Bridges on QFN and QFP

[ktownsend]

I'm curious how reflowing QFN parts with solder paste been working out for other people here? I get the occasional bridge with QFP, but it's simple to fix it up with an iron ... the QFN's (ADXL335, etc.) are more of a pain in the ass to inspect and repair when you do get a bridge, though, even with elongated pads to make them iron-friendly. (One of the problems with lead-free solder is that you need to work with a ~350°C iron, and it's really easy to lift the pads at those temperatures so you want to avoid manual rework as much as possible.)

I've been thinking about manually making my paste layer on the individual pads in Eagle smaller than the pads themselves to reduce bridging? By default Eagle makes the paste (cream) layer the same size as the pads, but I think this promotes bridging since you have more paste than you need on the pads and it has no where to go except next door in that case. I've been meaning to go into all my footprints and do this (I think a paste layer 1/2 the size and in the middle of the pad should be ideal), but it isn't that fun so I've been putting it off and I'm not sure how the smaller holes will work out on the stencils. Just wondering what other people are doing to reduce bridging on fine-pitch parts before reworking my entire Eagle library. I'm keen to avoid manual rework, or at least minimise it as much as possible, but maybe other people have some tips on optimising the paste layer?

[adafruit]

huh! i do lots of adxl's (QFN 16) and never had bridging. my problem is more opens. but i read a cool hint on QFN pasting which helped. the chip was literally tilting on its side!

http://www.adafruit.com/blog/2009/09/11 ... uidelines/

[Zener]

Maybe just get away from the lead free solder.

[adafruit]

ktown is in paris

[freaklabs]

I get significant bridging on very fine pitch pargs because I'm not having professional solder paste masks made. I'm making them on my CNC with available materials. However a bit of flux and a few seconds going over the QFN with a soldering iron gets rid of the bridges so it's not a big problem for me. The real issue is how to deal with through hole components like headers since they can't be automated with a pick and place. Wave soldering may be possible, but need to wait to get some money to try that out.

[adafruit]

headers we do by hand. or you can leave them off and have the end user put em on. half the time, the headers are the wrong gender anyways

[freaklabs]

Yeah, I might end up needing to do something like that. Gonna try to install the headers myself as long as possible since most of the people on my site are looking for something to just get up and running quickly. Will probably experiment with leaving the headers off tho.

[ktownsend]

I think the rule of thumb I'm settling on for headers is that I'll solder any header pins used for jumpers since they're essential, but I'm leaving all the breakout pins empty since I don't know if people want male or female and with the leads on the top or the bottom. Plus it takes way to long to do properly on 30 boards.

[mojo]

I just use an ordinary iron with 1mm tip and leaded solder (I know...) I use a flux pen too.

I can usually get about 90% of the pins done without any bridges, and then just use an ordinary solder pump to remove them. Works well with AVRs, some other ICs may be a bit more heat sensitive. I think you really need leaded solder too as the melting point for lead-free is too high.

My fingers are stiff with banned but somehow I still manage to solder these fairly small pins so it can't be that hard

I have done about 40 Retro Adapter boards this way and maybe 20 other ICs for prototypes over the years. In the future I am thinking about asking SeeedStudio (who do all the PCBs) about buying stencils as those are what they use when assembling the same Retro Adapter boards.

[AdShea]

If you're doing it by hand (with a syringe) I've had good luck laying a tiny (0.5mm to 0.7mm) bead of paste perpendicular to the pads. This reflows without a problem and after a bit of practice no bridges.