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5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutio...
5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutio...
5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutio...
5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutio...
5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutio...
5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutio...
5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutio...
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PTH Fatigue

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PTH Fatigue, IPC Failure Analysis

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Transcript of "PTH Fatigue"

  1. 1. 5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutions.com Validation Document Version 2.0 Module: Plated Through Hole (PTH) Fatigue Date: April 5, 2011
  2. 2. 5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutions.com Abstract The purpose of this validation document is to provide the user with an introduction to the design issue or failure mechanism of concern, a clear explanation of the analysis or technique the software uses to assess the risk brought upon by the design issue or failure mechanism, and the data that demonstrates the analysis or technique accurately predicts the behavior of the mechanism and is in line with industry best practices. DfR represents that a reasonable effort has been made to ensure the accuracy and reliability of the information within this report. However, DfR Solutions makes no warranty, both express and implied, concerning the content of this document, including, but not limited to the existence of any latent or patent defects, merchantability, and/or fitness for a particular use. DfR will not be liable for loss of use, revenue, profit, or any special, incidental, or consequential damages arising out of, connected with, or resulting from, the information presented within this document.
  3. 3. 5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutions.com What is Plated Through Hole (PTH) Fatigue? Plated through holes (PTHs), also known as plated through vias (PTVs), are holes drilled through multilayer printed circuit boards (PCBs) that are electrochemically plated with a conductive metal (typically copper). These plated holes provide electrical connections between layers. Because these plated holes are metallurgically bonded to annular rings on the top and bottom of the printed circuit board, they act like rivets and constrain the PCB. This constraint subjects the PTH to stresses when the PCB experiences changes in temperature. PTH Fatigue is the circumferential cracking of plated through holes (PTHs) due to the differential expansion between the copper plating (~17 ppm) and the out-of-plane coefficient of thermal expansion (CTE) of the printed board (~45 to 70 ppm) during temperature variations. This failure mechanism was first reported by Bell Laboratories in 1976 What are the drivers for PTH Fatigue? PTH Fatigue is influenced by maximum temperature, minimum temperature, PTH diameter, PTH copper plating thickness, copper plating material properties (ductility, yield strength), printed board thickness, printed board out-of-plane material properties (CTE, elastic modulus), and defects within the copper plating (voids, folds, etch pits, etc.).
  4. 4. 5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutions.com How does the software assess PTH Fatigue? The software calculates a time to failure using the industry-accepted model published in IPC-TR-579, Round Robin Reliability Evaluation of Small Diameter Plated-Through Holes in Printed Wiring Boards1. Life calculation for PTHs subjected to thermal cycling is a three step process. The first step is to calculate the stress being experience by the copper barrel of the PTH. This is provided by the equations below, where a is coefficient of thermal expansion (CTE), T is temperature, E is elastic modules, h is board thickness, d is hole diameter, t is plating thickness, and E and Cu correspond to board and copper properties, respectively. Once stress is determined, the strain range is calculated by where Sy is the yield strength of copper. This strain is adjusted by two constants: a strain distribution (Kd) factor and a quality factor (KQ). 1 IPC-TR-579, Round Robin Reliability Evaluation of Small Diameter Plated-Through Holes in Printed Wiring Boards, September 1988
  5. 5. 5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutions.com While the strain distribution factor tends to be set to a value of 1.6, KQ is dependent upon the quality of the PTH (i.e., the presence of defects such as voids, cracks, folds, etc.). The quality factor can range from 0 to 10 with the following delineations  Extraordinary (KQ = 10)  Superior (KQ = 8.7)  Good (KQ = 6.7)  Marginal (KQ = 4.8)  Poor (KQ = 3.5) Once the strain range is defined, the cycles to failure (Nf) can be calculated iteratively with Su being the ultimate tensile strength and Df being ductility of the plated copper. Reviewing the equations above, it can be seen that the designer and PCB manufacturer have the following controls over the reliability of PTHs  Out-of-plane CTE of the printed board  Plating thickness  Aspect ratio (hole diameter over board thickness)  Plating material properties (strength and ductility) and plating quality An important limitation to the IPC model is its sensitivity to the quality of the PTH. This influence is captured in a quality factor. Quality factors are identified in one of two ways. The first is assuming a conservative quality factor, either good (KQ = 6.7) or marginal (KQ = 4.8). The second is by calculating the quality factor from the results of interconnect stress testing (IST) or highly accelerated thermal shock (HATS) qualification testing. The probability distribution used to describe the failure behavior of the PTHs is a 2 parameter Weibull. The Weibull distribution is generally accepted as a good distribution for reflecting low cycle fatigue behavior of ductile metals. Two parameters, the characteristic life and the shape factor (slope). The slope typically describes the quality and the higher the slope the closer the time to failures are. The software uses different slopes that correspond to the quality factors. As the quality factor decreases the slope decreases to reflect the reduced quality.
  6. 6. 5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutions.com Validation Data One of the challenges in presenting validation data is that the predicted performance of the PTH is dependent upon a quality factor that may not be known in the design state. Instead, purpose of the validation data below is to demonstrate quality factors derived from published test data to provide guidance on values that should be selected during design evaluation. Table 1: Quality factors calculated from published test data Reference [1]2 [1]3 [2] [3] [4] Year Published 1988 1988 1997 2000 2002 Hole Diameter (mm) 0.25 0.33 0.35 0.30 0.34 Plating Thickness (m) 20 32 25 30 25 Board Thickness (mm) 2.28 2.28 1.5 1.5 2.36 Out-of-Plane CTE (ppm) 70 83 63 50 N/A Mean Time to First Failure (thermal cycles) -35 to 125C 300 N/A N/A N/A N/A -40 to 125C N/A N/A N/A N/A 750 -55 to 125C N/A N/A 925 2190 N/A -65 to 125C N/A 371 N/A N/A N/A Calculated Quality Factor 6 8 7.5 7.2 9 2 For a Class 3 product per IPC-A-600 3 For a ‘good’ vendor. In the round-robin study, vendors were rated superior, good, marginal and poor
  7. 7. 5110 Roanoke Place, Suite 101, College Park, Maryland 20740 | Phone: (301) 474-0607 | Fax: (866) 247-9457 | www.dfrsolutions.com References 1. IPC-TR-579, Round Robin Reliability Evaluation of Small Diameter Plated-Through Holes in Printed Wiring Boards, September 1988 2. D. Goyal, H. Azimi, K.P. Chong and M.J. Lii, Reliability of high aspect ratio plated through holes (PTH) for advanced printed circuit board (PCB) packages, Proc. IRPS, 129-35 (1997). 3. T. Kobayashi and S. Hayashida, A study on reliability modeling for through hole cracking failure in thermal enhanced PBGA laminate, Proc. ECTC, 1658-60 (2000) 4. J. Smetana, Plated through hole reliability with high temperature lead-free soldering, The Board Authority, 4, 50-64 (2002)

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