ADSP-2148x E-pad question

Question

Hello 
 I have just finished reading the EE- 352 Engineering note regarding E-pad packages and mounting techniques. One thing I am not clear of is you don't say whether or not you use thermal relief's or direct connects on the ground plane connecting via's. Obviously direct connection would give you better electrical connection and lower thermal resistance to the ground plane but would it alter the temperature profile needed for reflow ? ie which would be better from the point of view of manufacturability ? 
 regards 
 david

AlClark · Accepted Answer

I always use many multiple direct connects. You want the impedance to be very low. 
 On some boards, I also use a large center hole as well. This makes prototyping and rework easier in cases where you are not going to have an automated reflow. The stencil will apply paste in four quadrants, so that it doesn't go into the center hole. 
 If the profile is set up correctly, you will not have a soldering issue. 
 BTW, The same issue applies to many buck regulators used for the core supply - No reliefs. 
 Al Clark 
 www.danvillesignal.com

AlClark · Answer

I use a 2.2mm center hole. This is sufficient for hand soldering. A 5mm hole is much too big. 
 On the 100 pin 21489, I used 24 sounding vias with 0.2mm holes 
 For hand soldering, we start with a small heater, Hakko FR-830. This preheats the board for better solder reflow. 
 It is possible to use solder, but we usually opt for paste. We solder a few outside pins on the LQFP first to insure alignment, and then use either a good iron or hot air. 
 A similar process is used for switchers. These are actually a little more difficult. Layout is critical for switchers. There is a small triangle area that includes the switch, inductor and output capacitor that should be very low impedance as well as the high current grounds.. You also want to avoid noise on the voltage divider and compensation. Read the datasheet many times and look at similar devices. They all have similar layout issues. 
 The ADI ADP2119 or ADP2120 is excellent for the core. It eliminates the need for external FETs. 
 Of course, all these hand soldering tricks are less than ideal compared to using a stencil and reflow. It's OK for prototyping, etc. but not what I would want to do in production. 
 We have never had a process problem with 21489 or 21479 devices in production. The profile is not that special. Any good contract manufacturer should get it right. 
 I have never heard of preheating parts for 24 hours before using. I doubt it a real big issue. 
 Al Clark 
 www.danvillesignal.com

AlClark · Answer

We apply paste to the pad before we place the part. 
 I think hot air works better than an iron. If the small vias are not completely covered by the mask, you can see some outgassing which helps you know you are melting the solder. 
 I should point out that I don't actually do any of this myself. I have skillful people. This means that this is the method I observe, but I could be a little off on the details. I know the hole size is correct since I lay out the PCBs. 
 Al