Manufacturing of electronic equipment

Transcription

1 Manufacturing of electronic equipment Objective: To give a generic understanding on the manufacturing methods and 2 nd level interconnection materials of electronic equipments Literature: K. Puttlitz, handbook of Lead-Free Solder Technology for Microelectronic Assemblies S. Ganesan, Lead-free Electronics Department of Electrical Engineering and Automation 10/12/ Soldering processes 1

2 Design, Mfg and Use Design Aspects: Manufacturing Configurations of components and substrate Configuration choices are made primarily by electric functions + heat dissipation + mechanical reliability requirements. Other factors are The need for high packaging density The number of the IC-circuits (actives) Component and package availability Testing characteristics of the circuits Economical aspects Environmetal aspects 2

3 Design Design Aspects: Components Dimensions, forms and materials Thermal requirements (soldering heat) MSL Sensitivity to static charges Packaging types of the components Packaging density of the components on the board Materials Basic materials of the leads Metal finishes (SnPb, Ni/Au, Ag, OSP, Bi, Pd) Electrochemical coating, hot tin, immersion coating Storage Wettability (requirements, test methods) Method and equipment Soldering Reflow soldering (heating method) Wave soldering (wave type) Fluxing and preheating methods Dross forming, oil adding Conveyor transportation and in-line arrangements Process conditions (time, temperature, atmosphere) Possible flux removal Maintenance 3

4 SMT Process- Reflow Thermal Aspects of Soldering Good thermal design of the soldering minimize soldering defects. Quite often solderability or machine settings are blamed although it is often thermal difficulties which cause the problem Two most important requirements for soldering are: The parts to be soldered (e.g., circuit board contact pads and component terminations) should be raised to a temperature high enough for them to be wetted by the solder The temperature to which the components (and substrate) rise during the soldering should not be so high as to affect their operating characteristics Each component has its typical temperature + time range, within which both of these requirements can be satisfied In order of simultaneous connecting (soldering), there must be a good overlap between the soldering ranges of the various types of components used 4

5 Design Aspects: Mixed Technology Mfg Surface Mount Technology 5

6 Surface Mount Technology,... Paste Printing 6

7 Outline Solder Paste Composition Characteristics Tixothropy Rheology Purpose of the flux medium Methods for Paste Printing Screen Printing Stencil Printing Types of stencils Stencil design Printing process Proflow Dispensing What is Solder Paste? A homogeneous dispersion of alloy particles suspended in a flux medium. 7

8 Solder Paste: Thixotropy Viscosity of solder paste during stencil printing Paste Applied to Stencil End Of Print Stroke Start of Print Stroke Release From Stencil Aperture Fill Screen Printing 8

9 Printing process Stencil-Paste Printing The printing process is a complex interaction between the parameters of stencil, board, paste, design and printing operation The paste printing process contributes significantly to the failure level of the entire assembly process (more than half of the total failures caused by printing failures are common). The volume of solder required to form a correct joint is dependent on lead shape, solder land dimensions, coplanarity, board warpage, wetting behavior, etc. Paste properties for stencil printing For printing process: Good rolling behavior Low viscosity during printing High viscosity after printing Proper release from the stencil No tendency to stick on the squeegee Delayed drying For reflow soldering: Paste Printing Sufficient tackiness to hold the components No slump during heating up Removal of oxides during reflow soldering Non-corrosive residues after soldering Paste Applied to Stencil Start of Print Stroke Apert ure Fill End Of Print Stroke Release From Stencil 9

10 Solder Paste Stencil printing of solder paste Solder paste is pressed by a squeegee through defined openings in a metal foil on the top of the substrate (showed in picture) Important steps in the process are: The moving of the paste ahead of the squeegee The flowing of the paste into the openings The leveling of the solder paste deposit The release of the paste on the substrate Stencil printing Dispensing Stencil printing is less attractive for small batch sizes, because of higher relative cost => Dispensing can be used to deposit the solder paste on the PCB contact pads When changing product/layout only new programming is needed Problem usually long cycle time 10

11 Design Aspects: Paste (incl. fluxes and solvents) Solder composition (melting point/range) Lead-free Allowed impurity content Form (solid, wire, flux core wire, paste, preform) Material dissolution into molten solder Flux type (rosin-flux, water soluble) Flux content: solvent, activators, resin/rosin, thixotropes, corrosion inhibitors) Rheology (viscosity, density, density control) Efficiency (activity) contra corrosion Cleaning solvents Environmental impacts ( CFC-compounds prohibited) and corrosion testing Interaction with the components Component mounting 11

12 Component mounting SMT Process- Reflow 12

13 SMT Process- Reflow SMT Process- Reflow The reflow profile means time-temperature dependency of the component board during different reflow stages Temperature profile is by far the most important parameter in reflowprocess Solder paste manufactures include profile model with their products, with which their paste produce the best reliable solder joints 13

14 SMT Process- Reflow Peak temperature TEMPERATURE Ramp Activation stage Melting stage Cooling stage TIME Wave soldering 14

15 Wave soldering Solder materials 15

16 Solder materials Lead [Pb] was eliminated from electronic industry the 1 st of July Drivers were Environmental Legistlation Environment conscious Marketing advantage Driving forces for Lead-Free solders Health-related aspects Contaminated drinking water Leachate from landfills Lead concentration limits Japan 0.3 mg/l in landfills USA mg/l drinking water It has been demonstrated that the concentration of lead leached from solders can be several hundred times higher than the acceptable limit Market advantage Mechanical properties It has been reported that many lead-free candidate solders exhibit significantly better strength and fatigue life properties than Sn37Pb 16

17 Toxicity Supply and Abundance Cost Selection Criteria for Suitable Lead-Free Solder Candidates Metal Density Factor Metal Processing Factor Melt Temperature and Range Minimize pasty range Wettability Effect of elemental additions on surface tension of Sn Minimum Complexity in Materials Fabrication Minimum Metallurgical Complexity Enhanced properties required Effect of elemental additions and impurities Adequate Properties for Operational Requirements 17

18 Toxicity Supply and Abundance Cost Selection Criteria for Suitable Lead-Free Solder Candidates Metal Density Factor Metal Processing Factor Melt Temperature and Range Minimize pasty range Wettability Effect of elemental additions on surface tension of Sn Minimum Complexity in Materials Fabrication Minimum Metallurgical Complexity Enhanced properties required Effect of elemental additions and impurities Adequate Properties for Operational Requirements 18

19 Resources Potential Candidate Elements ELEMENTS WORLD PRODUCTION (TONS) WORLD CAPACITY (TONS) Cu 8,000,000 10,200,000 Zn 6,900,000 7,600,000 Sn 160, ,000 Sb 78, ,300 Ag 13,500 15,000 Bi 4,000 8,000 In Ga

20 Toxicity Supply and Abundance Cost Selection Criteria for Suitable Lead-Free Solder Candidates Metal Density Factor Metal Processing Factor Melt Temperature and Range Minimize pasty range Wettability Effect of elemental additions on surface tension of Sn Minimum Complexity in Materials Fabrication Minimum Metallurgical Complexity Enhanced properties required Effect of elemental additions and impurities Adequate Properties for Operational Requirements 20

21 Lead-free solders, their mass prices (per kg) and volume prices. Solder alloy Melting ( o C) Price of metal Density (25 o C) Rel. price Sn37Pb Sn3.5Ag Sn0.7Cu Sn3.0Ag0.7Cu Sn3.4Ag4.8Bi Sn2.5Ag 0.8Cu0.5Sb Sn58Bi Sn9Zn

22 Pb-Free Alloy Groups ALLOY ADVANTAGES DISADVANTAGES Lowers melting point By-product of lead mining Improves wetting Poor thermal and electric conductor Bi Radioactive; embrittlement concerns Low to moderate toxicity Forms low melting eutectic with Sn and Pb In Lowers melting point High toxicity; Short supply Good elongation and strength Expensive; Corrosion concerns Minimal toxicity Highly reactive Inexpensive Oxidizes readily Zn Lowers melting point Reduces wettability Readily available Available supply High toxicity (when inhaled) Ag Expensive; Corrosion concerns Oxidizes easily Sb Strenthens allloy Available supply High toxicity (oral) Cu Slightly improves wetting Brittle intermetallic compounds Available supply High toxicity 22

23 Patents for lead-free solders SnAgCu No. Owner Sn Ag Cu Bi Sb Zn USA 4,879,096 Oatey USA in processing Kester Japan Ishikawa rest Japan Matsushita rest USA 4,778,733 Engelhard USA 5,527,628 Iowa State rest Japan Ishikawa rest Japan Senju rest

24 Why Industry prefers Sn-Ag-Cu? Sn3.5Ag SnAgCu SnAgCuSb Sn0.7Cu SnBiAg SnZnBi Melting/process temperature 5 3,5 3, Fillet lifting resistance 2,5 2,5 2,5 2,5 5,5 5,5 Solderability Processability 3 1,5 1, RELIABILITY 3 1,5 1, Recyclability 2,5 2,5 2,5 2,5 5 6 Cost 4,5 4,5 4,5 1,5 4,5 1,5 Alloy availability 1, ,5 5 6 Total score Average score 3,3 2,6 2,9 3,5 4,0 5,6 1-good 6-weak 10-poor 24

25 Properties of SnAg-based solders as compared with Sn37Pb solder. Sn-Ag-Cu Sn-Ag-Bi Sn-Ag-In Sn-Ag-Zn Manufacturability Wettability Dross formation Reliability Shock/Vibration Thermomechanical fatigue resistance Creep Corrosion resistance Toughness Strength Health effect Availability Price Acceptable Equal Stronger Good Equal Better Better Better Worse Higher Better Good Acceptable Good Equal Equal Good Equal Equal Better Better Worse Higher Better Worse Acceptable Good Worse Stronger Worse Not known Worse Better Not known Equal Equal Better Worse Higher Not known Worse Stronger Worse Not known Worse Better Worse Worse Equal Better Good Equal The challenge- New Combinations of Solder Alloys and Metallizations Protective coating 2 Protective coating 1 Conductor PWB Metallization Solder filler Al Cr, TiW Ni Cu Si Solder bump PWB Coating Component Coating Lead-Free Solder Ni/Au pure Sn Sn0.7Cu Ni/Pd/Au Ag/Pd Sn3.5Ag OSP (on Cu) Ni/Au Sn58Bi pure Sn Ag Sn3.4Ag0.8Cu Cu, Ni Au Ag Bi Bi UMBs Sn3.5Ag3Bi Sn9Zn 25

26 Preferred lead-free solder alloys in Europe Study carried out within ELFNET Preferred lead-free surface finish materials in Europe Study carried out within ELFNET 26

27 Table 1 - Scientific and Technical Lead-Free Soldering Implementation Issues Electronics Production Stage Design Bare Board Manufacture Component Manufacture Solders Surface Mount Processing W ave Soldering Hand Soldering Inspection End-Product Recycling Issues Requirements for disassembly Reduced use of hazardous materials Temperature sensitivities in view of increases processing temperature Compatibility of lead-free solders with component and board finishes Labelling with the solder composition Reduced use of brominated flame retardants and solder masks Lead-free board finishes Pure tin etch resists in place of tin-lead Degradation of low-quality boards at high processing temperatures Possible additional board support to avoid warping and sagging Component stability at higher soldering temperatures (eg. popcorning ) Melting point hierarchy in multichip modules Lead-free finishes Tin whiskers on pure tin finishes Obsolescence labelling, shelflife Alloy selection SnAgCu for general purpose, other alloys for specific operations Fluxes compatibility with alloy used, flux type Optimisation of reflow temperature profile Narrower process windows Temperature control including cooling rate Convection rather than IR ovens Possible use of inert gas atmosphere Optimisation of wave design Bath erosion Increasing copper levels in bath Fillet-lifting with lead components Possible use of inert gas atmosphere W orking practices and equipment for higher temperatures Retraining for operators Compatibility in repair and rework Higher dissolution rate of solder tips Optical retraining for operators re duller joints, recalibrate automated equipment X-Ray reconfiguration of equipment Reliability new test methods, modelling, harsh environments Green Labelling Testing and standards Increased Bi and Sb contents in recycling processes Technoeconomic impact of increased Ag content Solder joint disassembly Single Side Wave Soldering Alloy Selection TIN SILVER BISMUTH ANTIMONY Tin- Bismuth Eutectic Double Side Wave Soldering Tin Copper & Tin Silver Eutectic Reflow Soldering TIN ZINC TIN ZINC BISMUTH TIN SILVER ZINC TIN ANTIMONY TIN-SILVER-COPPER EUTECTIC ALLOY + ANTIMONY? NO TECHNICAL ADVANTAGE; TOXICITY? + BISMUTH POTENTIAL STRENGTH ADVANTAGE? LIMIT TO <2.5% 9 C REFLOW ADVANTAGE Repair & Rework TIN SILVER EUTECTIC LEAD PCB AND/OR COMPONENT LEAD FINISHES LEAD-FREE PCB AND/OR COMPONENT LEAD FINISHES 27