As usual, I agree MOSTLY with Joyce, but ...
I've not seen a lot of success trying to increase joint height by using preforms to add more solder--usually that just makes shorts. You can try to use a spacer, which will work as long as you pay attention to the relative thermal expansion coefficient of the spacer vs. the solder.
In previous posts, I've railed against the IPC cop-out expression "evidence of wetting"--especially for class 3 assemblies which have parts with pads completely beneath the part. Also note that the "evidence" does not need to include wetting to the toe of the lead (the only part you have a prayer of seeing). HOWEVER, history seems to show that these concerns aren't as serious as some of us old geezers thought they would be. At least, I'm unaware of huge numbers of solder joint failures on QFN parts attributable to inadequate joint height, even in prototyping runs. And other parts with no leads, such as large ceramic capacitors do not seem to suffer from pre-mature failure due to inadequate solder beneath the part.
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Yuan-chia Joyce Koo
Sent: Tuesday, August 30, 2016 2:58 PM
To: [log in to unmask]
Subject: Re: [TN] Solder under SMD Pads thickness Mins and Maxes.
why you need max min? if you need for reliability reason using Pb rich solder to relief some strain for example, you will need to have preform to define min. thickness. Unless your design is pushed to the limit - extreme temp included, or try to fix someone's crappy design at late stage, you don't use that... you will need (1) process control data, (2) FEM for thermal expansion analysis for example, etc.- not difficult if only a bit off, but it would very difficult if it is way off the mark (like someone selected a low Tg, high CTE board to cut cost... scream bloody murder...).
On Aug 29, 2016, at 4:12 PM, Decker, Scott UTAS wrote:
> For some reason, I thought I saw a spec that mentioned what the
> minimum and maximum solder thicknesses are or can be between the
> component termination and the board pad. I looked through the J-
> Standard for soldering and could only find a reference to Dimension
> "G" note 3 which reads "Must show evidence of wetting." but no minimum
> or maximum. Can someone point me to the spec if there is one, for this
> soldering aspect of an assembly, or tell me I'm seeing things that
> don't exist... Thanks in advance...
> Happy Monday to all!
>
> Scott Decker - Senior Analyst, Drafting & Design Services CID+ -
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