Specification Reflow soldering of SMD assemblies - Solder paste / lead-free

Transcription

1 Previous Edition Specification Class: Electrical technology Class No.:53 Reflow soldering of SMD assemblies - JED solder paste of lead-free alloy 842 Part name (for databases) Solder paste / lead-free Scope This specification is complementary to the manufacturer s specification. It describes the WABCOspecific purpose of use of and the requirements to solder pastes with lead-free alloy SAC Sn96,5/Ag3/Cu0,5 to be applied on SMD assemblies. The pastes are no clean pastes, i.e., residues of flux are left on the assembly. The fluxes contain colophony and / or are synthetically modified, and the pastes are suited for stencil printing. The reflow soldering may take place in nitrogen atmosphere or in air. 2 General requirements Solder pastes are mixtures of metal powder and flux which after melting establish a connection between the assembly carrier and the component. Residues of flux are left on the assembly; they shall not cause any chemical, corrosive or electrochemical reactions. The metal powder, uniformly mixed with the flux, has a considerable influence on the paste stability and processability. The oxidation of the metal has to be low in order to ensure good wetting at low flux activity and to avoid the formation of solder balls. After soldering, the printed circuit board will not be cleaned. The flux with its additives serves as the carrier of the paste, with the flux components having the following functions: Resin as carrier material, consisting of colophony or synthetic resins, Solvent for etching the resin, stabilization and wetting agents, Rheological additives prevent evaporation of the solvent during the storage and ensure the cohesion of the paste until preheating phase, Wetting additives control the dispersion of the flux, Stabilization agents ensure the compatibility of the components among each other, Activators are the most important additives; they remove any oxides and enhance the adhesion of the solder on the different types of component terminals. The paste is applied to the assembly carrier in the stencil printing method, resulting in the following requirements: Stability of contours until preheating in order to avoid bridging, particularly if small solder distances are combined with high stencil thickness Adhesiveness to ensure attachment of the components until soldering. Continuation page 2 to 9 Compiler Approved Product Group: 4961 Doc. Code Language Rev. Udo Schleusner Jens Gröger JED.dot Template version en -

2 JED-842 : Page 2 3 Properties In addition to the specified properties, there are unquantified material and processing requirements. The WABCO-specific tests are comparative tests. The properties of the solders released to date represent the minimum requirements. 3.1 Material Solidus / Liquidus : 217 C C Metal content Density Particle size Purity/alloy Flux : typ. 89 %, ± 1 % (weight percent) : 3,9 g/ml : powder type 3, 25 up to 45µm, J-STD-005 (Joint Industry Standard) : according to J-STD-006 : according to J-STD-004, L0 3.2 Processing Storage temperature : 2 up to 10 C Storage life Stencil thickness Printing speed Squeegee roll behaviour Endurance on the stencil Printing volume : 6 months/pot, 3 months/cartridge : 150 µm -175 µm and fine-pitch spacing 0,65 mm : 10 up to 140 mm/s : soft and supple during the printing process, no smearing on the bottom of the stencil : min. 8 h : clear high-contrast printing ensuring good detachment from the stencil to obtain high solder volume on the printed circuit board Stability during printing : up to 30 C Reflow profile Reflow under Distribution of flux Flux residue Solder beads Soldered connection Blowholes : linear and saddle : air atmosphere or nitrogen : concentric around the solder pad : for board-test pins (tip and crown); - suitability for WABCO protective coatings and potting compound : solder beading on the component and on the assembly carrier required to be minimal in order to avoid any risk of wetting bonding surfaces : uniform and smooth, facilitating visual inspection : formation of blowholes in BGAs and surface soldering required to be minimal

3 JED-842 : Page 3 Wetting Safety : comparative assessment, especially on open copper surfaces of punched component lead-frames : the material is processed in standard reflow systems equipped with air extraction systems. Any additional requirements have to be specified. 4 Requirements due to the assembly The properties of pastes that have already been released for use are considered as the minimum material and processing requirements. 4.1 Humid atmosphere In combination with humidity, flux residues may cause corrosion and migration on the substrate and on the surfaces of the component. Test conditions: Operation in humidity for 168 h at 85 C / 85 % relative humidity 4.2 Mechanical load caused by temperature / change of temperature The quality of the solder and its wetting properties influence the cycle stability of the solder connection. Temperature cycles and the different coefficients of elongation of the assembly carrier and the component subject the solder connection to tensile and shear stresses. This is particularly true if mechanical vibration or shock loads add to the effect. This leads to fatigue failure caused by cyclic plastic deformation of the solder. Test conditions on typ. assemblies: Temperature shock air/air with 2 chambers -40 C / +120 C, 30 min each Transition time approx. 5 s Number of cycles: Protective coating / potting compound Particular attention must be paid to the interaction with protective coatings and potting compounds applied later in the process. Some types of coatings delaminate from the solder paste flux residues. This will uncover those areas that, if combined with humidity, tend to have the highest concentration of ions, thus provoking electromigration. The causes of delamination have not yet been investigated. Therefore, a compatibility test has to be made. Since the solder development performed by the manufacturers has not aimed at this requirement, the compatibility of certain types of solder pastes is mere coincidence. The delamination might occur immediately or after several hundreds of temperature cycles. The change of temperature test conducted under the following conditions has proved to be reliable: Temperature shock air/air with 2 chambers -40 C / +120 C, 30 min each transition time approx. 5 s number of cycles: 500

4 JED-842 : Page 4 5 Guarantee 5.1 General The manufacturer is obliged to prove by regular random test that the requirements are observed, and to keep the documents referring to this inspection accessible to the customer. 5.2 Initial Sample On each first delivery some initial samples and the initial sample inspection report are to be submitted for inspection. 5.3 Changes The change of the materials or process parameters regardless of the kind of change and its reason is only permissible after previous release by the technically responsible WABCO expert teams.

5 JED-842 : Page 5 Pictures of good and poor quality examples: Good wetting of protective coating on the flux residues after 1000 temperature cycles No wetting; delamination of protective coating on the flux residues after only 100 temperature cycles

6 JED-842 : Page 6 Good wetting of protective coating on the flux residues after 1000 temperature cycles No wetting; delamination of protective coating on the flux residues after only 100 temperature cycles

7 JED-842 : Page 7 Good adhesion of coating on flux residues Poor adhesion of coating on flux residues

8 JED-842 : Page 8 Good adhesion of coating on flux residues Poor adhesion of coating on flux residues

9 JED-842 : Page 9