Friday, November 11, 2016

Building Chipstick

Recovered from an accidentally deleted blog!

Nobody is expecting you to build a ChipStick from scratch - unless you are very keen.

The ability to work with tiny SMT parts is not one everyone possesses, but it can quite easily be learned, and with practice, good results are possible with the simplest of tools and equipment.

This post follows the construction process step by step - for the curious - or for those brave or foolish to wish to follow in my footsteps.


SMT Hot Air Rework station with choice of nozzles - I used 8mm
Soldering iron with "needle" tip
Soldering sponge
Flux pen or water soluble SMT flux
Tweezers (stainless steel - non-magnetic)
0.38mm 60/40 solder (Don't struggle with lead-free - it's dreadful).
x10 magnifying eyepiece
good overall lighting - use spot light or bright daylight
scalpel or craft knife
3rd Hand
Bluetack - stick the pcb to the bench to stop it moving
De-Solder braid - accidents can happen
Patience - set a day aside for this, without interruptions
Use a Flux Pen to flux the footprint of the main IC
Flux is essential for good solder wetting.  Use it liberally on the multipin footprints - such as mcu and other ICs.  I use a rosin flux pen for convenience, but there are now some water soluble fluxes that wash off the board with water and an old toothbrush.

Use a soldering iron and add some solder to the IC pads
but not the centre one!
MCU footprint with solder applied
These boards have been "hot air solder levelled" or HASL.  They are easy to solder, but for IC pins you need to build up a little more solder thickness on the pads - and don't forget to flux well.

Flux the underside of the IC package
and also reflux the pads on the board 
I first position the mcu centrally on the footprint pads with tweezers.  The flux heps to hold it in place.  Check from the sides that al the pins line up, and check again that Pin 1 is in the correct position!!

Set the temperature of the hot air tool to between 370C and 400C.

When happy with positioning -  use the hot air tool about 20mm above the IC and swirl the air around the IC in slow circular motions - about once every second.  I count to 20, and usually the solder has melted and the IC is sitting squarely on the board - and not moved to either side.   Don't spend more than 30 seconds with heat on any one component.  Leave to cool - it will be hot!

The mcu successfully soldered in position using hot air reflow tool
Now go around the pins with soldering iron checking every joint is good
Next I use the needle point tipped iron and rework every pin.  I check that the solder reflows and that the soldered joints are bright silver an "fresh". There should be a neat solder fillet between the board and every contact of the QFN package.

Repeat 13 more times!
At this stage - I put the boards into a programming jig and made sure that they accepted the programmed code. If not I inspected carefully and reworked where necessary.

Voltage regulator and decoupling capacitors added - USB next 
I then proceeded to put the remainder of the smaller components onto the top of the board - leaving the bulkier mini USB socket until I had completed all the work on the underside of the pcb first!

Underside of board with USB converter on left and 128K SRAM on right
Topside completed - undergoing testing

Four completed units - with penny for scale

Choice of pins - left are for breadboard, right are to fit DIL 20  IC socket

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