Steve,
Besides what Joyce has mentioned, I wonder also about the quality of the
balls they are using, the size variation of the solder balls and the
difference between the diameter of the solder balls and the XY dimensions of
the pads they are attaching the balls to.  All these things could influence
the possibility of getting Head ON Pillow. The other thing, is where are you
seeing HoP?  Random, everywhere, the middle, one end?  If it is concentrated
in the middle or end then you will have to consider warpage of the connector
and/or the board.  And how good is your printing process?

Regards,
Bev  

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Kelly
Sent: Thursday, August 24, 2017 2:12 PM
To: [log in to unmask]
Subject: [TN] Dried flux

Hi All,
I have a new hi-density BGA connector I am trying to assemble. When the
supplier makes the connector they "solder balls onto the pin" (lead free).
We see dried flux on the balls and when we solder the connector we are
getting a few joints with head in pillow. The connector manufacturer says
the dried flux is not an issue - is this true? Thanks. Steve Kelly