This question has been asked before and the archives are bursting but the answer
can vary depending on specifics, so let me be specific. What is the maximum gold
thickness that can be used on SM pads without causing solder joint reliability
problems?
SPECIFICS:
Bare Board -  0.062",  4-8 layer, 1/2oz outer foil, 1-2mils electroplated Cu,
100-200u" electroplated Ni, 20-35u"? electroplated hard Au.

Assembly - solder paste no-clean, minimum paste thickness 6 mils, % by weight
metal(Sn/Pb)in paste 50%, Reflow oven forced air convection, temp 455 F peak, 20
mil pitch QFP pads worse case, one and/or two sided reflow.

End product use environment - operational range temp -20 to +120 F, max rate of
temp change 5 F/min, impact resistance 10ft drop to concrete, minimum service
life 10+ years.


Because of certain design constraints the 20u" minimum specified in the 20-35u"
of Au plating on the board is the lowest allowable thickness. The 35u" max is to
give the board fabricator an acheiveable target with a thickness range of 15u",
as a result of variation in current density across the panel.


Typically 3-4% by weight(BWT) seems to be the industry max for Au in the solder
joint. If I back this down by 30-40% for a safety factor and lack of uniform
dispersion of Au in the joint it works out to 2-2.5% BWT.

A gold thickness of 35u" calculates out to about 2.2% BWT in the solder joint.
This is based on the following:
Au thickness - 35u" max
solder paste - 6 mils minimum
%Sn/Pb in paste - 50% BV
Density Au - @18
Density Sn/Pb - @9

However it seems that most people are using 5-15u" of Au and start wringing
their hands and getting nervous if required to go to 20u" and above. Is this
really a problem or a perception? I'd like some real world feedback. Please when
possible back up your statements with facts/data.

We will be doing solder joint reliability testing on gold in these thickness
ranges for product verification. This will include thermal cycling, vibration
and impact testing based on our internal specifications. If there are particular
test conditions/procedures that you have found meaningful I would like to here
about them.

                                                Regards
                                                        Michael Barmuta
                                                        Staff Engineer
                                                        Fluke Corp.
                                                        Everett Wa.
                                                        425-356-6076

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