Modeling Printed Circuit Boards with Sherlock 3.2

Transcription

1 Modeling Printed Circuit Boards with Sherlock 3.2 DfR Solutions September 23, 2014 Presented by: Dr. Nathan Blattau Senior Vice President 9000 Virginia Manor Rd Ste 290, Beltsville MD

2 Who is DfR Solutions? The Industry Leader in Quality-Reliability- Durability of Electronics Fastest Growing Companies in the Electronics Industry - Inc Magazine Best Design Verification Tool - Printed Circuit Design 2012 Global Technology Award Winner

3 Printed Circuit Boards (PCB), also known as Printed Wiring Boards (PWB) Circuit Cards, etc.. Provides mechanical support and electrical interconnects to the electronic components Basic Materials Conductors Dielectric or insulator Structural reinforcement Woven glass cloth Fibers Must be able to provide the necessary: Electrical performance Structural performance Survive manufacturing IPC Specification for Base Materials for Rigid and Multilayer Printed Boards Covers the requirements for base materials that are referred to as laminate or prepreg. These are to be used primarily for rigid and multilayer printed boards for electrical and electronic circuits.

4 Printed Circuit Board Composite material Reinforcement (glass cloth) Polymer (resin) Copper Glass fibers polymer copper

5 Glass Style PCB laminates (and prepregs) are fabricated with a variety of glass styles Problem: All datasheet properties are for laminate with 7628 glass style Most laminate (and prepreg) in complex PCBs have a low volume fraction of glass (i.e., 1080 or 106) Glass Style Resin Volume Content Fiber Volume Content

6 Copper Thickness specified as a weight: Weight rolled out over a square foot, determines the thickness Thickness (mils) = Copper weight (oz) * oz copper = mils, 17.4 µm 1 oz copper = 1.37 mils, 34.8 µm 2 oz copper = 2.74 mils, 69.6 µm 3 oz copper = 4.11 mils, µm During manufacturing of the copper layer starts out as complete and is etched away to define the interconnects

7 Two types of resins (not including composition) are used: Pre-preg partially cured resin that flows and fills in all the etched away copper features, cures during the pressing process, does not have copper on it Laminate fully cured resin that typically has copper foil already attached to it There are many different types of resin systems Dicy Phenolic, etc.. Resin

8 PCB Materials / Laminate Selection PCB materials are frequently under specified! Some common issues: PCB supplier frequently allowed to select laminate material No restrictions on laminate changes Generic IPC slash sheet requirements used Laminates called out by Tg only and with no measurement method specified No cleanliness requirements specified Failure to specify stackup Not all laminates are created equal Failure to put some controls in places opens the door to failure

9 PCB Materials and Reliability Historically, two material properties of concern Out-of-plane coefficient of thermal expansion (CTEz) Out-of-plane elastic modulus ( stiffness )(Ez) These drive fatigue of the plated through holes Key Assumption: No exposure to temperatures above the glass transition temperature (Tg) (field environment) The two material properties (CTE and E) are driven by choices in resin, glass style, and filler Additional concern is in-plane properties (solder joint fatigue)

10 Coefficient of Thermal Expansion Is the amount a material expands when exposed to a change in temperature It is unlikely that the designer or end user will be able to influence the component properties Component packaging is typically driven by the die and assembly Passing of JEDEC level package tests May be able to pick parts with different lead frame materials Printed wiring board properties Designer can influence printed wiring board properties Glass style Laminate type Copper Thickness This is one of the main factors that drive solder joint fatigue

11 Influence of Board Properties In the past most electronic packages had CTE values closer to that of copper, 17.6 ppm/ C Larger die and smaller packages have driven a reduction in the component CTE, examples: Leadless ceramic chip resistors 5.6 ppm/ C QFN (quad flat no-leads) 8 to 12 ppm/ C The CTE of the laminates has decreased over the years The PCB laminate manufactures do not make it easy to determine the CTE of their laminate

12 Solder Joint Fatigue Elimination of leaded devices Provides lower RC and higher package densities Reduces compliance Cycles to failure -40 to 125C QFP: >10,000 BGA: 3,000 to 8,000 CSP / Flip Chip: <1,000 QFN: 1,000 to 3,000

13 Effect of Glass Style Realistic target for board CTE is between 15 and 17 ppm/ C Most laminate suppliers provide CTExy values Key concern, these values are typically for a low resin content laminate (46%-50% resin content by weight, 7628 glass style) However the most popular laminates have much higher resin contents Higher resin content = higher CTE Lower modulus

14 Calculations y x & Solving for E m Consider Positive value of E m as solution.

15 Effect of Glass Style, Exy and CTExy Modulus decreases as resin content increases CTE increases as resin content increases Copper content plays a significant role These values can be now used to predict solder joint fatigue

16 Examples - Glass Style, Modulus Ez and CTEz Glass Style Modulus of Elasticity Ez (MPa) CTEz (ppm)

17 Additional Factors to CTE Another key factor is copper thickness CTE of copper is about 17 ppm/ C One would expect that as the copper thickness increases the CTE of the board would approach this This is rarely the case Thicker copper means larger gaps when the copper is removed, more resin is required to fill those gaps When faced with thick copper layers board shops will dramatically increase the prepreg resin content to make manufacturing easier This dramatically increases the CTE of the Board

18 Examples CTE 15 ppm/c Boards with thick copper have a lot of resin (higher CTE) CTE 21 ppm/c

19 Virginia Manor Rd Ste 290, Beltsville MD

20 Sherlock 3.2 Adds additional tools for more detailed modeling of the printed circuit board Stackup tool now allows for the selection of Glass Style Fiber Material The material selected determine the overall material properties used during FEA analysis for the selected laminate Laminate manager tool Detailed FEA modeling options for PCBs New 3D part viewer Lead modeling Improved ODB++ parsing

21 Improved Stackup Tool When specifying glass style Sherlock back calculates the properties of the resin based upon the entered datasheet values (7628 glass style) The effect of different glass styles can then be calculated The properties of the fibers can also be changed Sherlock assumes resin in the copper layer gaps

22 Lead Modeling Sherlock now allows one to add through hole leads to select components As expected adding features greatly increases the FEA model complexity 3D part viewer shows the part

23 High Fidelity PCB Newest Feature Sherlock can identify and mesh copper feature within a PCB or substrate Provides unrivaled insight into risks due to warpage, thermal issues, mechanical loads, etc Virginia Manor Rd Ste 290, Beltsville MD

24 Detailed FEA PCB Modeling Mosaic technique, material properties are averaged over the individual elements

25 PCB Meshing Techniques

26 Meshing Techniques Uniform model, homogeneous properties for the whole board Layered model, homogenous properties per layer (layers have different mechanical properties)

27 Meshing Techniques, cont Uniform elements model, mechanical properties vary only in-plane Layered elements model, each element has properties computed based on location and layer

28 Lead Modeling

29 Lead Modeling (cont.) Bonded model with leads on all through hole components

30 Lead Modeling (cont.) Sherlock automatically post-processes the FEA results to make predictions for lead vibration fatigue

31 Sherlock 3.2 More accurate modeling of printed circuit boards The ability to conduct more complex simulations

32 Questions?? Thank you! Nathan Blattau Tom O Connor toconnor@dfrsolutions.com 32