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1 Effectiveness of Baking to Remove Moisture from Between PCB Ground Planes Martin Wickham & Owen Thomas 18 th March Your Delegate Webinar Control Panel Open and close your panel Full screen view Submit text questions during or at the end NPL Management Ltd - Commercial 2

2 Effectiveness of Baking to Remove Moisture from Between PCB Ground Planes Martin Wickham & Owen Thomas 18 th March

3 NPL & Electronics Interconnection About NPL The UK s national standards laboratory 36,000 m 2 national laboratory Founded in 1900 World leading National Measurement Institute 600+ specialists in Measurement Science State-of-the-art standards facilities The heart of the UK s National Measurement System to support business and society Experts in Know ledge Transfer W orld leading measurement science building 6

4 Our Technical Breadth Electronics Interconnection Group Multidisciplinary R&D and technical services for public and private sector Acoustics Advanced Materials Air Quality Biometrics Biotechnology Corrosion Dimensional metrology Environmental measurement Lasers Mass and force Micro/Nanometrology Neutron measurements Photonics Photometry/Colour Pressure RF/Microwaves Radiation Dosimetry Radioactivity Radiometry Scientific Software Sensory metrology Statistics Surface analysis Thermal measurements Time & Frequency Electrical Standards 7 EI Group Research Areas Interconnect Reliability Plastic electronics & WEEE SIR testing MWCNT Interconnect Tin Whi skers CAF Tin Pest Smart Textiles Conformal Coatings PCB Delamination Tuesday, 18 March PCB Reliability

5 Introduction Baking removes moisture from PCB and reduces risk of delamination during the reflow stage. Since going Pb-free, higher reflow temperature has increased risk of delamination Moisture content usually determined from measurement of mass increase which does not determine where moisture is present. This presentation covers delamination detection and measurement of moisture with capacitance sensors, experimental results for removing moisture from under ground planes and modelling to discuss further the effectiveness of baking in various situations. 9 Moisture in PCBs Four phase project Detecting delamination RIE measurements Capacitance measurements Modelling moisture diffusion behaviour Project Partners Aero Engine Controls Artetch BAe Systems ESA GE Fanuc MBDA Hansatech Polar Instruments Rolls Royce Marine Selex Galileo 10

6 Detecting delamination Scanning Acoustic Microscopy Interesting results indicating delamination in areas not visually apparent System needs tuning for each PCB design Unable to image areas underneath components Capital cost relatively high 11 Detecting delamination Principle of Pulsed Thermography Fast sy stem but results require interpretation Cannot v iew underneath components Some promise in comparing test board to golden board Possible to use digitial image correlation with this technique but set-up can be time consuming 12

7 RIE Measurement Route Impulse Energy Uses Time Domain Reflectometry (TDR) to measure the difference in loss between two identically constructed PCB transmission lines of different lengths Longer test line & shorter reference line, both with far ends open circuited Identical step pulses are injected into each line and reflected step pulses captured Ratio between the energies contained in reflected step pulses is proportional to extra loss introduced by additional length of test line 13 RIE Measurement 2. 3 RIE for 2116 before and after reflow 2. 2 RIE Value Results show good correlation between RIE and moisture ingression Test pattern relatively large Opportunities for size reduction (50%) Use of existing structures within PCB? Possible use for delamination measurement if delam occurs between signal line and ground plane 14

8 Capacitance Measurement Moisture ingress changes the dielectric constant and hence also the capacitance between two electrodes separated by the PCB material. Boards initially in 85ºC & 85% R.H. damp heat oven for 2 months. Agilent 4263B LCR Meter 100kHz 15 Baking Temperature 3 Bake Temperatures : 80 o C, 110 o C and 125 o C Moisture diffuses out faster at higher bake temperatures Moisture diffuses out from smaller parallel plate capacitor fastest. 16

9 Capacitor Design - Size Slotted design to allow water ingress 5 Capacitor Areas: 28, 60, 105, 180 & 410 mm 2 28 mm mm 2 All capacitor sizes tested proved to be capable to detect moisture content 17 Capacitor Design Pattern Grid design with gaps for holes 5 different designs: No Holes 25 non PTH 25 PTH 13 PTH 4 PTH Patterns not affecting ability to detect moisture content. These patterns can be used to detect moisture under ground planes. 18

10 Effect of Resist on Mass and Capacitance Measurements Capacitance change correlates well to mass change. With resist, capacitance lags mass due to unable to accurately reflect the initial decrease in mass from moisture exiting the resist layer. 19 Test Board C 4 hole densities. 4 mm, 5.5 mm and 8 mm pitch between holes PTH and nonpth 20

11 Water removal from under ground planes (1) Moisture diffuses out faster for nonpth boards and for greater hole density due to greater surface area. 21 Water removal from under ground planes (2) Moisture diffuses out of corners of 3 by 3 array faster than at the center due to the edges contributing to moisture removal 22

12 Non-PTH density- Experimental Results Effect of non-pth hole density on average capacitance (of the 9 capacitors) decrease for the test boards baked at: (a) 80 C, (b) 110 C and (c) 125 C. 23 PTH vs non-pth, 125 C Non-PTH faster desorption Comparison of bake time at 125 C for three capacitor hole densities, corresponding to (a) 25 holes, (b) 13 holes and (c) 4 holes 24

13 Experiment to modelling Experimental data can be fitted from simple diffusion models. Diffusion coefficient fitted to data at 80, 110 & 125 C. Using an Arrhenius relationship, the diffusion coefficient can be extrapolated to 20 C. With this material property, three scenarios have been studied to illustrate the effect of baking, and moisture desorption. Plated and non plated through holes Meshed ground planes Simple ground plane which has been exposed to moisture but has not had time to fully equilibrate. 25 PTH and non-pth model Cross section of through holes for a) non-pth with copper plane on outer layers, b) PTH with copper plane on outer layers and c) PTH with internal copper planes (dimensions in mm). 26

14 PTH and non-pth model Non-PTH PTH outer ground planes PTH inner ground planes Equivalent baking at 125 C Clearly the nonplated area is key to desorption As the desorption path becomes more tortuous and restricted, so does the bake time increase Hole density, and copper plane mesh open area have a significant impact 27 Modelling desorption in meshed ground planes Initially saturated with moisture Dif fusion coefficient of 33 mm 2 s -2 (simulate 125 C baking of FR4) 28

15 Mesh Ground Plane Example Time Lapse 2 hours 1 second 29 Mesh Ground Plane Example Time Lapse 1 day 1 second (12 faster) ~26 days to reduce to <5% initial moisture 30

16 Mesh Examples Snap shots showing l oss of moisture during 125 C bake 31 Mesh pitches & track width For different mesh pitches, the bake time increases as the track width increases due to the decrease in the open surface area from which moisture can escape. The bake time is also seen to increase as the pitch increases because moisture must travel a longer horizontal distance before it can escape through one of the open areas in the mesh. 32

17 Baking can increase local moisture content Simulation runs for equivalent of 120 days at 20 o C Dif fusion Coefficient 0.25 mm 2 s -2 Shows the uptake of moisture 33 Baking can increase local moisture content Baking at 125 o C Diffusion Coefficient 33 mm 2 s -2 34

18 Baking can increase local moisture content Simplified double sided PCB with copper plane 35 Baking can increase local moisture content It takes a long time to drive moisture in between ground planes Baking initially increases the moisture level at the centre of the ground plane 36

19 Store in a cool dry place If the external water vapour pressure is greater than that in the PCB, then moisture will diffuse into the PCB. Similarly, if the internal vapour pressure is greater than that externally, then moisture will diffuse out of the PCB. The saturation water vapour pressure increases with temperature, such that boards that are stored in high temperature and humidity environments absorb greater amounts of moisture than those stored in cooler environments making them more prone to delamination during the reflow process. 37 Store in a cool dry place (2) 38

20 Conclusions Capacitance suitable for measuring moisture change in PCBs, 10% change in capacitance compared to 0.35% change in mass. Capacitance sensors as small as 28 mm 2 have been shown to be adequate in moisture detection. Resist has little effect on the moisture uptake. Measurements under ground planes has shown PTH and lower hole density both increase bake times. Variation in board thickness, composition and measurement conditions also effect capacitance such that capacitance can only really be used for looking at moisture change. E.g. comparison between as received, pre bake and post bake. 39 Conclusions Modelling faster than experimental measurements Modelling allows visualisation of moisture diffusion process under ground planes within a PCB Complicated ground plane structures can be modelled and bake out times predicted. If a board is left in the environment, moisture will ingress up to the external vapour density. This may take weeks/years/decades depending on the design of a PCB. If the PCB is kept in a dry environment it won t need to be baked! 24 hour baking is not a panacea for removing moisture 40

21 Coating & Cleaning Seminar April 8-10 th Tuesday 8th April Design Rules for Conformal Coating and Cleaning Clean or No Clean for Conformal Coating Reliability 1.30 Selecting a Coating Based on Product and Environment 2.30 Reworking Conformal Coating and Component Damage 3.30 Process and Field Failures with Coatings Wednesday 9th April Dip or Spray Coating Advantages and Disadvantages IPC, ESA and NASA Inspection Requirements for Coating 1.30 In Process Quality Control for Conformal Coating PCBs 2.30 Cleaning Printed Board Assemblies for Coating 3.30 Confirming Component Compatibility with Cleaning Materials Thursday 10th April In House or Contract Coating Cost and Implementation PCB Contamination Testing Options for Coated Boards 1.30 Test Methods for Conformal Coating Reliability 2.30 Conformal Coating Defects Causes and Cures Presentations are mins maximum during the show

22 The coating & cleaning posters have been used in Europe by Lars Wallin and your presenter on cleaningworkshops. Free IPCcolour poster guide oncleaning standards andcoating plus your chance to winasetofipcstandardsatnew NPL Webinars Tues 10 June 2014 Solderability Assessment Te s ting, Ageing and Practical Impact on Assembly Yield Wed 10 September 2014 Using Surface Insulation Resistance (SIR) to Qualify Production Processes & Materials

23 Effectiveness of Baking to Remove Moisture from Between PCB Ground Planes Martin Wickham & Owen Thomas 18 th March