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PWB/Substrate Design Tutorial

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  1. PWB/Substrate Design Tutorial Larry Smith, Ph.D. Chi-Shih Chang, Ph.D September 8, 2003

  2. Organization of Tutorial • Product Design Considerations (Chi-Shih Chang) • Evolution to System-in-a-Package (SiP) • Chip-on-Board (CoB) • System-on-a-Chip (SoC) and Stacked-Die Tradeoffs • SiP Implementation • SiP and stacked die design considerations • Substrate technologies • Electrical design • Thermomechanical design • Bare substrate testing • Assembled substrate testing • PWB Design (Larry Smith) • What’s different about PWB design using die products? • General design issues • Wirebond designs • Flip-chip designs • CAD Tools

  3. Introductions • Larry Smith • Design Manager, K&S Substrate Division • BGA substrates for high IO flip-chip • Program Manager, MicroModule Systems • MCMs, SiPs, microBGAs • Module Manager, Dell Computer • MCM for high-volume notebook computers • Technical Director, MCC Packaging/Interconnect/HVED Program • R&D Consortium: design, fabrication, assembly, test, program management • Background • High-density thin-film interconnect • Electrical, thermal, mechanical modeling

  4. Introductions (cont’d) • Chi-Shih Chang • SMS Micro, Inc. provides consulting services for electronics packaging and signal integrity • VP of Adv. Products, High Connection Density, Inc. • Strategic Applications Manager, K&S • Senior Fellow, Sematech • Member of the IBM Academy of Technology • Background • Semiconductor devices, IC designs, and testing • Electromagnetics and transmission lines • Electrical design and signal integrity • Packaging technologies • ITRS AP-TWG member since 1995

  5. Organization of Tutorial • Product Design Considerations (Chi-Shih Chang) • Evolution to System-in-a-Package (SiP) • Chip-on-Board (CoB) • System-on-a-Chip (SoC) and Stacked-Die Tradeoffs • SiP Implementation • SiP and stacked die design considerations • Substrate technologies • Electrical design • Thermomechanical design • Bare substrate testing • Assembled substrate testing • PWB Design (Larry Smith) • What’s different about PWB design using die products? • Design issues • Wirebond footprint creation • Flip-chip escape routing • CAD Tools

  6. Chip-on-Board (CoB): WB ICs • Bond fingers beyond die edges, allowing less dense PWB line width and spacing • Footprint on PWB much less than that of QFP • Reduced connection length between ICs, less series resistance, inductance & parasitic capacitance • Use low modulus die adhesive to buffer the mismatch of the coefficient of thermal expansion (CTE) of IC and that of PWB • Programmable wire bonding accommodates future die shrink without PWB redesign • Wafer-level test & burn-in, if needed

  7. WB IC Example Source: M. Roston, et. al., “Assembly Challenges Related to Fine Pitch In-Line and Staggered Bond Pad Devices,” Proc. 53rd ECTC, May 28-30, 2003, New Orleans, pp. 1334-1343.

  8. Short Wire for High Speed Applications

  9. CoB: WLP ICs • Area array solder pads underneath die at 0.5-0.4 mm pitch, allowing relatively small number of I/Os • Footprint on PWB less than that of WB ICs • Further reduced connection length between ICs • Should reduce I/O array footprint (thus # I/Os) to a fraction of the die size to facilitate future die shrink • Wafer-level test & burn-in preferred. They may be made at die-level, if necessary

  10. CoB: Solder Flip-Chip ICs • Area array solder pads underneath die at 0.15-0.25 mm pitch, allowing a large number of I/Os • Large number of I/O pads available for V/G connections, capable of carrying current for high power IC. They also reduce inductance and switching noise • Large number of signal I/Os for wide data bus • Very small footprint on PWB • High density PWB needed • Mismatch in CTE of IC and that of PWB presents a problem for large IC, requiring underfill encapsulation

  11. Solder Ball Flip Chip IC Example

  12. CoB: Adhesive Flip-Chip ICs • Peripheral as well as area array connection pads underneath die at 0.1-0.2 mm pitch, allowing the maximum number of I/Os • Extremely high density PWB needed, unless number of I/O rows being limited • Minimum footprint on PWB • Adhesive serves the function of underfill encapsulant to mitigate CTE mismatch concern • Relatively low temperature at assembly • Limited current carrying capability, not suitable for high current IC

  13. Organization of Tutorial • Product Design Considerations (Chi-Shih Chang) • Evolution to System-in-a-Package (SiP) • Chip-on-Board (CoB) • System-on-a-Chip (SoC) and Stacked-Die Tradeoffs • SiP Implementation • SiP and stacked die design considerations • Substrate technologies • Electrical design • Thermomechanical design • Bare substrate testing • Assembled substrate testing • PWB Design (Larry Smith) • What’s different about PWB design using die products? • Design issues • Wirebond footprint creation • Flip-chip escape routing • CAD Tools

  14. SoC & Stacked-Die Tradeoffs • SoC benefits • Integrating additional functions/features, thus reducing the required number of ICs • Extremely wide bus between functions on IC • High speed connections between functions on IC • Reduced board area occupied • SoC Product Consideration (next chart)

  15. SoC in a Competitive Marketplace • Product life & market volume may be limited • Commodity uP, uC, DSP, flash, SRAM have higher volume & lower cost • SoC targets a specific system product • Mask cost adder (approx. $ 1M per mask set) • Wafer cost • Additional process steps required to integrated analog, DRAM, flash, … etc. • Yield loss associated with additional process steps • Design time / Time-to-market • Require new IC design for each new product • Adding new product features for product upgrade • Require new IC design when adding new features

  16. Stacked-Die Packages • Commodity ICs: uP, uC, DSP, SRAM, DRAM, Flash, Analog, RF, GaAs, ... • Ease of use • Higher level of reuse/lower cost • Faster time-to-market • Examples • Flash on SRAM (smaller die on top) • Flash on flash (same die size, a spacer needed) • Memory on baseband IC • Processor on processor • Stacked-die enabling technologies • Stacked-packages alternative

  17. Stacked-Die Example -1

  18. Stacked-Die Example - 2

  19. Stacked-Die Enabling Technologies • Wafer thinning • Die-to-die bonding • Low loop height wire bonding • Die attach control • Thin and dense substrate • High yield assembly process • High quality die products • Mechanical stress management • Testing

  20. Stacked-Packages Alternative • Larger footprint and larger height than stacked-die package • Higher profile than stacked-die packages • Flexibility in supply chain • Test and burn-in at individual package level

  21. Stacked-Package Comparison

  22. Organization of Tutorial • Product Design Considerations (Chi-Shih Chang) • Evolution to System-in-a-Package (SiP) • Chip-on-Board (CoB) • System-on-a-Chip (SoC) and Stacked-Die Tradeoffs • SiP Implementation • SiP and stacked die design considerations • Substrate technologies • Electrical design • Thermomechanical design • Bare substrate testing • Assembled substrate testing • PWB Design (Larry Smith) • What’s different about PWB design using die products? • Design issues • Wirebond footprint creation • Flip-chip escape routing • CAD Tools

  23. SiP Implementation • SiP applications and examples • Portable products – Cell phone, digital camera, … • Baseband processor, application processor, flash • Include passives on SiP, instead of on the motherboard (improved signal integrity) • SiP benefits

  24. SiP with WB & Passive

  25. SiP with WB & CSP & Passive

  26. SiP with FC & CSP & Passive

  27. Embedded Passives in Substrate

  28. SiP Benefits • Small form factor (lower PWB cost) • Light weight for portable products • Less aggressive I/O pitch than that with COB (lower PWB cost) • Improved performance (reduced interconnect length) • Mixed IC technologies • Faster time-to-market • Ease product upgrade

  29. Organization of Tutorial • Product Design Considerations (Chi-Shih Chang) • Evolution to System-in-a-Package (SiP) • Chip-on-Board (CoB) • System-on-a-Chip (SoC) and Stacked-Die Tradeoffs • SiP Implementation • SiP and stacked die design considerations • Substrate technologies • Electrical design • Thermomechanical design • Bare substrate testing • Assembled substrate testing • PWB Design (Larry Smith) • What’s different about PWB design using die products? • Design issues • Wirebond footprint creation • Flip-chip escape routing • CAD Tools

  30. Substrate Technologies • Multi-layer FR-4 laminate • Add buildup layers • Photo via (photo-sensitive dielectric needed) • Laser via (general dielectric materials) • Reduce coefficient of thermal expansion (CTE) • Copper-invar-copper (CIC) to replace copper V/G reference planes • Glass ceramic • Low dielectric constant / loss materials • Reduce capacitive loading • Reduce dielectric loss for serial data communications

  31. Laser Microvia on Buildup Layer

  32. Two Laser Via Technologies

  33. Substrate with Teflon on CIC

  34. Laser Drilling through Reference Plane

  35. ALIVH Structure & Technologies

  36. Organization of Tutorial • Product Design Considerations (Chi-Shih Chang) • Evolution to System-in-a-Package (SiP) • Chip-on-Board (CoB) • System-on-a-Chip (SoC) and Stacked-Die Tradeoffs • SiP Implementation • SiP and stacked die design considerations • Substrate technologies • Electrical design • Thermomechanical design • Bare substrate testing • Assembled substrate testing • PWB Design (Larry Smith) • What’s different about PWB design using die products? • Design issues • Wirebond footprint creation • Flip-chip escape routing • CAD Tools

  37. Electrical Design • Power/ground distribution • Multiple voltages on one plane • Signal lines (To be considered a transmission line?) • Controlled impedance • Function (Line width, dielectric thickness, dielectric constant) • Signal crosstalk • Function (edge-to-edge spacing, dielectric thickness) • Noise • Magnitude of received signal depending on data pattern (noise immunity) • Propagation time depending on data patterns (timing skew) • Migrate to “differential pair”

  38. When is 10 mm wiring a transmission line (TL)? • Time-of-flight (TOF) • 3.33 (ps/mm) x sqrt(4.0) x 10 mm = 66.7 ps • 4 TOF = 0.267 ns • A 10 mm signal wiring is to be treated as a TL when the signal rise time is less than 0.267 ns, or • f (GHz)  0.35 / 0.267 = 1.31 GHz • Important parameters: • Characteristic impedance (Zo) • Propagation time constant • Or line capacitance & inductance

  39. Effect of a transmission line (TL) • To get a 1.0 volt signal across a 50-ohm TL, 20 mA of current is required. • When the signal line width in the PWB reduces, it may become a 60-ohm TL. At this discontinuity, the voltage becomes 1.091 volts, and the current becomes 18.2 mA. • Excessive changes in voltage and current along a signal line may increase circuit delay time, reduces noise margin, or even impact circuit functionality. • This is a burden to circuit and system designers

  40. Zo of a Transmission Line • Zo = (377/R)/[(WEFF/H1)+(WEFF/H2)+2.62(WEFF/H1)1/4] • WEFF  (W + T) / 1.5, when 0.3  T / W  0.6 Source: C.S. Chang, “Electrical Design Methodologies,” in Electronic Materials Handbook, Volume 1 Packaging, ASM International, 1989, pp. 25-44.

  41. Zo Dependence on Design Parameters • Zo is inversely proportional to sqrt (R) • Zo increases when H1 increases • Zo decreases when W and/or T increases • Zo has weak dependence on H2, where H2 > H1 • In typical PWB, any change in W would change Zo. This discontinuity in Zo would cause signal reflection (noise)

  42. Design Consideration for Parallel TLs • To reduce cross-talk noise, a second reference plane is very beneficial. • Reduce signal line width to maintain the same Zo. • Low cross-talk noise also reduces the effect of data patterns (in data bus and address bus) on: • Noise margin of receiver circuits • Effective signal propagation time

  43. Two Parallel Transmission Lines One line active Adjacent line picks up noise Both lines active Common mode: both switching the same polarity Difference mode: switching on opposite polarities

  44. Low Voltage Differential Pair • Point-to-point wiring net with far-end termination • Eliminate reflected signal (multiple bits sent before the first bit arrive at far-end) • Two signal lines for each signal port • Eliminate common mode noise (Simultaneous switching noise) • Drastically reduce signal cross-talk between adjacent pairs • (Minimize delay time dependence on data patterns in the bus) • Receiver circuit needs very small voltage swing • Reduce power consumption • Tolerate signal attenuation, less concern with skin-effect and dielectric losses (Accommodate longer line or higher frequency)

  45. Differential Pair Transmission Lines • Two signal lines for each signal (2X wiring requirements, but reduce voltage/ground I/O#) • Immune to noise on reference planes (Tolerate a reference plane split into multiple voltages) • Low cross-talk noise from adjacent signal pairs. • Spacing within a signal pair needs tight control.

  46. Differential Pair Transmission Lines • Low voltage swing & low power consumption • Differential receiver circuit can correctly sense an input signal even when it is attenuated to 10% or less (Single-ended receiver circuit allows attenuation at about 70%.) • Tolerate higher attenuation • Longer distance between input & output • Higher frequency • Skin effect loss proportional to square root of frequency (smooth copper surface beneficial) • Dielectric loss proportional to frequency (low loss material desirable) • Often used for broadband data communications

  47. Orthogonal Fan-out Wiring in PBGA

  48. Organization of Tutorial • Product Design Considerations (Chi-Shih Chang) • Evolution to System-in-a-Package (SiP) • Chip-on-Board (CoB) • System-on-a-Chip (SoC) and Stacked-Die Tradeoffs • SiP Implementation • SiP and stacked die design considerations • Substrate technologies • Electrical design • Thermomechanical design • Bare substrate testing • Assembled substrate testing • PWB Design (Larry Smith) • What’s different about PWB design using die products? • Design issues • Wirebond footprint creation • Flip-chip escape routing • CAD Tools

  49. Thermomechanical Design • Total power of all dice in a stack converted to power per unit area • Low CTE(eff) of the stacked-die causes thermomechanical stress on substrate • Not a problem for a small die • Use low modulus die adhesive, spacer and encapsulant • Use low CTE substrate

  50. Thermomechanical Stress