Gary

In our project LEADFREE we made our first production trials. Just some statements about what we found. The tests where intended to get our feet wet much more work has to be done and a lot has been done in the IDEALS project and in the NCMS project:
Drawbacks:
- No complex boards, no large BGA's on SMT, no real heavy components (SMT and wave)
- only test boards
- No mass production just 3-10 PCB's

Materials
- Boards: standard FR4, 2 inner layers completely plated with copper
- Components SMT. Plating: whatever was available on the market, whenever possible the same component leadfree and with tin lead : 
SO 014
SO 016
SO 020
SOT 23
C1206
C0603
C0805
- Components wave
C1812
C0603
R0603
R0402
MELF0204
MELF0102
TO 220
Elco axial
Connector V96
Resistor axial 2W

- Board finishes
NiAu
Immersion tin
SnPb HAL

- Solder
SnPbAg
SnCuAg
SnAg
SnZn ( only reflow)
Solder pastes of the same alloy from various suppliers where used.

- Results Reflow
Equipment: Full convection reflow
A quick and dirty approach with using a standard profile for SnPbAg and rising the peak temperature to 233°C gave excellent solder joints with all alloys on all surfaces. The influence of the flux composition was more significant than the one from the alloy on a given surface.

- Results Wave
Equipment: Double wave, preheating IR plus convection, fully closed N2 system, nozzle geometry adapted to lead free solder.
Preheating: Top of PCB 118°C, TO220 Package 92.2°C
Flux: Synthetic flux ( no composition available)
Conveyor speed: 1m/min.
Solderpot temperature: Test Tmax= 255°C; Test Tmin=245°C
Soldering with a solderpot temperature of 245°C gave perfect solder joints. Even under air. Earlier trials with 280°C gave very poor results.
Trials with table top wave solder equipment gave "icicles" and shorts even with 280°C. Which makes sense, since the thermal energy in a small pot is much smaller than in a big one a fact that is more important with lead free solders than with SnPb. We will have to confirm these results with a complex PCB. The problem is to find a sponsor that supplies us with a large THT board that is a challenge to solder since such a thing tents to be quite expensive.

- Problems to come
Reflow equipment doesn't like the temperatures needed for lead free solder. Even 240°C seems to be too much and wear out has to be expected.
Tin is a good solvent. Thus all parts made of stainless steel that are in contact wit liquid tin erode. There is equipment available suited for lead free solder.
When using VOC free flux in wave soldering the preaheating of the PCB's becomes a problem to get the water out of the flux. Convection is rcommended.
Dross formation and its costs ( better with N2)
Higher process temperatures migt be necessary in reflow with large components. I have no experience so far.
Reliability has the potential to become a problem. Our slow thermal cycling tests show a tendency of faster degradation of lead free solder compared to Tin Lead.


I hope this helps a bit.


Best regards

Guenter

.



EMPA
Swiss Federal Institute for Materials Testing and Research
Centre for Reliability
Dipl. Eng. Guenter Grossmann

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1823 4054
mail:     [log in to unmask]

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