if you have MEMS or accelerometer, even QFN (large parts with MCM),   
flatness  is critical... warpage too... in addition, few can  do HASL  
well (high speed air knife, or even coinning after to keep  it flat,   
plus well clean  after HASL - ionograph stablized and meet spec, not  
just for few minutes... if you get from far east - asia, watch out...  
some board  final rinse is still city water - the one you need  to  
boiling before drinking type)... If you have a wonderful vendor can  
coining the board  for  you, HASL is better (hopefully, they don't  
over do  it to get extra thick IMC on the  board...you might have   
problem at re-work).
On Jan 23, 2018, at 10:48 AM, rkondner wrote:

> Bev and John,
>
>  Thank you. So that is two votes for flatness issues.
>
>  Any others?
>
> Thanks,
> Bob
>
>
> On 1/23/2018 10:27 AM, John Burke wrote:
>> 	
>> 		
>> 		
>> 	
>> 		I believe this is referring to “random highs” where most of the  
>> pads are relatively flat but on some the solder is still on the  
>> pad typically high in the direction of the air knife
>> 		
>> 		
>>
>> 		Sent from my iPad Pro
>> 	
>>
>>
>>
>>
>> On Tue, Jan 23, 2018 at 7:18 AM -0800, "BEV CHRISTIAN"  
>> <[log in to unmask]> wrote:
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> Bob,
>> Please explain what in your case you mean by “warts”.  Did you  
>> mean that literally or as you using the term as Wayne was?  HASL  
>> is by its nature is not flat.
>>
>> I would contend that HASL flatness CAN be an issue that solder  
>> paste is not always going to fix.
>>
>> Regards,
>> Bev
>> HDPUG
>>
>> Sent from Mail for Windows 10
>>
>> From: rkondner
>> Sent: Tuesday, January 23, 2018 10:08 AM
>> To: [log in to unmask]
>> Subject: Re: [TN] soldering problems
>>
>> Hi,
>>
>> Can someone explain to me the warts associated with HASL? I know some
>> would say flatness but since solder paste goes on all my boards  
>> that is
>> not am issue.
>>
>> I am an EE and I typically run 5 to 25 boards for protos. A run of  
>> 100
>> or 250 is a big job for me. I cannot keep up with validating every
>> vendor. I cannot run and verify coupons for every run.
>>
>> HASL has been very good to me even with low cost vendors and simple
>> board storage. What am I missing. (Other than soldering problems! :-)
>>
>> Thanks,
>> Bob K.
>>
>>
>> On 1/22/2018 7:25 PM, Wayne Showers wrote:
>>> IAg is not a horrible idea for a surface finish, but it does come  
>>> with its own warts.
>>> 1) Creep Corrosion: DFR Solutions has done much research on  
>>> this.  Primary recommendation to mitigate it is to paste all  
>>> component pads whether populated or not.
>>>>> https://link.springer.com/article/10.1007/s11664-004-0025-x
>>> 2) Move away from WS fluxes.  Because RMA fluxes utilize the  
>>> Rosin as a suspension agent, you will get must less secondary  
>>> issues with the flux penetrating into unwanted areas which  
>>> include under the solder mask, between plating layers (This is  
>>> critical for immersion silver as the silver immersion process  
>>> creates micro-voids and micro-pitting of the underlying copper.)
>>>
>>> 3) Storage and Handling: Always wear clean cotton or latex gloves  
>>> (preferred).  Cotton can trap oils and if the gloves are cleaned,  
>>> many detergents and fabric softeners have phosphor and sulfur  
>>> compounds in them.
>>> And Silver saver paper is a must.  Silica desiccant and IAg are  
>>> not compatible.
>>>
>>> And as a final aside, unfortunately the days of Tin-Lead are  
>>> limited thanks to the tail wagging the dog over in the EU with  
>>> Electronics as the target.  In 22 years of Electronics  
>>> Manufacturing, I have had 4 issues with bad ENIG or ENEPIG  
>>> plating, and Roughly 40-50 issues with flux penetration or  
>>> entrapment including 1 recall that totaled over $1.5 Million  
>>> Dollars.
>>>