solderability and hasl thickness| 9 September, 2003
We would be very wary of allowing 50 uinches, because this will likely result in reduced shelf life due to intermetallic growth. As a guesstimate, at least 10 uinches of that 50 uinches, and maybe more, is already intermetallic when you receive the boards from your fabricator. So, that leaves you less than 40 uinches as solderablity protection to protect the pads during the storage life of the board.
solderability and hasl thickness| 10 September, 2003
Intermetallic compounds [IMC]: * Form during the alloying process of some metals. In soldering these are Sn, Ni, Ag, Cu, Au. * Do not accept solder. * Increase in depth in logarithmic proportion to both time and temperature. [So during the soldering process, staying on the joint twice as long, or with an iron twice as hot, causes the intermetallic layer to become 10 times thicker.] [So, the less solder on your pads, the less time it takes to consume the solder on the pad.] * Are brittle and form between the copper pad and the solder holding the component on the board. Mechanical stress can cause solder failures at the IMC / solder boundary.
Unlike hand and reflow soldering, wave soldering sweeps the IMC from the pad.
solderability and hasl thickness| 11 September, 2003
Sure it's conceivable that thin HASL can cause tombstoning.
Harkening back to the ample discussion in the fine SMTnet Archives, tombstoning is caused by an imbalance in solder surface tension between the two pads of SMT component. The source of this imbalance in solder surface tension could be pad layout selection, reflow temperature or something like that.
If we squint our eyes real tight and ignore the dog out the yard barking, we can sort of imagine a weird HASL coating / pad coincidence that might cause tombstoning, but try as we might, that barking sure drags us back outside and makes us want to exhaust all other more conventional causes of tombstoning before we go running down the road, waving our hands over our heads, yammering something like "Eureka, it's the thin HASL!!!"
solderability and hasl thickness| 12 September, 2003
Around 1989 I had this probelmm with a SMT CCA that would not solder in one area consistently. I had it cross sectioned by the bare board supplier and found that under their metallurgical microscope the thickness of this HASL bare board could not be determined.
I thought that this was the case as the intermetallic would "grow" to the surface and be oxidized thereby rendering the pads unsolderable.