Basics of Temperature Profiling Heat Transfer Standards

Transcription

1 Basics of Temperature Profiling Heat Transfer Standards University of Technology, Brnó, Czech Republic Dr. Hans Bell

2 Basics of temperature profiling Basics of heat transfer Development of a 0402 solder joint 2

3 Basics of temperature profiling Basics of heat transfer Heat quantity Q = c m (T PCB - T RT ) Heat transfer Q = h A t (T Oven - T PCB ) c Specific heat capacity h Heat transfer coefficient m Mass A Surface T PCB T RT Temperature difference t Time T Oven T PCB Temperature difference 3

4 Convection vs. Condensation Convection Nitrogen (inert) Separate heating zones Effective cooling zones h ~ 20 to 70 W/m²K Condensation Condenso - Principle Injection of inert medium Horizontal transport Vacuum option h ~ 100 to 300 W/m²K Preheating Peak Cooling section 4

5 Condensation Soldering The Condenso Process 5

6 Convection vs. Condensation Soldering results Convection SnAgCu Condensation SnAgCu Convection SnPb Condensation SnPbAg 6

7 Convection vs. Condensation Solder spreading Convection Air Wetted area a [mm 2 ] Interaction Solder paste Soldering process Soldering process Conv./N 2 Conv./Air Cond./Air Cond./Vac. Condensation Air 25 Type A Type B Type C Solder paste type Source: TU Dresden H. Wohlrabe, T. Herzog, S. Schröder, Bericht zur Untersuchung des Einflusses von verschiedenen Löt- und Materialbedingungen auf die Qualität von gefertigten SMD-Baugruppen, Versuch 2,

8 Convection vs. Condensation Summary Reflow profiling flexibility Cycle time T P : Maximum board temperature T M : Individual board temperature difference Adherence to limit gradients Convection (VXP) Very good (a lot of heating zones) Very good Depends on process parameters Depends on the number of heating zones Depends on process parameters Condensation (CondensoX) Good (only one medium available) Limited (Batch process) Limited Determined by the selected medium Relatively small Can be easily exceeded Depends on process parameters Soldering of large masses Limited Very good Inert process atmosphere Vacuum process Nitrogen A separate vacuum chamber is necessary Medium Galden (PFPE Perfluoropolyether) Very good, in the very same process chamber 8

9 Influence of heat Solder joint formation Effect of false temperature profiles, too cold: Solder deposition not or not completely remelted Picture: B. Müller, Siemens CT Berlin,

10 Influence of heat Heat resistance of components Switch before reflow after reflow Pictures: D. Birgel, Endress+Hauser, 11. EE-Kolleg Sant Jordi

11 Influence of heat Heat resistance of PCB material PCB after Reflow Source: Rainer Taube, Risikofaktor Basismaterialien, BFE Seminar , Böblingen 11

12 Basics of temperature profiling Solder joint formation Copper-tin reaction, SAC, formation of intermetallic phases Cu 6 Sn 5 Cu 3 Sn Cu Source: Matthias Hutter, FhG IZM, Auswirkungen von zusätzlichen Elementen auf die Eigenschaften von Lötverbindungen, WeichLöten 2009, Hanau

13 Influence of heat Influence of Time above Liquidus t above Paste: SnAgCu Time above Liquidus < 20 s Time above Liquidus ~ 30 s Time above Liquidus ~ 120 s 13

14 Influence of heat Influence of peak temperature and time above liquidus on the intermetallic layers (IMZ) Solder paste: SnAg3,0Cu0,5 PCB: OSP [µm] Thickness of IMZ 2,5 2 1,5 1 0,5 0 t above 30 s 90 s Temperature d = k t Löt E k( T ) = k0 exp R T d t k 0 R E T Layer thickness Soldering time Diffusion constant Gas constant Activation energy Reflow temperature Source: John Pan u. Andere, Effects of Reflow Profile and Thermal Shock on Intermetallic Compound Thickness for SnPb and SnAgCu Solder Joints, Proceedings APEX Feb. 2006, Los Angeles CA 14

15 Influence of heat Influence of diffusion Basic copper dissolving (after storage 240 C / 24 h, solder paste Pb95Sb) T = const. 15

16 Influence of heat Influence of heat quantity Integral above Liquidus Integral above liquidu us (QT) [Ks] 1250,0 1000,0 750,0 500,0 250,0 T max 235 C t above [s] QT = t t above above ( 2 ) T ( t) dt (1) 0, Temperature above liquidus [K] 16

17 Standards Qualification of moisture-sensitive components IPC/JEDEC J-STD-020D.1 of March 2008 The reflow profiles in this document are for classification / preconditioning and are not meant to specify board assembly profiles. 17

18 Standards Process window as per IPC/JEDEC J-STD-020D.1, March 2008 Leaded Lead Free 260 C 235 C 220 C small package 245 C large package small package 230 C recommended soldering temperature large package SnAgCu 217 C 200 C recommended soldering temperature 195 C SnPb 183 C dt = 20 K dt = 15 K 18

19 Standards Process compatibility of moisture sensitive components Delamination after lead-free reflow soldering (240 C), US-Microscopy Before reflow After reflow 19

20 Standards Qualification of non-ic components IPC/JEDEC J-STD-075 of August 2008 Classification of Non-IC Electronic Components for Assembly Process Definition of a Process Sensitivity Level PSL (for reflow and wave) (number of reflow, flux applications, temperature profiles, washing processes) 20

21 Standards Process compatibility of components Electrolytic capacitors (small size) Life-time test after reflow 0-5 initial 2 runs 400 h 1000 h 1500 h deltac/c in % L Convection Conv.Linear Vapourphase KLS inline Limit Source: Franz Wieser, SMD Alu Elkos f ür bleif reie Prozesse, 8. EE-Kolleg Sant Jordi März 2005 Elektrolyte boils at 200 C Max. soldering temp. 250 C 21

22 Influence of cooling Cooling gradient -dt/dt Break of a FC solder joint as a result of too fast cool down 22

23 Basics of temperature profiling Envelope profile for a lead-free SAC process Temperature [ C] s 10 s 255 C 230 C 217 C 210 C < 240 s > 20 s < -1,4 K/s 0,7 K/s 120 s 3 K/s 3 K/s -6 K/s -3 K/s 300 s Time [s] 23

24 Thank you and wish you the right profile every time! 24