NimbleSig III PC Board, rev 1.3, Required Modifications
                                                                            73  de VA7TA
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Please note four modifications below are required for the version 1.3 boards.
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Modification 1 (below) is a closeup view of the solder bridge needed between 0.0 Ohm resistor R21 to the
grounded terminal of C48. Without this bridge the RF detector ground pin is left floating.


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Modification 2 shown below is needed to power the RF detector from 4.7V instead of 3.3V as originally
planned. Without this modification the RF Detector will saturate for higher power levels (above 0 dBm). This
will negatively impact attempts to calibrate the detector. The mod requires soldering RFB3 standing on end
which exposes the upper terminal of the ferrite core. A short, 30 AWG, wire (wirewrap type) jumper is then
connected between the cathode of protection diode D1 and the exposed terminal of RFB3.


Modification 3 - Very Important!!!  20090405 Update

                     As mentioned in my article these boards were manufactured without solder mask over the vias. This was done to facilitate debugging and experimentation as it provides exposed connection points to many of the the traces.  However, there are four, non analog ground vias under the DDS chip which easily can become solder paste shorted to the analog ground pad under the chip if they are not masked. These exposed vias normally have a minimal clearance to the analog ground pad under the DDS due to the thickness of the surrounding solder mask however any excess solder can easily fill that gap. With manual soldering of the analog ground pad from under the board using a minimal amount of solder I have not had problems with solder reaching these vias. The clearance from the thickness of the surrounding solder mask has provided me sufficient gap isolation. However if these vias become exposed to solder they will short out to analog ground. Solder paste applied in the normal fashion from the top side of the board would, in all probability, be also applied to these exposed vias and would surely short them to the DDS analog ground pad during soldering. Even with manual soldering it is easy to accidently get solder on these vias as they are not protected. These vias must be masked before applying paste for oven soldering and for manual soldering I highly recommend they be mask protected as a precautionary measure.

     I have recently become aware that there is a high temperature rated Epoxy OverCoat product manufactured by Chemtronics, part number CW2500, specifically intended for coating bare PCB pads or traces prior to high temperature processing. I bought mine from DigiKey and have now given this overcoat a try on multiple boards. I first applied it to the pads of a spare PCB to try it out. After it set I found I could not solder through it with a 300C iron and there was no sign of the overcoat lifting or softening. I sure wish I would have known about this product earlier!

    The applications steps are as follows - refer to the photos below for reference:

• Scrape off any coating applied earlier (high temperature enamel or nail polish) with a suitable tool such as a scapple or exacto knife. This is a tedious job!
• Mask the area of concern off with cellophane tape
• Mix the epoxy as per the instructions. Only a tiny amount will be needed.
• Applied the epoxy overcoat with a sharp, round wooden toothpick. One can pick up tiny droplets of glue on the tip of the toothpick and then smear it over the vias. I overfilled the rectangular area exposed by the masking tape and spread the expoxy around with the toothpick. Then with fresh toothpicks I removed any excess epoxy leveling the surface of the overcoat flush with the masking tape.
• Place the board on a slightly too hot to touch warming tray for about 10 minutes.
• Using the microscope examine the vias carefully. The expoxy tends to spread out and can become too thin over the vias. Any bare copper or gold should be clearly covered with the green pigment otherwise the coating has become too thin. As needed add a small droplet to each via and spread it around to ensure good coverage.
• Place the board back on the warming tray for about an hour.
• Using the microscope carefully remove the masking tape in reverse sequence.
• Carefully examine the IC pads and scrape any overcoat off the pads. Usually the corner pads get covered becaus of the gap in the masking tape there.

    If you happen to have a fresh board that has never had any previous product applied to the vias your job just got a whole bunch easier as you don't need to bother with scraping or masking. As shown in the bottom right photo below vias can be easily covered.

    Using the microscope and toothpick just apply a droplet of expoxy to each of the 4 critical vias under the chip, let it set a little on the warming tray for about 10 minutes at which time some of the epoxy will spread and or drain into the via  hole, then finally add another droplet to each via to ensure there is sufficient converage. If the four critical vias are well covered with green pigment once the epoxy has harden your job is finished.



    Although I think the high temperature enamel mentioned below is marginally OK I would have more confidence in using this epoxy product meant for the job. I found with the high temperature enamel that if I heat it with a 300C iron it will soften and then I can solder through it.

    The high temperature rated enamel paint (second choice alternative to Chemtronics epoxy overcoat) is sold in spray cans and is readily available in hardware stores. Krylon makes a spray paint intended for barbaques that is rated at 650C, is readily available in most locations and is inexpensive. I sprayed a small amound of this paint into a plastic cup to form a small puddle. After applying this paint to a few boards the easiest method I found was to first mask the IC contacts and most of the centre pad area off with regular stationary clear cellophane tape leaving only the area to be overcoated exposed. Then the enamel can easily be applied with a cotton gauze tip (Q Tip). After a minute or two some of the enamel will drain into the VIA holes at which time a second application is needed.

    I will be protecting all the bare boards I have left before sending them out. But for those that have already received earlier bare boards please make sure to protect these vias, preferably with the high temperature epoxy overcoat specifically intended for PC board trace and pad solder masking.






Modification 4:
                    The EEPROM WP (write protect) input pin 7 was left floating. Although this has not caused any known problems with NS3 operation to this date it is a poor design practice to leave an input pin floating. The data sheet states that this pin should either be grounded for normal operation or pulled high to implement write protection. Fortunately it is very easy to ground this pin with the existing version 1.3 PC board artwork design. Since the ground plane circles the floating pad it is just a matter of scraping a small amount of solder mask off  the ground plane and solder bridging the narrow gap with some solder. The schematics have been updated on this web site to show the WP pin grounded. Also the artwork for the next board version has been updated. Credit goes to Jim, KC2QFE for identifying this oversight.  The photo on the right below illustrates how I grounded this pin with a miniscule length of #30 awg bare wire bridging the gap under the solder blob.