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I've found that BGA balls actually melt. I've read that some say that it's advised to add some paste, some say to add flux and some say just to put the chip in place.

At the hobbiest level, without any special PCB manufacturing equipment and with hand placement, do you advise to add some solder paste in very small quantity, to be sure that all pads will have a contact and to allow correct alignment?

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You can get small bottles of liquid flux suspension that don't make a mess of the array, the flux is usually suspended in isopropyl, additional benefits are that you can use a syringe with attached needle to add flux and reheat if all the balls didn't connect properly during initial heat-up, and you can usually just apply the stuff and heat the chip without having to change the balls.

The greasy stuff can sometimes join the balls together and can short the chip or board.

Tip: I noticed a guy put a (not too heavy) small weight on the chip after heating and it helped the joints to cool down whilst properly joined.

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    \$\begingroup\$ I agree with this solution. Use preferably some liquid flux but in no case any solder paste. \$\endgroup\$ – Julien Oct 1 '16 at 15:01
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    \$\begingroup\$ Professional PCB houses do add solder paste (125 microns typically) , but the purpose is for the flux (which will therefore be the same across the PCB) as the amount of solder in the paste is dwarfed by the amount of soler in the balls. \$\endgroup\$ – Peter Smith Oct 1 '16 at 16:00
  • \$\begingroup\$ @Peter Smith you're right, I remember know, we needed a little paste to make the balls stick in the stencils. \$\endgroup\$ – abz Oct 1 '16 at 19:29

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