On 17 December, James queried:

>> Could someone tell me the advantages of using Gold/NI vs HASL and
>> what are the common (if any) process problems I may encounter when
>> screening/placing parts/soldering to these type boards using
>> 63/37 eutectic solder paste? Any wave solder issues? Thanks in advance!



James,

    There ain't many process problems as far as assembly goes with gold/nickle 
plating. There was some concern in the past with gold because of the amount that
was plated on the pads...the gold would leech into the solder joint and could 
cause the joint to be brittle, or maybe form an excessively thick intermetalic 
layer. But the amount that's plated on the pads nowadays doesn't warrant the 
concern that there was in the past...which also explains why the cost of gold 
plating has gone down _A BUNCH_ over the past 4-5 years.

    You really shouldn't have to change your process at all, although (I know 
there may be some debate about this), you *MAY* want to add 2% silver and get a 
SN62/PB36/2AG paste. Now I know this is just a cosmetic suggestion, but the 2% 
silver _REALLY_ does make the solder joint a heck of lot "sparklier and shinier"
when soldering to gold than if you didn't use it. The solder joint from a 63/37 
solder looks okay mind you, and you might not even notice anything just looking 
at a 63/37 joint on a gold plated PCB by itself. But if you held another gold 
plated PCB up next to it that was soldered with a 62/36/2, you would easily be 
able to pick the two apart. So if you have a fetish about shiny stuff, go with 
the 2% silver. Wave soldering doesn't have the same "shiny" issue because most 
of the gold that does go into solution gets carried away into the wave solder 
pot, unlike SMT reflow where you're dealing with a finite amount of solder on 
each pad.

    If anything, using gold plating will solve some problems fer' ya', such as:

1. Shelf life of yer' PCB's will be extended with a gold finish...

2. Printing solder paste will be less of a pain on yer' fine-pitch 'cause yer' 
   gonna' have some nice flat pads now...

3. Yer' boards will have that appearance of class now...ya' know that Tiffany's 
   look with the gold plating n' all? HA! HA! (just joshin' ya')

         Seriously, I think it's a good move "SMT-wise" in my opinion. 

            Now, go just another step further and switch to OSP's...

  
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