HMC441 and HMC451 suggested layouts solder mask

Question

Why do the HMC441 and HMC451 suggested layouts pull back the solder mask around the leads of the parts? We had one part on our board with the ground and 5V rail bridged with solder.

SMcBride · Accepted Answer

Hi kris565, 
 For the HMC441LC3B I see that solder-mask does not appear between Vdd and the adjacent NC pins (see the grey rectangles below, which overlap the device pads...at right, side-by-side with the Functional Diagram): 
 
 . 
 Those NC pins (as defined on the Functional Diagram) actually connect to the device ground paddle on the PCB and thus to the device GND. If you were not careful with your dispensation of solder, then shorting between Vdd and the N/C pins/GND could occur during reflow. It looks like this HMC441LC3B PCB is a multi-product PCB meaning, it gets used for more than just the HMC441LC3B...thus the lack of solder mask optimized for this particular part. 
 The situation is similar for the HMC451LC3 and HMC451LP3E; both are multiproduct PCBs on which the solder mask did not extend between Vcc and adjacent pins. For that reason care must be taken with dispensation and during reflow of the solder. 
 Please let me know if you have additional questions. 
 Regards, 
 SMcBride