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

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Abraham Brown
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1 FYS4260/FYS9260: Microsystems and Electronics Packaging and Interconnect Production of Printed Circuit Boards

2 Learning objectives Recap Printed Wiring Board manufacturing process Assembly processes Background literature: The HDI handbook Malestroem: The printed circuit handbook 6 th ed.

3 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 3

4 Steps described in the EuroCircuits video FYS4260/FYS9260 Frode Strisland 4

Gerber production files Preparing the phototools Photoplotter creates films to be

5 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 5

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.

6 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 to keep is covered by resist FYS4260/FYS9260 Frode Strisland 6

7 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 7

8 Copper etching in alkaline solution Usually done in iron chloride solutions in a twostep redox reaction to copper(i) chloride and then to copper(ii) chloride in the production of printed circuit boards. Step 1: FeCl 3 + Cu FeCl 2 + CuCl Step 2: FeCl 3 + CuCl FeCl 2 + CuCl 2 Here, CuCl 2 is dissolved in the etchant solution. FYS4260/FYS9260 Frode Strisland 8

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.

9 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 9

10 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 10

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 together. 2.

11 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 together. 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 11

12 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. Seed with palladium particles iii. Rinse iv. Electroless copper deposition (approximately 1 µm) 12

13 Steps in making a 4-layer PCB 9. Image the outer layers (and eletroplate holes) 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 adds copper on all the exposed panel planes, not just the holes

14 14 Background: Electroplating Once the entire surface is coated by electroless copper, it is possible to do electroplating: The panels are biased in a bath containing dissolved CuSO4 (Cu 2+ and SO 4 2- ions). The Cu 2+ ions are driven towards the conductive panels due to the electrical bias. - + Cu -> Cu e - Cu e - -> Cu CuSO 4 /H 2 SO 4

15 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

16 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

17 Steps in making a 4-layer PCB 11. Electroless gold over nickel coating A surface coating is 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)

18 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

19 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

Steps in making a 4-layer PCB 14. Final electrical testing, profiling and testing 1. Logical testing that the different conductive paths are connected where required and non-conductive elsewhere. 2.

20 Steps in making a 4-layer PCB 14. Final electrical testing, profiling and testing 1. Logical testing that the different conductive paths are connected where required and non-conductive 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

21 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 21

22 FYS4260/FYS9260 Frode Strisland 22

23 Panel vs. pattern plating Panel plating: All surfaces are electroplated This requires considerable power Afterwards, you often end up etching a lot of the plated copper away again Pattern plating: Masking off areas where plating is not needed gives reduced surface area Less copper consumed and less copper needed to be removed in subsequent etching Increased complexity because you need to tin plate surface prior to etching

24 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.

25 Discuss Is the "EuroCircuits process" an example of a subtractive or an additive process? Would it be possible to realize the PCBs using the opposite approach? How? FYS4260/FYS9260 Frode Strisland 25

26 How to construct and assemble PCBs that work Internal details of a two head, gantry style pick-and-place JUKI SMT machine. In the foreground are tape and reel feeders, then the (currently empty) conveyor belt for printed circuit boards, and in back are large parts in a tray. The gantry carries two pickup needles, flanking a camera (marked "do not touch" to avoid fingerprints on the lens). "Pick and place internals of surface mount machine" by Peripitus - Own work. Licensed under CC BYSA 3.0 via Wikimedia Commons ediaviewer/file:pick_and_place_internals_of_surface_mount_machine.jpg

Electronics designers' role in product development: Designers are key personnel in the development of electronic products, but is not the only player, and must work in intimate cooperation with:

27 Electronics designers' role in product development: Designers are key personnel in the development of electronic products, but is not the only player, and must work in intimate cooperation with: Sales, marketing and customers Subcontractors Production process experts Cost engineers Logistics and purchasing staff etc. And a product's success relies first of all on it's ability to meet end user needs! Vscan Pocket Ultrasound: Pocket-sized ultrasound that may transform the way physicians see their patients. Developed in Horten, Norway, by GE Vingmed.

28 CAD Electronics design Advanced PCB CAD tools a neccessity, e.g. CadStar Schematics Component Library Critical Parameters (Placement Constraints, Electromagnetic Compatibility, Thermal Limitations, etc) Automatic Routing Final Touch Manual Routing (Verification by Final Simulation and Back Annotation) You learn the game from the project work I will skip details in the lectures FYS4260/FYS9260 Frode Strisland 28

29 Electronics construction: If you expect problems - you have a chance to get it right! Know your target requirements specifications Target the Right Quality The right quality is rarely "the best available" Select the Best Suited Technology/Technologies Select the appropriate Components: Right Compromise between Performance, Reliability, Cost, etc. Design for Production Design for Testability Design for Repair FYS4260/FYS9260 Frode Strisland 29

30 PCB production virtual tour Large scale automated production FYS4260/FYS9260 Frode Strisland 30

31 Automated Thru-Hole Component Mounting Fig. 7.5: Simplified process in the axial inserter: 1): Cutting the components from the tape 2): Lead bending 3) - 4): Insertion 5): Cut and clinch 6): Return to starting position.

32 Surface Mounting: Deposition of solder paste or adhesive Soldering by wave solder process or by reflow process Fig. 7.13: Application of adhesive for SMD mounting by: a): Screen printing b): Dispensing c): Pin transfer

33 Main industry level soldering techniques Reflow soldering Wave soldering FYS4260/FYS9260 Frode Strisland 33

34 Reflow Solder Process Print solder paste Mount components Dry solder paste Solder by heating to melting of paste

35 Solder Paste Consists of: Solder particles (~ 80 % by weight) Flux Solvents and additives to give good printing properties (rheology) Typical mesh count in screen: 80 per inch Area ratio: A o = a 2 /(a+b) 2 Paste volume deposited: V = V o A o t

36 Solder Paste, continued Fig. 7.17: Microphotograph of Multicore solder paste type Sn 62 RMA B 3. The designation means 62 % by weight of Sn, 35.7% Pb, 2%, Ag, 0.3% Sb, RMA flux, 75 µm average particle size, 85% metal content, viscosity centipoise.

37 Reflow soldering Heating of solder on a conveyer belt according to a prespecified temperature profile "Mark Reflow Oven" by Heller Industries Inc php. Licensed under CC BY-SA 3.0 via Wikimedia Commons - k5_1826_reflow_oven.jpg Example reflow temperature profile for eutectic lead solder "RSS Components of a Profile1" by Zithan - Own work. Licensed under Public Domain via Wikimedia Commons - svg FYS4260/FYS9260 Frode Strisland 37

38 Failure mode: Tombstoning FYS4260/FYS9260 Frode Strisland 38

39 Surface Mounting: Wave Solder Process Apply adhesive by dispenser, screen printing or pin transfer Cure by heat or UV Turn board Wave solder Double-wave soldering machine common for SMT Not all SMD components suitable for wave soldering

40 Wave Soldering principle Fluxing Pre-heating Soldering (Cleaning) Fig. 7.9: a): Principle of foam fluxer. b): Control system for density and level of the flux bath.

41 Wave Soldering principle (2) Fig. 7.10: a): Principle of wave soldering. b): The real shape of the wave.

42 "Double Wave" Wave Soldering Lambda wave Fig. 7.15: Double wave for SMD soldering. The first is a turbulent wave that wets, followed by a gentle lambda wave that removes superfluous solder.

43 Double Wave Soldering Temperature Profile Fig. 7.16: Temperature profile during wave soldering in a double wave machine.

44 Failure modes in Wave Soldering Fig. 7.14: a): Shadowing in SMD wave soldering. b): Solder bridging on fine pitch package.

45 PCB Design: Design for Manufacture Few layers Coarse pattern Standardisation Robust design (coarse tolerances) Orderly placement

46 PCB Design: Design for Manufacture Fig. 6.1.a: Proper component placement for hole- and surface mounted components FYS4260/FYS9260 Frode Strisland 46

47 Orderly Placement, continued Fig. 6.1.b: Proper component placement for hole- and surface mounted IC components

48 PCB Design, continued Important Guideline for "Robust Design": Circuits should function with large parameter tolerances: Design windows allowing for variations in component parameters. Process windows allowing for variations in each process step. Regulatory requirements on safety and EMC should be passed within the specified design and process windows.

Design of Hole and Surface Mounted PCBs: Design Parameters Minimum Dimensions: The conductor cross section areas and resistivity of the material determine maximum current capacity and

49 Design of Hole and Surface Mounted PCBs: Design Parameters Minimum Dimensions: The conductor cross section areas and resistivity of the material determine maximum current capacity and thereby minimum dimensions. Current capacity is limited by excessive heating of the conductors and the PCB. Maximum allowed ohmic voltage drop along the conductor also determines minimum dimensions.

50 Design Parameters: DC Line Resistance: DC Line resistance: RR = ρρ LL tt bb = RR LL bb ρ Ωm for Cu foil ρ is resistivity of the conductor material (ohm m) L is conductor length b L t t is conductor thickness b is conductor width Current I Sheet Resistance [ohm/square]: R sq = ρ / t R = ρ/t R = R L b R = R sq L / b 18 um copper: R sq ~ 1 mω/sq 35 um copper: R sq ~ 0.5 mω/sq

51 Design Parameters: Minimum Dimensions Fig. 6.2: Current capacity and temperature increase in conductors on PCBs. The upper figure shows the temperature increase (labels on each curve) at different combinations of cross-sections and currents). The lower figure shows the conductor cross-section (along the x-axis) as a function of the conductor width for different Cu-layer thicknesses.

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

FYS4260/FYS9260: Microsystems and Electronics Packaging and Interconnect Printed Circuit Boards Interior detail from an Apple iphone 5 printed circuit board Learning objectives Understand how printed wiring/circuit