ELEC 6740 Electronics Manufacturing: Chapter 13 Flux & Cleaning

Transcription

1 ELEC 6740 Electronics Manufacturing: Chapter 13 Flux & Cleaning R. Wayne Johnson Alumni Professor Auburn University

2 Outline 1. Introduction 2. Flux 3. Contaminants 4. Selection of cleaning methods 5. Cleaning processes & equipment 6. Cleanliness Test Methods & Requirements 7. Design for cleaning

3 Introduction! Flux residue " Cleaning dependent on flux type " Clean: # Water soluble organic acid fluxes # Rosin based fluxes with halides # Resin based fluxes with halides " Cleaning optional # Rosin based fluxes without halides # Resin based fluxes without halides

4 Cleaning Difficulty

5 Cleaning Difficulty

6 Function of flux! To chemically react with oxides and quickly produce a fresh, tarnish-free surface at soldering temperatures so that intermetallic bonding can take place! To prevent re-oxidation during the sodlering cycle

7 Flux Classification! L = low or no flux/flux residue activity! M = Moderate flux/flux residue activity! H = High flux/flux residue activity

8 Flux Classification

9 Inorganic fluxes! Hydrochloric acid! Hydrofluoric acid! Stannous chloride! Sodium fluoride! Potassium fluoride! Zinc chloride! Not used for electronics assembly!!!!

10 Organic Acids! Citric acid! Lactic acid! Oleic acid! Stronger than rosin fluxes! Require cleaning, aqueous or semi-aqueous! Can be used for military applications

11 Rosin Fluxes! Extracted from the stumps or bark of pine trees! C 19 H 29 COOH! Abietic acid (70-85%)! Pimaric Acids (10-15%)! Rosin fluxes are inactive at room temperature but become active when heated to soldering temperatures! The melting point of the rosin is o C

12 Rosin Fluxes RCO 2 H + MX $ RCO 2 M + HX RCO 2 H = is the rosin in the flux M = Sn, Pb or Cu X = oxide, hydroxide, or carbonate

13 Rosin Fluxes

14 Rosin Fluxes! Rosin fluxes (R and RMA) do not have to be cleaned, especially if halide free! Sticky and can attract particles! Residue can impact bed of nails testing " Penetration of probe " Coating probe

15 Low Residue or No-Clean Fluxes & Solder Pastes (Resin)

16 Low Residue or No-Clean Fluxes & Solder Pastes (Resin)! Resin content varies from 1-35% " Solids content! High solids content $ more residue! Low solids content resin fluxes leave no visible residue

17 Low Residue or No-Clean Fluxes & Solder Pastes (Resin)! Generally not as aggressive as rosin fluxes! Boards need to be handled properly to avoid contamination! Boards should be used FIFO to minimize storage time (aging of finish)! Solder balling more of a problem especially with wave soldering! Compatibility of flux with solder mask and finish must be determined

18 Low Residue or No-Clean Fluxes & Solder Pastes (Resin)! Impact of residue on bed-of-nails testing " Similar to rosin fluxes! Compatibility with conformal caotings must be determined " Conformal coatings are typically silicone or urethane coatings dipped or sprayed onto the board surface after assembly to provide environmental protection in the field

19 Contaminants

20 Contaminants! Particulates " Dust " Lint " Solder balls! Nonpolar " Rosin residues " Synthetic resin " Organic compounds from low residue/no-clean flux formulations " Plasticizers from core flux! Nonpolar (cont.) " Oil used in wave soldering " Greas or oil from palcement of insertion equipment " Hand lotions or makeup " Release agents on components " Insoluble inorganic compounds (oxidation products) " Rheologoical additives to solder paste

21 Contaminants & Solvents

22 Polar Contaminants! Residues that form ions when dissolved in water " Salts # NaCl " Halides " Acids! Increase the conductivity of water

23 Polar Contaminants! Main cause of electromigration " Produces dendritic growth between conductors and causes shorts " Requires: # A voltage difference between conductors # The presence of moisture # Ionic material

24 Polar Contaminants! Corrosion " Pb + ½O 2 = PbO " PbO +2HCl = PbCl 2 + H 2 O " PbCl 2 + H 2 O + CO 2 = PbCO 3 + 2HCl " CuO + 2HCl =CuCl 2 + H 2 O

25 Environmentally Harmful Solvents

26 CFC Phase-out

27 Other Selection Considerations! Economics of usage " Materials costs " Equipment cost " Energy cost " Disposal/treatment costs! In-house technical expertise! Compatibility with equipment & components

28 Other Selection Considerations! Performance! Surface tension " Gaps and spacings! Streaking properties! Odor! Boiling point! Flash point! Fire hazard

29 Cleaning

30 Cleaning Chemicals

31 Batch Solvent Cleaner

32 Semi-aqueous Batch Cleaner

33 Three Stage Semi-aqueous Batch Cleaner

34 Semi-aqueous Batch Cleaner

35 In-line Semi-aqueous Cleaner

36 Effectiveness of Aqueous Detergents

37 Effectiveness of Aqueous Detergents

38 Effectiveness of Aqueous Detergents

39 Chelation! Chelation is a process mwhereby an insoluble product is dissolved with the aid of a complexant! EDTA (ethylenediaminetetraacetic acid) and its sodium salts are typical chelating agents! Chelation is used to dissolve heavy metal salts which are otherwise insoluble in water! The resultant complex or chelate is water soluble

40 Closed loop Design with/without Chelating Agent

41 Aqueous Cleaner

42 Cleanliness Test Methods! Visual inspection " 2-10X microscope! Solvent extraction " Test solvent alcohol and deionized water " Using agitation, submerge board into solvent mixture to dissolve contaminants " Measure the NaCl equivalent in the solvent per square inch of board area " J-STD-001 requires less than 10.06µg NaCl equivalent per square inch

43 Cleanliness Test Methods! Surface Insulation Resistance! J-STD-001! IPC-T-650 Test Method

44 SIR Test Patterns

45 SIR Test Patterns

46 SIR Test Patterns

47 Effect of Humidity on Insulation Resistance

48 SIR Test Conditions

49 Bare Board Cleanliness

50 Example Results

51 Example Results