An SDF file is a SQL CE (compact edition) database. Your connection string in the case of CE should just be the path to the file, without “Data Source=”, for example, simply this:
D:\bubbleData\BubblePos3-gp-08-09-16.sdf
You should consider upgrading to SQL Express. You will need to convert your CE (sdf) database to SQL Express (mdf). SQL CE will give you problems eventually, so the upgrade is highly recommended. Then your connection string will look different, using the “Data Source=” part.