Saturday, December 31, 2011

PSoC3 Custom Board - Soldering the chip

For any first timers that stumbled onto this post,the chip here refers to a 48 pin SSOP package with a 0.635mm(25mils) pin pitch.

Things didnt go that well on the first board I tried out,mainly due to me using the wrong type of tip.After this,I was able to use sharper tip (from my brother,all credits to him.),which did the trick and I was able to do chips with little effort.The equipment and sequence of events that worked for me,in order are as below.The method I describe below is inspired from this post by Arup Basak of CircuitsDIY ,so credits to him :-).

Equipment Used:
1.Any cheaply available 1mm solder wire,(don't recall the price,bought this last year)
2.Liquid Flux,I used a locally available liquid flux,its a pinkish liquid in an unmarked dropper bottle.
(A 30ml bottle cost me Rs.10)
3.A chisel tip and a sharp pin like tip for your iron,(Soldron tips cost me around Rs.40 per tip,a few months ago.)
4.The soldering iron itself.Many claim that cheap 25-30W irons that connect directly to your AC mains supply wouldn't be used for SMD work,but that is all I had around,so that is what I used.It is a 25W one from Soldron,the most common one you'll see in India.
5.Solder wick(Rs.10 for a 1m roll)
6.Patience,which is extremely critical for this number of pins and fine pitch we are working with here.


1.Flux the pads.

2.Use a chisel tipped soldering iron and 'relay' solder onto it.What I mean is,no applying solder directly to the pads,rather melt some solder onto the tip and then lightly pass the thick(relatively) tip lightly over 5-8 pins,applying solder this way.The ideal height would be where only the solder blob grazes the pins,getting consumed while you pass it over the pins.

3.After you're done adding solder to the pads this way,align the chip and use some means to hold it in place while you solder the pins.Many people use Kapton Tape,or let the viscous flux do that,but I just held it down with my index and forefinger.

4.While you make sure its aligned,come in with a sharp tipped iron and solder the 4 corner pins,in the order of top left,top right,bottom left and finally bottom right.When I write solder,I mean, NO SOLDER on the tip.Clean your tip and just touch the pins to melt the solder on the pads into a nice union of pin and pad.DO NOT keep your iron there for more than 2 seconds,or come in for consecutive 'tacks' on the same pin,doubting the joint or just wanting to make it look shinier(the pad(s) will overheat,and you will cause it to come off),you could wreck your PCB.Solder each of the 48 pins this way.

5.After you've patiently done one row of 24 pins,flip it around,do the other row and then test the pins' joints by prodding them individually with sharp forceps(I use the kind used for dissection of lab animals).

In case you get solder bridges between pins (which you wont,if you follow the no-solder-on-tip-while-tacking-pins very strictly) use the solder-wick,but don't keep your iron there for more than a second or two,since the tracks there will be fine,and can come off.That is how I made those 2 mistakes earlier.

Here are some pics of my end result:

I have currently populated all the parts I have,so progress will pause on this till I get more parts from local dealers+KitsNSpares.

No comments:

Post a Comment