SPECIFYING STENCILS TO OPTIMIZE PRINT PERFORMANCE Upper Midwest Tech Expo June 30, Chrys Shea Shea Engineering Services

Transcription

1 SPECIFYING STENCILS TO OPTIMIZE PRINT PERFORMANCE Upper Midwest Tech Expo June 30, 2016 Chrys Shea Shea Engineering Services

2 PCB Layout Drives Stencil Design PCB Layout DFM Feedback loop Component type, size, location Stencil Design Foil thickness, steps, aperture sizes Overview of available options Stencil Mat l & Mfg Process SS or Ni E-form or Laser cut Coating Print Quality Stencil design, foil material, cut quality, finishing SMT Yields

3 Stencil Design Keys to a successful print process: Aperture design Material selection Quality manufacturing Flux-repellent coatings A good print does not guarantee the PCBA will not need rework. But a bad print does.

4 Tradeoffs Between Aperture Size and and Foil Thickness Broad range of component sizes on PCB design Big ones that requires higher volume solder paste deposits Power components, PTH, SMT connectors Rf shields High I/O BGAs and LGAs Small ones that requires high-precision, lower volume deposits ubgas, some QFNs, LGAs and BTCs 0201s, 01005s Put extreme demands on stencil printing process Larger deposits require thicker stencils Smaller deposits require thinner stencils Optimum print parameters change with feature size & density

5 Managing the Mix Stepped stencils Different foil thicknesses accommodate different paste deposition requirements Max step is 2mil (50um) Preforms Add extra solder when printing can t achieve necessary volume Stencil design Calculate volumes for Pin-in-Paste and other large solder joints Calculate volumes for BGAs, QFNs and small solder joints Determine tradeoffs in stencil thicknesses Traditional Approaches Stepped stencils Solder preforms in tape and reel

6 Solder Paste Release from Stencil Paste PCB Pad Stencil PCB After the aperture is filled, the solder paste sets up and sticks to both the stencil walls and the pads. At separation, the forces holding the deposit to the pad must overcome the forces holding the deposit to the stencil walls Depending on area ratio, a portion of the paste will release to the PWB, while some will stay in the aperture. Some paste may also stick to the bottom of the stencil due to stringing, bad gasketing or pump out The smaller the AR, the lower the TE 2015 Shea Engineering Services

7 Impact of AR on Deposit Volume ARs: Sizes (µm): Sizes (mil): What releases better? Uncoated stencil, Paste C 2015 Shea Engineering Services

8 Stepping Stepping is critical in many processes, especially when stencil design calculations are being performed based on aperture volumes and area ratios Steps can be chemically etched, milled, or welded prior to laser cutting Step Types: Step Up: Thickens stencil locally Step Down: Thins stencil locally Top or Bottom side steps, or both Stepless steps: Smooth the transition (used w/encl print heads) Angled steps: Reduce squeegee damage (also w/encl print heads) Cavity relief: For labels or other PCB topographical features Precision steps are often required for high-density assemblies

9 Precision Stepping From top or bottom May have very tight keepout zone Needs well defined walls May have irregular shape Low tolerance on thickness variation Fine Grain (FG) stainless steel is best choice Image Source: HP Etch

10 Step Up Stencils Used when thicker paste deposits are required on a small portion of the stencil. A ceramic BGA where it is necessary to get 8mil (0.2mm) paste height because of ball coplanarity but 6mil (0.15mm) height on all other surface-mount component pads. stencil PCB Courtesy of LaserJob GmbH

11 Step Down Stencil Used when thinner paste deposits are required on a small portion of the stencil. Fine pitch BGA of 0.5mm (20mil) pitch that requires a stencil thickness of 4mil (0.1mm) to achieve an area ratio At the same time there are devices on the same board that need thickness of 6mil (0.15mm). stencil PCB Courtesy of LaserJob GmbH

12 PCB Contact Side Relief Used when labels, circuit repair or snugger tooling will cause gasketing problems for nearby prints. Label is located close enough to fine feature to prevent the stencil from seating properly on the PCB stencil PCB label Courtesy of LaserJob GmbH

13 Step-in-Step Base thickness of stencil: t base =150µm Thickness of heat sink area: t hs =180µm QFP: t leads =120µm 180µm 150µm 120µm 180µm 120µm Pitch = 400µm Pad width: 200µm Keep-out distance: 700µm Courtesy of LaserJob GmbH

14 Laser Cut and Welded Steps Manufacturing Process: Laser cut of required size of step area Welding process of an accurate fitted step in desired thickness User Benefits: Exact thickness and surface roughness Highest precision in stencil step technology Can step up/down on same stencil and still maintain surface quality Can create stencils for cavity PCBs Courtesy of LaserJob GmbH Cutting of apertures in step area and stencil

15 IPC 7525 Stencil Step Guidelines As a general design guide K1 should be 0.9mm [35.4mil] for every 0.025mm [0.98mil] of step- down thickness. K1 K1 Ste p Step Depth 0.010mm, 0.4mil 0.025mm, 1mil 0.030mm, 1.2mil 0.050mm, 2mil 0.080mm, 3 mil 0.100mm, 4mil K1 is distance form the step edge to the nearest aperture in stepped down area 0.36mm, 14mil 0.90mm, 35 mil 1.08mm, 42mil 1.80mm, 71mil 2.88mm, 113mil 3.60mm, 142mil Courtesy of LaserJob GmbH

16 Step Design Guidelines Depth: no more than 2mil per step Will lose fill pressure on solder paste Keepout zone: distance from aperture to edge of step Minimum recommended: 25mil Preferred: as much as possible Keep out perimeter 25mil (0.625mm) Paste Buildup Larger keepout zones: SQUEEGEE Enable better squeegee deflection into recess Keep the dried paste buildup in the corner of the pocket, away from the apertures 2mil (50µm) MAX per step 2015 Shea Engineering Services

17 BTC/QFN Stencil Design This is a very economical, reliable and popular package Some chipsets are only available in this package type Some assemblers have up to 15 years experience with package; some have 0. Thermal/ground pad causes issues: Too much paste on center pad prevents perimeter joint formation Not enough paste on center pad limits thermal transfer Themal vias in pad rob paste from bond, causing voids and creating printing problems on second side Flux in solder paste causes voids Voiding in pad may affect thermal and electrical performance Image Source: Digikey

18 Bottom Termination Components IDEAL SOLDERING CONDITION Good wetting on terminations 2-3mil standoff Good wetting, low voiding (<25%) on thermal/ground pad Outside goes to liquidus before inside; should set standoff 2015 Shea Engineering Services

19 Bottom Termination Components TOO MUCH SOLDER ON PAD Open or insufficient solder joints Component floats on solder mass during reflow NOT ENOUGH SOLDER ON PAD Good wetting on terminations 2-3mil standoff Insufficient contact, thermal and electrical performance 2015 Shea Engineering Services

20 QFN Stencil Design Need to balance solder volume on land terminations with solder volume on center pad Calculate solder paste TE and predicted volumes for lands Reduce by 50% to estimate volume of metal Aim for 2-3mil standoff (better cleaning and reliability) Calculate volume for pad Solder paste TE will be close to 100% Reduce volume by putting window panes in stencil (also prevents scooping or tearing) When in doubt, aim for 0.5-1mil lower standoff Understand that the calculations are estimates and may require fine tuning

21 BTC/QFN Stencil Design Tips Use stencil webs to cover thermal vias on ctr pad If possible, keep paste 50mil away from vias Will prevent vias from robbing paste and limiting contact Robbed paste shows up on side 2 as solder bumps that interfere with printing process Provides outgassing paths to minimize voiding Aids in cleaning when cleaning is necessary Use solder mask to window pane ground pads Maintains outgassing paths to limit process voids Do NOT cover thermal vias

22 BTC/QFN Stencil Design Tips Overprint on terminations Extend apertures by up to 30mils on outside of lands Extra solder paste will pull back and help compensate for float from center pad Don t connect ground pins to ground pads on the stencil Will cause stencil and squeegee damage Squeegee damage will affect other products Don t connect ground leadpad stencil apertures Will cause premature stencil wear and squeegee damage Squeegee Damage Image courtesy of PARMI

23 QFN Aperture Design Center Aperture Usually divided Provides outgassing paths to limit voiding Reduces height of center solder joint to allow perimeter joint formation Avoid printing over or near thermal vias Define pad with solder mask to maintain outgassing paths and control coverage Goal: 2-3mil standoff Option: prefluxed preforms Lead/Land Apertures If 0.5mm pitch or smaller, need to calculate predicted paste deposit volume transfer efficiency based on AR, TE and paste type If stencil thickness changes, so does AR, TE and volume deposited If aperture size changes, so does AR, TE and volume deposited Iterative process, may also need DOE to optimize Suggested center pad aperture designs for Amkor MLF68

24 Preforms Add solder where printing can t Tape and reel preforms come in standard chip sizes: 0201, 0402, 0603, etc, for easy vision recognition and different thicknesses to achieve desired volumes Help eliminate voiding under BTCs Solid solder mass had less volatiles to create voids and offers better contact to both surfaces Is placed on small paste deposits on ground/thermal pad Preform Paste BTC image courtesy of Indium Corp

25 PTH Stencil Design Calculations Through Hole/PiP Solder volume needed = Hole vol pin vol + solder fillets (assumption) Solder paste deposited = Aperture volume (overprint) + solder volume pushed into hole (assumption) Aperture volume changes with changes in foil thickness Preform volume (if used) =LxWxH, also available from on-line chart Solid solder volume =~50% of paste volume + 100% of preform volume Spreadsheets from suppliers help with the calculations: Fine features/ubga/0201 Deposit volume = Aperture volume * TE for the aperture s AR and paste type AR and TE change with changes in foil thickness Solder preforms placed in solder paste add volume to PTH and other large solder joints Image Source: Alpha

26 Stencil Foil Materials, Manufacturing and Coating Processes The stencil is the 2 nd most important piece of tooling in your printer (board support is 1 st ) Stencil quality is essential to good print performance Always purchase stencils based on quality, not price

27 Alloys/Foil Materials & Mfg Processes Stress relieved stainless steel (9 yrs) Fine grain stainless steel (7 yrs) New high-performance, low cost stainless steel (now) New electroforming processes (always a new one!) New nickel plating processes (5+ yrs) Laser-cut Ni (not new at all) Fiber lasers in cutting machines (5 yrs) New generation of laser cutters now out Image courtesy of Datum Alloys

28 Stainless Steel vs Nickel How to tell the difference? The finish on the PCB side This is a reflection of the phone that took this picture SS: Moderately shiny; reflects diffused colors and blurred shapes Ni: Mirror-like finish with clear reflection

29 Stress-Relieved Stainless Steel Cold rolling work hardens SS and imparts residual stresses Annealing removes them Stress-relieved materials deform less during cutting and enable tighter aperture density Standard 304 SS used for stencils Stress-relieved 304 SS used for stencils Images courtesy of Datum Alloys

30 Studies on Foil Materials & Mfg Processes 2010 FG outperforms std SS, electropolished SS, Laser-cut Ni 2011 FG outperforms stress-relieved SS, E-form, Laser-Ni Nanocoating* improves quality 2012 SS outperforms E-form and Ni-plated SS Nanocoating* improves release 2013 New nanocoating* better than previous nanocoating FG still better than E-form, Experimental SS shows promise Reducing under wipes with nanocoating improves quality * Three different nanocoatings were used in three different tests

31 Transfer Efficiency % Results of Mat l & Mfg Studies Effect of Foil Material on Transfer Efficiency Circular NSMD Pads FG SS NI EP Area Ra o FG=301SS 1-2um grain, Ni=Laser cut Ni, SS=304SS, EP=Electropolished 304SS =Eform Ni, 2=Laser-cut Ni, 3=Stress Relieved 304SS, 4=301SS 1-2um grain, 5=304SS 2013 All 4 studies performed & published independently by Shea Engineering Services and PCB assemblers.

32 Electropolishing Removes rough peaks from walls and smoothes them out Standard SS from Same Lot, Cut on Same Cutter Electropolished Non- Electropolished Rounded corners on apertures can cause gasketing problems Results in more print variation New EP materials and processes promise to improve output by controlling chemistry better than current electrochemical processes

33 New Tension SS Alloy New alloy developed for high tension applications was tested (at std tensions) from 6 different stencil cutters. Performance was compared to Fine Grain (current best) as benchmark TE s slightly better in most cases Print variation comparable Source: The Effects of Stencil Alloy and Cut Quality on Solder Paste Print Performance, C. Shea and R. Whittier, Proceedings of SMTA International 2014.

34 SS Foil Alloy Selection Stress- Relieved 304SS Fine Grain 301SS Tension Miniaturized or high-density assembly Area ratios <0.66 General SMT, lead pitches 0.5mm, leadless pitches 1.0mm Stepped stencil for µbga, CSP, QFN, BTC Uniform foil thickness 150µm Powder size Type: 4,5,6 Powder size: Type 3 Buyer Beware! The #1 provider of Fine Grain stencil foils has Discontinued the product and replaced it with Tension. FG material does not necessarily provide the high print quality it once did. Based on empirical information & laboratory tests

35 Cut Quality is Critical The finer the feature, the more important the cut quality Rougher walls do not release paste as well as smoother walls Burrs can impair solder paste fill and release flow Slag on contact side can create gasketing problems Best print performance in recent study Worst print performance in recent study

36 Analysis performed by Lyncee Tec Wall Roughness Comparison Holographic Microscopy Cut on same cutting parameters by stencil supplier E, the best performer Stencil #9 Fine Grain Stencil #10 Experimental SS

37 Analysis performed by Lyncee Tec Wall Roughness Comparison Holographic Microscopy Cut on same cutting parameters by stencil supplier B, the worst performer Stencil #3 Fine Grain Stencil #4 Experimental SS

38 Laser Cutting Produces most accurate stencils Stencil quality depends on machine quality, age, calibration, maintenance Newer cutters have very fine laser beams, on-board optical inspection to make sure the hole is fully cut, on-board aperture measurement for SPC, and remote control for cals, tune-ups or troubleshooting Ask your stencil vendor about his cutter Image source:

39 Electroforming Stencils are grown in electroplating tank Nickel is very hard material Good for high pressure or high volume processes Can exhibit dimensional problems Plating processes are notoriously hard to control Entire stencil grown with apertures Very smooth walls Nickel blank grown then cut on laser cutter Improves dimensional accuracy Modern lasers can cut very clean walls Can do ½ mil thicknesses: 3.5, 4.5, 5.5, etc.

40 High Tension Foils and Frames Typical tension N/cm High tension 50+ N/cm Promise to have less snap back and cleaner release Need a more rigid frame to carry higher tension without warping May need a harder steel to carry higher tension on thin webs, hence the idea for Tension alloy Image source:

41 Nanocoatings are a Repellency Treatment Water repellent: hydrophobic Oil repellent: oleophobic Examples of Common Water and Oil Repellency Treatments On fabric On carpet On paper food containers Image source: Daikin Industries (UNIDYNE web page)

42 Flux Repellent: Fluxophobic Example of Fluxophobic Stencil Treatment Untreated stencil Flux wicks out on the bottom surface away from the apertures Treated stencil Flux is repelled from the bottom surface and is contained primarily within the apertures

43 Flux Repellency on Stencils Flux Treated with UV Tracer Dye Untreated stencil Flux wicks out on the bottom surface away from the apertures Treated stencil Flux is repelled from the bottom surface and is contained primarily within the apertures

44 Print Definition Improvements QFN and 0201s after 10 prints with no wipe Same board, same stencil, same print stroke Print definition Bridge No Nano Nano

45 Nanocoatings A very thin coating of fluxophobic material applied to the stencil Different products are applied differently Wipe-on Polymer Heat/vacuum cured Polymer Heat cured Plasma Different availabilities, lead times and costs All are relatively new products Few complete head-to-head comparisons performed No complete head-to-head comparisons published to date Image source: Aculon

46 Nano Coatings 2 Basic Attribute Fluoro-Polymer Types SAMP Application Spray and thermal cure Wipe on Fluoro-Polymer Coating Applied by Stencil supplier Supplier or self Thickness 2-4 µm 1-2 nm Hydro- and oleophobic Abrasion resistant Chemical resistant Visible Accessibility Selected suppliers Stencil suppliers or internet Reworkable if worn off SAMP Coating (Self-Assembling Monolayer Phosphonate) Reduces frequency of underside cleaning Solder paste volume 15 25% increase in TE Slight decrease < 5% Aperture Redesign? Maybe No Minimum Area Ratio 0.10 lower than foil Same as foil Cost >$100 supplier dependent $20-50

47 Wipe-On NanoCoating Originally introduced in 2011 Marketed as DEK NanoProTek New and improved formula in 2013 Marketed as Aculon NanoClear Prevailed over predecessor in head-to-head tests 2-Part system Primer brings up oxide layer on metal Molecule bonds to fresh oxide layer & sets up immediately Called a Self-Assembling Monolayer Phosphonate (SAMP) Molecule

48 SAMP Nanocoating Functional Tail Group Repels flux ~4 nm Phosphonate Head Group Bonds to stencil

49 Nanocoating Payback and Cost Savings Cost Savings of NanoClear SMT Stencil Treatment Enter information into the white cells; calculations appear in the yellow cells Quality Payback Period Current First Pass Yield, % 80 Savings per print $ 0.35 Projected First Pass Yield, % 90 Cost of Nanoclear $ % Improvement 10% Cost of Application $ % of defects requiring simple rework 90 Payback - # of Prints 128 % of defects requiring complex rework 10 Savings in Yield Improvement, per print $ 0.35 Productivity Annual Savings per SMT Line # of prints per hour 60 Current Wipe Frequency 5 # of production hours per week 80 Projected Wipe Frequency 10 # of paper roll changes per week 10 % Reduction 50% Time to change wiper roll, minutes 5 Savings in under wipe consumables, per print $ Annual Cost Reduction $ 87,984 Cost Reduction >>> PLUS <<< Cost of simple rework $ 1.20 Additional Prodution Uptime, hours per year 43 Cost of complex rework $ Additional PCBs assembled per year 2600 Cost of wiper paper, per wipe cycle $ 0.03 Cost of solvent, per wipe cycle $ 0.02 Modify cost information on the "Cost Calculator" tab

50 Calculate Your Own Cost of Defects and Consumables Rework and Consumables Cost Calculator Cost of Simple Rework Cost of Wiper Paper Time required, min` 4 Cost per roll 20 Labor rate, per hour 12 Length of roll, m 10 Benefit rate, % 25 Advance per wiper pass, mm 5 Overhead Rate, % 25 # of wiper passes in cycle 3 Cost of Simple Rework $ 1.20 Cost of Paper per Wipe Cycle $ 0.03 Cost of Complex Rework Cost of Wiper Solvent Time required, min` 60 Cost of solvent container 30 Labor rate, per hour 16 Capacity of solvent container, liter 4 Benefit rate, % 25 Volume of solvent used on each wipe, ml 2 Overhead Rate, % 25 # of solvent passes in wipe cycle 1 Cost of Complex Rework $ Cost of Solvent per Wipe Cycle $ 0.02 These calculations feed into Cost Savings sheet Calculator free download at:

51 Polymer Nanocoating Sprayed on and cured Colored to make it visible Available only from select stencil manufacturers No rework Can be sensitive to water soluble fluxes Sensitive to abrasion

52 Nanocoated Stencil Release Higher ARs: Clean stencil appears to have slightly flatter tops Clean stencil shows slightly less sticking Dirty 2015 Shea Engineering Services Clean

53 Nanocoated Stencil Release Lower ARs: No discernable difference in release in this comparison Dirty 2015 Shea Engineering Services Clean

54 Nanocoating vs. No Nanocoating Nanocoating is always better. ALWAYS Videos at low ARs Nanocoated 2015 Shea Engineering Services Not Nanocoated

55 2015 Shea Engineering Services Nano-Coated Walls The polymer nanocoating used in this test Fills the valleys in the cut and smoothes the walls Repels solder paste Improve Transfer Efficiency: Less adhesion between paste and wall Less surface area of wall Uncoated Nanocoated

56 Nanocoating Summary Helps the print process many ways Improves release Reduces variation Lowers wipe requirements Prevents bleed out More forgiving to bad gasketing Costs less than rework

57 Dirty vs. Clean Uncoated Stencil Dirty condition is 4 th print without wipe Not an exceptionally long wipe interval Clean condition is 1 st print after solvent wipe 5 prints before wipe to get squeegee in right position ARs: Sizes (µm): Sizes (mil): Dirty Clean

58 Solvent vs Dry Wipe on Nanocoated Stencil Solvent Wipe Dry Wipe Tests were performed using wipe-on (SAMP) nanocoating with solder paste treated with UV tracer dye and a black light to show the flux smearing

59 Understencil Wiping Criticality increasing due to Component miniaturization Density / Placement of components Need for improved yields Wipe options Dry/vacuum Solvent IPA Specially engineered materials Historically, not a lot of science was applied to the process

60 Isopropyl Alcohol (IPA) Common wipe solvent Fast evaporation Ineffective on many lead-free solder pastes

61 Engineered Wipe Solvent Matched to the solder paste flux (like dissolves like) Fast Evaporation Non-flammable Environmentally friendly Low odor

62 Under Wiping Continued interest will be placed on underwiping: Need for process control Tenacious fluxes that are hard to remove Smaller solder paste particles that are harder to capture in paper/fabric More work on solvents More work on papers More work on compatibility with nanocoatings

63 Stencil Cleaning Solvents ph <9.0 High alkalinity solvents will attack Many ph neutral solvents now available Raw paste solvents can be used for under wipe Paper can be abrasive to nanocoating If using nanocoating, use softer wiper paper

64 Any Questions?

65 Thank You Chrys Shea Please note that this presentation contains copyrighted material and no portions of its contents may be copied or distributed without prior consent of Shea Engineering Services.