As the installation of the AD9958 chip is probably the most critical step in populating the NimbleSig III PC board I have decided to describe the manual soldering technique I use in a bit more detail with reference to some recent macro lens photos I have taken.

    The AD9958 chips, which apparently are moisture sensitive, are usually distributed in an hermetically sealed anti static package along with a small desicant pack and a humidity level measurement paper strip gage. If the humidity level exceeds the prescibed level as shown by the paper strip gage ADI recommends driving the moisture out of the chip by baking it at 125C for 24 hours prior to soldering with a high temperature process. As long as the AD9958 is left within the sealed package along with the desicant package until installation time I don't think there would normally ever be a need for baking.

    Photo 1 illustrates the soldering supplies I use which consist of very fine (0.015") rosin core solder and a flux-pen both from Kester plus a coil of very fine (0.025") de-soldering braid.

Photo 1: Soldering Supplies

    Prior to placing the chip on the PC board I coat the AD9958 PCB foot print pads with flux from the flux pen. I also coat the chip contacts and analog ground pad on the AD9958 chip bottom side with flux. I find coating the pads and chip contacts with flux makes soldering the chip into place noticeably easier.

    To start I carefully orientate the AD9958 chip to the PC board foot print ensuring that the chip contacts are properly aligned with the pads on all four sides. I double check the orientation against the pictorial prior to proceeding. 

     With the chip in place I tack solder the bottom left corner of the chip (pin 29) using my very fine tipped, temperature controlled soldering iron set to 290C. Pin 29 is a good place to start as pins 29 to 39 are all bonded together to the AVDD supply (see photo 2). If a little extra solder here bridges adjacent contacts it doesn't matter as they are connected together anyway.

    After pin 29 is soldered I re-check the overall alignment of the chip ensuring that the contacts are centered with the pads on all four sides. As long as the chip has remained closely aligned after the soldering of pin 29 the chip can usually be pivoted slightly by using a needle as a subminiature pry bar to achieve near perfect alignment with the pads.

    Once proper alignment has been established I then solder pin 56 which is more or less diagonally oposite pin 29 and which happens to be a digital ground pin. With both pins 29 and 56 soldered the chip will become rigid. It is good idea to stop at this point and re-check the chip alignment. Also check to ensure the chip is flat against the board. If necessary re-heat the two soldered pins, one at a time, whilst pressing the chip against the board.

    After your fully satisfied the chip is properly in place continue with soldering the rest of the pins, one row at a time. It is probable that you will bridge some pins togehter with too much solder which is where the de-solder braid comes into play to sop up the excess solder. Once you have completed a row go back with the solder braid to remove any solder bridges (remember bridges between pins 29 to 39 don't matter as those pins are connected together anyway).


Photo 2: AD9958_SolderedPins_29_42


    It is also possible to not have sufficient solder to bridge the chip contacts to the associated pads. Sometimes the contacts might appear to be soldered when looking directly down on the chip from above but if examined from the side a small gap between the contact and pad might become viewable. Photo 3 below illustrates how the connections should look when viewed at a 45 degree angle to the plane of the chip. As shown solder should extend from the contact down to the pad.

    Prior to soldering the analog ground it is wise to do continuity checks to detect any adjacent pin shorts or open connections. A DVM with a continuity tone feature is a valuable tool for doing these checks as one can remain focused on the board whilst listening for the continuity beep. As the vias are bare and most connections have vias in the trace path continuity can be tested from contact to via. The vias can also be used for checking most pins for adjacent pin shorts. I also check continuity from directly from the MPU pins to the associated DDS chip contacts. I have found problems with floating MPU pins doing this test. This testing step is a bit tedious and time consuming but it can save a great deal of time and possibly even an expensive DDS chip should a problem be discovered and resolved prior to final soldering of the analog ground pad.



Photo 3: Closeup View of Soldered Pins 43_56

    Photo 4 below shows the specially filed down soldering iron tip that I use for soldering the analog ground connection from underneat the PC board. I procurred an inexpensive soldering iron for this purpose which I power from an adjustable variac transformer. When I power this iron from about 65% of full line voltage after about 20 minutes it settles with a tip tempurature of 280-290C. The diameter of the tip is a bit less than 1/8" and it fits loosely inside the analog ground plated through hole under the DDS chip. This soldering iron has a fair amount of thermal mass thus it will maintain a temperature well above the solder melting point of  220C during the soldering process which only takes a second or two. It is important to load the tip with lots of solder so that the tip does not draw solder away from the PCB through hole when the iron is removed.


Photo 4: Soldering Iron Tip Used for Soldering Analog Ground Pad

    While the iron is heating up I anchor the NS3 board within a rubber clawed vice and mount the board bottom side up in a manner that allows me to apply pressure against the DDS chip from underneath to hold the chip against the board.



Photo 5: Solder Coil Preparation

    I then prepare a small coil of solder as shown in photo 4 which will stand inside the throuh hole as shown in photo 5. Once the soldering iron is up to temperature and the tip is loaded with solder I get ready to solder the ground "branding iron" style. It is essential that the hot iron be applied for only one or two seconds. If you linger too long with the hot iron applied to the bottom of the chip you risk danger of de-soldering the DDS from the board or damaging the DDS from heat shock. As a precautionary measure I hold the DDS chip up against the PC board from underneath with a gloved finger. I then insert the butt tip of the 280C soldering iron against the solder coil and then into the analog ground hole as the solder quickly melts. The heat should be only applied for about a second or two maximum.The solder melts and bonds to the chip almost instantly and the result is a nicely soldered ground connection. Caution! The DDS package will quickly become hot and can burn a bare finger! I have learned the hard way to ware a thin leather glove.


Photo 6: Solder Coil Ready for Soldering AD9958 Analog Ground Pad

 

73 & Enjoy!
VA7TA