SMT in Practice. Special Seminar III. Dr. Peter W. Pachowicz Department of Electrical and Computer Engineering Volgenau School of Engineering

Transcription

1 Special Seminar III SMT in Practice Dr. Peter W. Pachowicz Department of Electrical and Computer Engineering Volgenau School of Engineering This seminar is dedicated to students taking senior design project or attempting to take it very soon The focus is on practical aspects of engineering necessary to complete senior design project successfully and professionally

2 Goals To use modern technology available only as SMT components To understand SMT To learn SMT soldering and rework techniques To handle SMT devices with ease To use senior design lab resources properly and efficiently

3 Topics ECE SMT fabrication capabilities SMT components SMT tips SMT soldering techniques SMT soldering tips SMT soldering tools Rework for SMT Note: Please listen carefully and keep notes. There will be a lot of information provided that is not on these slides.

4 Additional Resources Books: Mastering Surface Mount Technology, by V. Himpe Surface Mount Technology for PC Boards, by G. Blackwell & J. Hollmon On-Line articles: Intro to SMT Search youtube.com for soldering tips

5 ECE SMT Fabrication Capabilities PCB fabrication Solder paste disposer IR soldering and rework Hot air rework, hot tweezers Quality inspection

6 Why SMT? THT is phased out (!!!) Reduction of size when compared with THT Weight reduction Availability of advanced multi-pin chips Ability to populate a PCB on both sides Reduction on parasitic inductance, capacitance and resistance Better noise immunity (due to shorter interconnections) Easier to repair/rework (but when using proper tools) Lower manufacturing cost (less floor space, small machines) Lower overall cost (However, components can be more expensive, but there is no need to drill so many holes)

7 SMT in Senior Design Projects Limited availability of THT chips! This causes significant problems for teams Use of advanced Off-The-Shelf modules is frequently necessary Consider GPS, sensors, power management, etc. Many components are only available as SMDs New components are only SMDs (!!!) You need to avoid certain packages (BGA) More flexibility during the design stage Advice: Use a multi-pin MCU to speedup your design effort Less space required Easier to rework than THT components

8 SMT Dimensioning Imperial units are still in use for PCB design US is the only country left in the world using Imperial units Think about implications and confusions Metric units are used most of the time in SMT Migration to metric is accelerated due to miniaturization Virtually all new SMDs are metric-dimensioned (!!!) Plan accordingly when using PCB design software 1 mil = mm Divide your PCB design process into stages and work w/ single dimension units (do not mix them up); e.g., 1. Sketch board contour inch (or metric) units 2. Assign THT footprints inch units 3. Assign SMD footprints metric (or inch) units 4. Make connections metric (or inch) units

9 SMT Component Dimensions Units have implications on device dimension type and indirectly on pad/footprint size Standardization; e.g., Inch notation 1206 implies 0.12 x0.06 (3.048x2.032 mm) Metric notation 3216 implies 3.2x1.6 mm (0.126 x0.063 ) However, inch 1812 is very similar in size to metric 4532 Plan to work with chips at least 3mm long Good luck if you decide to work with ultra-small chips Plan to work with multi-pin chips of pitch >0.5mm Manual work with pitch 0.5mm or lower is very difficult If so, allocate a lot of time, be very patient, and learn soldering on a separate breakout board first (!) Plan to work with larger pads for easier soldering

10 SMD Resistor Technology Standard packages: 0805 (in); 2012 (mm): 0.125W, 150V 1206 (in); 3216 (mm): 0.25W, 200V 1210 (in); 3225 (mm): 0.50W, 200V 1812 (in); 4532 (mm): 0.75W, 200V Bigger chip higher the cost Larger chip ratings are really high but the size will make you comfortable to place and solder them Resistor technology: Thin film are of better quality, better tolerances, lower induced noise (E24 and E48 series) Thick film low cost, better for very high resistance values >5 MΩ (E12 series)

11 SMD Resistor Values No longer color coded Worse, certain manufactures avoid stamping values Use a 3 or 4 digit stamped code it s a bit of a madhouse Coding: followed by 1 zero = 100 Ω and 0 zeros = 10 Ω 103 = 10 kω 2212 = 22.1 kω 10R Most common notation = 10 Ω 8R2 = 8.2 Ω R100 or R10 = 0.1 Ω General denomination code 3 digit indicates a 5% part 4 digit indicates a 1% part or better

12 SMD Capacitor Technology (1) Standard packages the same as for SMT resistors Capacitors have a third dimension (their thickness) depending on the cap value Capacitor values are not stamped don t mix chips Ceramic capacitors (general use) come in different grade NPO, X5R, X7R, Y5V, etc. depending on dielectric material Highly depend on an application and have tolerances >=10% Film capacitors Better quality with tolerances within 5% Mostly for signal coupling applications in analog electronics

13 SMD Capacitor Technology (2) Tantalum Capacitors Needed, for example, to cut noise in a MCU package May have different footprints They have internal fuse and self extinguishing case material Because, they can malfunction, explode, and/or set fire They are polarized. It s important not to reverse voltage on them. Reverse voltage of 1.5V is enough to cause a catastrophic failure Electrolytic capacitors Generally more expensive than ones in THT when ordered in small quantities. Still price is low if you need few of them. Check footprint spec for one you need (typically: CAN-SMD)

14 SMD Diodes and Transistors Diode packages: SOD-xxx DO-xxx Small Outline Diode Outline diode Can be two-, three- or more pin packages (SOT-xxx) Transistor packages: SOT-xxx DPAK and D2PAK Small Outline Transistor Have minimum three pins (can have more, even to eight pins) Avoid sizes below 3mm (or <2.5mm) Attention: diode size can be below 1mm

15 SMT Integrated Circuits Contains by far the largest variety of packages Footprints vary Dual row style packages Four sided packages Complex BGA and QFN style packages Some of these packages cannot be handled without specialized equipment! However, there are tricks to solder them properly (ask before working on your own) I will highlight selected packages which 1. You can solder relatively easily 2. You will need to be very patient 3. You should try to avoid, if possible

16 SMT IC Easy Category SOIC package (Small Outline IC) A step forward from DIP SOIC-14 (specifies number of pins) Pitch 50 mil (1.27mm) TSOP type II Pins on the longer side, pitch 1.27mm (For larger pin#: pitch 0.8mm or 0.65mm!) QFP/TQFP package (Thin Quad Flat Package) For low pin# package: pitch 0.8mm to 0.65mm All of above can be soldered by hand (!)

17 SMT IC BeVeryPatient Category SSOP with pitch 0.65 mm QFP/TQFP package with larger number of pins (>=64 pins) Pitch 0.5 mm (you need to be extremely patient) This is an unavoidable package for many high-pin# MCUs Practice soldering these chips before the final job Practice using small breakout boards (cost $5-$7) rather than working directly with your PCB They can be soldered by hand and traditional iron when: You have experience, work very slowly, and use right tools HOWEVER: Soldering should be done in the ECE fabrication lab using solder paste and IR soldering station (!)

18 SMT IC TryToAvoid Category mini-soic: TSOP type I: pitch 0.5 mm and a very small chip pitch 0.5 mm and pins on the shorter side PLCC: not a SMT package, solderable, but unreworkable QFP/TQFP very high pin# (100 pins): pitch 0.4 mm QFN: requires special soldering equipment or extra skills, pitch 0.5 mm and extremely difficult access BGA: for extremely dense interconnections All above can be soldered in the lab using solder paste and IR station special care is needed (!!!)

19 SMT Component Tips Size at least 3mm: 1206 (in) package If you are good at soldering then eventually use 0805 (in) or 2012 (mm) they are popular and low cost Quality storage boxes are a necessity for storage of SMT components Never store a SMT chip without/outside its own package/tape Resistors: Buy 1% resistors no matter what (the cost difference is small) Try to buy the E24 series resistors (common default) However, E48 series resistors are made with more values Capacitors: Buy X7R type, if possible (caps of relatively stable parameters) Avoid Y5V type (very unstable)

20 More SMT Tips Never touch chips with your fingers only with tweezers Tweezers must have rounded tips Otherwise, you may pierce through a chip Open chip package right before soldering Moisture absorption chips; e.g., sensors Certain devices are sensitive to moisture try to avoid them They require soldering within a given time after opening a sealed package. Usually within 24 hours Check spec sheet before you buy Read labels carefully If time expires then they must be beaked first (!!!)

21 Zero Ohm Resistor Use 0Ω resistors to jump traces SMT resistor of a larger size will jump more than one trace Be sensitive to price large packages can be expensive This resistor will help simplifying your PCB design You can combine jump resistors but do it only in a series (no more than 45 angle between them)

22 Soldering Techniques for SMT It s difficult to solder SMT chips (!) There are many methods search the Internet and YouTube I focus on methods that work for someone without extensive practice You will need more than one soldering technique to use You need a plan! (it depends on your preferences); e.g., First, solder most difficult parts (low pitch multi-pin SMDs) Second, solder SMT resistors, caps, SOIC chips, etc. Finally, solder connectors and THT chips (see Seminar I) Have your plan written (!!!) Each step should be approached with a single soldering technique do not mix techniques at a single step

23 Soldering Tools Needed (1) Iron for soldering larger items (THT, connectors) Small tip iron for soldering SMT chips Best if short in length for precision touch Fine point tip vs. chisel tip Solder paste disposer IR soldering station

24 Soldering Tools Needed (2) Cleaner: Isopropyl alcohol Glue and toothpicks Quality tweezers (ESD rated, stainless steel, rounded tips) Dental picks Flux pen avoid flux dispensers (too messy) Solder wire as thin as possible Solder paste and needles Vise to hold your board; Fume extractor Magnifying lamp or Visor Magnifier

25 Preparation Before Soldering Clean milled PCB from dust, oil residues, and oxidation This will greatly improve solder wetting Isopropyl alcohol and a toothbrush Do not use acetone in the ECE lab Remember, re-oxidation starts shortly you only have about one hour time Place your board in a vise Understand how you can gain access from different directions Understand how you can adjust vise to improve the access Prepare the iron Define temperature to work with (!!!)

26 Soldering Methods We will discuss two classes of soldering methods, when applied to SMT devices: 1) Traditional manual hand-based methods 2) Modern semi-automated IR soldering methods (with equipment available in the ECE Fabrication Lab.)

27 Manual SMT Soldering Technique 1 Using iron and solder in coordination Process: 1) Apply flux to footprints only 2) Place (glue) chips to the PCB 3) Execute soldering steps circuitwork.com Slide and point iron at pin-2-pad contact Wait Touch with solder wire Release Requires an extreme precision when used on small pitch multi-pin chips. It s more suitable for SMT resistors, etc. It s easy for solder to spread around and join pins together If so, do not panic and use solder-wick to remove excess solder

28 Manual SMD Soldering Technique 2 Using iron only an effective technique Process: 1) Apply flux to footprints 2) Apply solder to footprints (not too much, not too little) Simply move iron with solder deposit through all pads 3) Inspect solder height across all pads must be uniform NO YES 4) Clean and reapply flux 5) Place and hold/press a chip 6) Execute soldering steps Push iron down vertically on a single leg Wait Release

29 Manual Soldering Tips (1) Do not use Technique 2 when soldering: Chips of a block structure such as resistors, capacitors, etc. You can easily damage copper trace and pads First, iron was applied here What is going to happen if you Push iron right here after the first step? Eventually, you can: 1. Apply solder on one pad only (!) 2. Solder a chip only on one side, so it will stay firm 3. Finally solder a chip on the other side

30 Manual Soldering Tips (2) Use narrow solder-wick to remove any excess of solder which joins adjacent legs together Run a sharp knife tip between pins to be sure there is no solder residue touching another pin Using Hot Air Station (more about it later) Will burn glue AND may blow chips out Joint quality??????

31 Modern Soldering Technique 1 Applicable to all SMDs Process: 1) Apply flux to cleaned footprints 2) Glue (if needed) a chip to PCB (do not glue over flux residue) 3) Dispose solder paste at the edge of chip leg (dots or a line) 4) Solder using IR soldering station 5) Use a solder-wick to remove excess solder Paste line Glue Paste dots Final result Glue

32 Modern Soldering Technique 2 The same as the Technique 1, but Dispose solder paste on footprints first, and next Align and place a chip over Difficult to do it precisely without spreading solder paste around Solder paste disposed: line vs dots Chip placed and aligned After IR soldering After excess solder removal

33 Modern Soldering Technique 3 Based on pre-soldering the footprints and following with IR soldering Process: 1) Apply flux to footprints 2) Apply solder to footprints (not too much, not too little) Simply wet the footprints by moving a larger iron with solder deposit through all pads 3) Inspect solder height across all pads must be of equal height (!!!) NO 4) Clean and reapply flux 5) Use IR soldering station (Optionally, push the chip down using tweezers) YES

34 Soldering QFN Chips Soldering these chips is possible but requires a special technique We soldered many of them already Ask for help/instructions

35 Solder Paste Disposer Zephyrtronics ZT-5100 instrument Uses externally supplied compressed air (pressure up to 90psi) Activation via a foot pedal Parameters to work with: Air pressure (do not exceed 70psi) Disposal time (Use extremely small time for small diameter needles and dots) Back suction Test your skills first

36 Solder Paste Disposal Tips Depending on your hand coordination small dots are preferred rather than a line (!) Small dots properly disposed give the best results Remember: Less is better Solder paste must be of room temperature Remove from refrigerator minimum 2 hours earlier Use small diameter needle Practice on separate piece of cardboard Experimentally test dispenser parameters Touch the pad decisively do not hold the needle above If there is too much mess, clean the PCB and start over Do not touch the solder paste with bare hands (!!!)

37 IR Soldering Station Detail instructions and tips will be provided at the time you will use it Safety first: Bring your sunglasses (!) the light is very intensive and you need extra protection in addition to the IR screen Wait until the board cools down this will take several minutes

38 More Soldering Tips (1) Design your PCB is such a way that chip placement has a pattern suitable for easy soldering Align all resistors, caps, etc. in one direction (maximum two) horizontal or vertical The same applies to multi-pin chips In summary, have an order in chip placement on your PCB This will speed up soldering and you will not miss joints Watch out for placement errors (it happened to me) Soldering should be done by one person (if you really need then second person can quietly assist) Do not touch chips with your fingers!!! They are already tinned

39 More Soldering Tips (2) Do not overheat SMT components Many are sensitive to heat (even resistors and capacitors) Remove the iron immediately after smoke from the flux disappears Approximately within 2sec after solder melts Do not tin-plate PCB Seriously, this is not needed for SMT PCBs Prototype PCBs do not need tin plating anyway

40 Rework for SMT (1) More difficult than soldering SMT components Do not use excessive heat/force no matter what remember Copper traces are glued Components are sensitive to excessive heat First step: Remove solder using solder-wick (very narrow) Second step: Use proper tool to remove entire chip Hot Tweezers for removal of: Resistors, capacitors, etc. Dual-row multi-pin chips You need to select proper adapter, if available

41 Rework for SMT (2) Hot air station for removal of: All-around multiple-pin chips Dual-row multiple-pin chips Other focused rework Use proper adapter IR soldering station for removal of: Larger chips Non-typical chips, etc. First, heat up PCB from bottom to 150 C Next, use top heater to gradually increase temp Remove a chip with thermally isolated tweezers

42 Rework Tools Rework tools are available upon request or can be accessed in the ECE Fabrication Lab. Contact Lab TA/Technician Tools must be returned immediately You need to babysit these tools they are sensitive to abuse Safety first!!! NEVER work with hot air tool at temp >400 OR lowest air flow You will damage this valuable station It may melt in your hand (!) Always use sunglasses in addition to IR screen when working with IR soldering

43 Questions