Making a connection to the EPAD on a 144 pin TQFP after it's been fitted!

[deephaven]

I've been a bit silly. I changed assemblers and forgot to issue the new assember with a solder paste mask. As a result, they manually assembled my boards including fitting the Altera EP3C5E144C8N. Unfortunately, this means that the EPAD underneath the chip has not been soldered.

From this undesirable state, what is the best course of action to remedy this situation? Would it be feasible for them to remove the chip, apply solder paste to the EPAD, and then apply heat to the chip with a hot-air gun?

I have 20 boards in this state!

[Balaur]

Although I know my way in assembling, I'm not an expert so please take my comments with a bit of salt.

(For this type of configuration, you would normally plan an array of vias or even larger hole(s) underneath the chip in order to allow for an easy assembly
or reworking. Of course, it doesn't help you right now...)

First, while I know that the EPAD should be connected to GND, are you sure that it has an electrical purpose?

If non, then it's only used for heat dissipation? Do you actually need it? If not, you could reasonably forget about this.

Otherwise, if you are really sure that you need to proceed, the correct procedure could be the following:
- remove the chip using a hot air station with an adequate nozzle
- put some solder paste on the PCB pad
- re-solder everything

However, I cannot recommend you a good method to actually solder the EPAD between the chip and the board. If there is no heat transfer between the bottom of the PCB (through vias) and the pad, things could get very rough. You could try to heat everything with the hot air for a limited amount of time.

Best of luck,
Dan

[deephaven]

Thanks for your reply, Balaur. I can confirm that none of my boards will work without the connection, and this is borne out by a quick Internet search showing other people who have had the same problem. The pad has 4 vias to an internal groundplane. I'm hoping my assembler add some solder paste, after removing the chip, and possibly heating selectively from both sides of the board.

[jahonen]

The exposed pad in EP3C5 is for electrical grounding purposes only, not thermal. We did similar thing at work a while ago to same chip, removed the chip and applied solder paste and re-soldered the chip. That worked ok.

Regards,
Janne

[deephaven]

Thanks Janne. Did you use a heat gun or put it in a reflow oven?

[jahonen]

Thanks Janne. Did you use a heat gun or put it in a reflow oven?

We soldered it using a Weller hot-air station.

Regards,
Janne

[reagle]

You should be able to do that. Considering that the footprint /mask opening is there, just use hot air to lift the chip, put past under, reflow and do the pins manually. Or if you have a reflow oven, just stencil the part itself and reflow.
We did something even crazier once when we forgot to tie a BGA ball to a ground . Had to lift the part, draw a trace with conductive pad, then resolder BGA