Cross section of a normal panel and a panel with metal dome.

49 End of Chapter 5: Technologies for Electronics Overview Important issues: Many circuit board technologies are used. You should understand and be able to explain the main processing steps in order to develop a PCB