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Component & assembly issues in PCB design

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  • The glass transition temperature (TG) of the resin system is the temperature at which the material transitions from a rigid state to a softened stateCTE is a measure of the material expansion both below and above the Tg expressed in parts per million (PPM).Decomposition temperature (Td) is a value derived by measuring the weight loss of the sample vs. temperature.
  • • Component Packages Are Getting Smaller • 0603 > 0402 > 0201>01005 • BGA > CSP > WL-CSP > ?• Assembly Design Density Is Increasing • Tighter Component to Component Spacing (Spacing Smaller Than 0.020” Common) • Smaller Copper (Pad) Interconnections• Assembly Process Margins Are Tighter • Thermal Balance @ Pad Level Is Critical (Trace/Via Connection Size to Pads) • Component Placement Accuracy
  • Lead has good electrical & thermal conductivities. The use of lead in electronics assembly operations has come under scrutiny due to health and environmental concerns associated with lead exposure. HASL-Hot Air Solder Level, OSP-Organic SolderabilityProctectants, ENIG-Electroless nickel immersion gold, Ag-Silver,Sn-Tin
  • Storage of PCBs prior to assembly is much more important. Drying may be required! Release of moisture may be more important than absorption.
  • If plastic packaged parts have been manufactured or stored under humid conditions, they can develop cracks on soldering, by whatever method this is carried out. Absorbed moisture turns to steam when heat is applied, building up a pressure of several atmospheres in the interior of the component, which causes the housing to ‘balloon’ and crack. The stages of this so-called ‘popcorn effect’ are shown in Figure 1. Ultrasonic microscope photographs confirm that delamination has occurred, providing an easy path to the chip and bond pads both for moisture and external sources of ionic contaminationDelamination and failure of packaging due to expansion of trapped moisture
  • Measling: Whitish, discrete spots or crosses below the laminate surface - usually induced by thermal shock / stress. Measling that bridges uncommon conductors is unacceptable.A whisker is a slender needle-shaped metallic growth between a conductor and a land. Typically the result of mechanical stresses in high tin compounds, it is a reliability concernPoor wetting is an indicator of poor solderability, improper flux, or contamination. Nonwettingresults in the solder forming a ball or beading on the termination surface. The fillet is convex; no feathered edge is apparentVoids are an indication of improper process control, and are typically caused by insufficient solder, solder wicking / thieving, or contamination.
  • Flux residue indicates improper / incomplete cleaning.Bridging is an indicator of poor process controls (i.e.: excess solder, smeared paste, improper lacement, incorrect heat)Excess solder / Solder flooding / is an indicator of improper / incorrect process controls, and is typically seen in wave solderingPinholes are typically small holes in the solder surface, leading to a void of indeterminate size within the solder terminationPorous solder exhibits an uneven surface and a spongy appearance that may contain a concentration of small pinholes and voids Voids are caused due to Surface tension is higher than Sn-Pb, flux residues can not escape
  • The glass transition temperature (TG) of the resin system is the temperature at which the material transitions from a rigid state to a softened state. Tg is the material property typically used by industry to compare thermal robustness of laminates; however, it is not a good indicator of lead free compatibility.Decomposition temperature (Td) is a value derived by measuring the weight loss of the sample vs. temperature. Td is an important material property used to assess thermal survivability.
  • Most design issues are tied to material choice and finish. Most likely, every market will need to go lead-free because device manufacturers are not likely to support two component finishes – one with lead and one without.
  • The presence of significant levels of solder residues, tin and lead, along with ionic material on all PCB areas after production is a latent defect. These residues are the essential feedstock for corrosion processes which require heat, humidity and applied EMF in order to proceed by several possible routes including redox reactions, galvanic processes, electrolysis, atmospheric oxidation, ion transport and ionic salt formation. The long term consequences of these processes are well known including metal filament and dendrite growth, potentially leading to contact resistance changes, arcing and complete circuit failure.
  • Transcript

    • 1. Review on component & assembly issues VSD 534 M.Sc. [Engg.] in VLSI System Design Module Title: High speed board design Date: 30 April 2012 M. S. Ramaiah School of Advanced Studies 1
    • 2. Contents• Introduction• Major issues• Lead free PCB• Lead free issues• Voids• Lead free materials• Conclusion• References M. S. Ramaiah School of Advanced Studies 2
    • 3. Introduction• As the industry has moved to lead-free assembly processing, the performance demands on the lead free compatible PCBs have significantly increased• This new lead-free arena brings different PCB materials and surface finishes to the table as well as specific changes required in the PCB manufacturing process to support lead-free compatibility• It’s important to understand the effects these different materials will have on PCB fabrication, as well as the need to respond to these changes• When exposing laminates to higher temperatures, glass transition temperature (Tg), coefficient of thermal expansion (CTE) and decomposition temperature are data points to consider M. S. Ramaiah School of Advanced Studies 3
    • 4. Major issues• Component packages are getting smaller• Assembly design density is increasing• Assembly process margins are tighter• Use of lead free materials• Solderable finish selection• PCB material selection• Reliability factors M. S. Ramaiah School of Advanced Studies 4
    • 5. Lead free assembly• Higher processing temperatures are required (20 C to 40 C higher)• Narrower reflow process window. Between liquidus and peak (was 400C, now 200C)• There are many lead-free patents. Alloy patent issues are now resolved by cross-licensing between major suppliers• The right finish choice for lead-free…OSP?...HASL?… ENIG?... Ag?… Sn…?• Formation of tin whiskers… Figure 1. ENIG finish [3] M. S. Ramaiah School of Advanced Studies 5
    • 6. Lead free design challenges• For lead-free assembly, peak temperatures have increased a minimum of 20ºC up to and sometimes greater than 65ºC which can damage and reduce the reliability of base materials• Increase in the frequency of blistering and delamination during lead-free assembly• Increase in the frequency of assembly related defects such as, voids, tombstoning, copper dissolution, out gassing, pad cratering, poor wet out, etc.• Transitioning to Pb-free soldering the industry faces significant risks of solder joint fragility associated with all the commonly used solder pad surface finishes. M. S. Ramaiah School of Advanced Studies 6
    • 7. Uncontrolled environment• All materials will absorb some level of moisture• Water vapor increases internal stresses significantly• The amount of water absorbed, depends on the type of package, material & the conditions under which the part is stored Figure 2. Moisture pick for different base materials M. S. Ramaiah School of Advanced Studies 7
    • 8. Popcorn effect• Delamination and failure of packaging due to expansion of trapped moisture• Moisture can be present as liquid in voids• Absorbed moisture turns to steam when heat is applied, building up a pressure of several atmospheres in the interior of the component, which causes the housing to ‘balloon’ and crack Figure 3. Illustration of popcorn effect & cracking [3] M. S. Ramaiah School of Advanced Studies 8
    • 9. Lead free issues (1/2)Figure 4. Lead free issues [4] M. S. Ramaiah School of Advanced Studies 9
    • 10. Lead free issues (2/2)Figure 5. Lead free issues [4] M. S. Ramaiah School of Advanced Studies 10
    • 11. Voids• Found with all lead-free solders• Occurs at the interface between solder & pad• Flux design & process control are critical• Not a reliability issue until 0.5 mm pitch• Voids are caused due to surface tension being higher than Sn-Pb, also flux residues can not escape Figure 6. Formation of voids [2] M. S. Ramaiah School of Advanced Studies 11
    • 12. Lead-free materialsTable 1. Lead free materials [1] M. S. Ramaiah School of Advanced Studies 12
    • 13. FR-4 vs. FR-370HR base materialFigure 7. Traditional FR-4 & high performance FR-370HR [1] M. S. Ramaiah School of Advanced Studies 13
    • 14. Field failure due to corrosion• PCB’s are often required to operate in extreme cycles of temperature and humidity• Corrosion areas between IC contacts are found to contain a complex mixture of tin, lead and copper oxides.• Copper chloride residue (in red) has formed in the gap between the IC contacts.• Tin from the solder process (in blue) has also accumulated on contacts and on all areas of the PCB Figure 8. Image overlay from a corroded area between IC legs [3] M. S. Ramaiah School of Advanced Studies 14
    • 15. Summary• The need for lead-free solder assembly has led to many investigations into PCB materials and finishes• Substitutes for tin/lead alloy in the electronic industry are tin-copper for wave soldering and tin-copper-silver for re-flow soldering• Most design issues are tied to material choice and finish• Most likely, every market will need to go lead-free because device manufacturers are not likely to support two component finishes – one with lead and one without M. S. Ramaiah School of Advanced Studies 15
    • 16. References1. Bob McGrath (2005) The Effects of Lead-Free on PCB Fabrication, [White paper] PCB East, Joseph Fjelstad SiliconPipe Inc., San Jose, CA2. Clyde F. Coombs (2008) Printed Circuit Handbook. New York: Mc-Graw Hill3. Dale lee (2011) Root Cause Failure Analysis of PCB Assemblies [online] available at <http://www.smta.org/chapters/files/Wisconsin_DFX-Root_ Cause_Failure_Analysis_Final.pdf> Retrieved on 29th April 20124. NASA Workmanship Standards (2002) Through Hole Soldering General Requirements [online] available at <http://workmanship.nasa.gov/lib/insp/2 %20books/links/sections/601%20General%20Requirements.html> Retrieved on 29th April 2012 M. S. Ramaiah School of Advanced Studies 16
    • 17. Thank YouM. S. Ramaiah School of Advanced Studies 17
    • 18. RemarksSl. No. Topic Max. marks Marks obtained 1 Quality of slides 5 2 Clarity of subject 5 3 Presentation 5 4 Effort and question handling 5 Total 20 M. S. Ramaiah School of Advanced Studies 18