voiding on my design with the toster oven - Page 1
Forum section: electronics / general

author: buton
author_rank: Newbie
timestamp: May 30, 2017, 04:00:36 pm
content: I am trying to reflow with my toster a QFN Part which is a RF part... some boards didnt work, some failed to send the RF signal or is sent with errors.i was able to get an xray machine and my friend told me i have some voiding on the solder...any recomendations? i am using lead free solder, and reflowing at 240C.. changing to lead solder will help with the problem?

author: KL27x
author_rank: Super Contributor
timestamp: May 30, 2017, 06:25:25 pm
content: You were able to get an Xray machine, but you reflow with a toaster oven?The first thing to try is to go around the part with a compatible flux and iron. There should be a little bit of exposed pad around the edges. Make sure you get these pads wet and try to see a filet. Many QFN parts have exposed lead up the side of the chip, and what you want to see is sometimes called "suckling piglets." This is what the beads will look like when they have a good filet between pad and the side-leads.Some other things to consider, not necessarily in order1. Enlarge the stencil holes/thickness and/or the part pads to get more solder on there2. improve the stencil pasting... dead flat jig, proper squeegee, etc3. try a difference solder paste.4. Perhaps slowing the ramp up a little. I'm not sure what "voiding" is but if you ramp temp too fast, solvent can "explode" which will move solder away from where it needs to be.FWIW, I have a $300.00 reflow oven. And my preferred way to put on QFN on a 2 layer board is with a $30.00 hot air station. If I use a stencil at all, I will remove the IC's from the paste layer and do them last. I just pretin the pads with regular rosin solderwire and a bevel tip. Then wipe with more liquid rosin flux. Then I pin the chip down with tweezers while hitting with hot air, to prevent it from sliding once the flux starts to boil. Then I put on the knife tip and go around the edges with more solderwire. I've done hundred plus chips this way, and it works fine. Somewhere along the way, you get a feel for how much solder to put on the center pad. And you figure out what is too crooked or high for the reflow. And you just add more flux and hit it again, nudging with tweezers if it too stubborn to realign by itself. Stereomicroscope is more valuable tool than reflow oven, any day of the week, but esp if you are making prototypes more than high volume. Personally, I would typically bother with solderpaste and oven only for multilayer board.

author: buton
author_rank: Newbie
timestamp: May 30, 2017, 06:56:45 pm
content: QuoteLet's say that my friend works for a CM and he has access to it.gonna switch to leaded solder and see what happens..this is voiding.. i might add flux then

author: KL27x
author_rank: Super Contributor
timestamp: May 30, 2017, 07:00:54 pm
content: Check datasheet for your solder paste. Even if it reflows at 240C, the board takes time to reach that. You might need to go higher or hold it longer or increase the soak time. I don't know why the voids matter, but I don't know much about RF. What I understand is that even flux residue can affect RF. But if you're getting no signal at all, I don't understand how these little voids or residue could do that.