chip scale BGA (csBGA) - Page 1
Forum section: electronics / general

author: codeboy2k
author_rank: Super Contributor
timestamp: April 28, 2013, 08:01:44 pm
content: Has anyone ever tried doing a chipscale BGA at home? I am laying out a board that will have a few csBGA parts on board. In production it will be pick and place, and through the oven, but the prototypes will be hand soldered by me.I am thinking that I will have to preheat the board on a hotplate and quickly reflow it with hot air, without blowing it away.The chip is about 1.7 x 2.3 mm. it has 6 balls in a 2 x 3 grid, with a 0.50 mm pitch.Is it doable at home? Or should I not even bother to attempt it?Thanks !

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 28, 2013, 08:26:24 pm
content: I'd go for hotplate or toaster oven - air is likely to blow it away, and heating may be uneven.Most important thing is a clean (leaded) paste print, and accurate placement - put some marks in the top resist or silkscreen to assist with placement - resist may be more accurate than silkscreen.You need to place it down right first time, as any sideways movement may smudge the paste pattern

author: codeboy2k
author_rank: Super Contributor
timestamp: April 28, 2013, 09:50:04 pm
content: Thanks Mike... I was thinking of some kind of top side marking to assist with placement, yes... I was thinking about silkscreen but your idea sounds better, use copper. I'll put some copper traces at the cornersI don't have an oven here, I was hoping to just get it done with the hotplate and a low flow air... I can use the hotplate alone and bring the temp up completely and reflow it that way, but that might be tricky with a board this small -- I just need to be careful not to burn the board

author: westfw
author_rank: Super Contributor
timestamp: April 29, 2013, 12:51:03 am
content: QuoteThat might work; it all edges. I tried an experiment with that tiny NXP ARM chip in the 16-pin csbga package, and I couldn't even get a PCB design that fit within typical prototype (Seeed, Itead, OSHPark) design rules :-(

author: gxti
author_rank: Frequent Contributor
timestamp: April 29, 2013, 02:03:11 am
content: Is it LM3691, perchance? I have used it before, and while it is certainly easy to screw up I eventually got something resembling a successful process for small-quantity assembly. I did not use solder paste, just smeared as little flux paste as I could on the area under the part. Soldering was done in a toaster oven by watching for it to settle. In my case it was the only lead-free solder on the board so once it was done everything was good to go.The really critical element was getting it aligned properly. I think I had 2 that "skipped" a row and had to be desoldered and tossed.

author: jmole
author_rank: Regular Contributor
timestamp: April 29, 2013, 02:12:19 am
content: most of these have solder balls on the device, so I don't think you need paste on there. Just be sure you use a flat plating solution, like ENIG. doing it by hand, adding a drop of flux is not a bad idea.

author: codeboy2k
author_rank: Super Contributor
timestamp: April 29, 2013, 04:33:34 am
content: it's not the LM3691, but I am going to remember that device for the future. This board will have a 78M05 in sot223, fed from 12V in. If I need the space I might consider the LM3691. It's costs about twice as much, but its much smaller total area at about 15mm2and the 78M05 is about 45mm2. I think I can fit everything on as it is, but if not I might use the LM3691. The board is supposed to be 2cm x 2cm or 400mm2The client has said he has some wiggle room on the board size, but not much. He also wants it to be a single sided board if possible. might be possible, since I don't have to break out the BGA, it's all in line.It's a dual P-channel FET,CSD75211W1723. Actually, I was wrong, its not 3x2 balls, it 4x3 balls so thereareinternal balls, but as westfw said, if I can line up the edges I should be ok. But in the course of writing this out to post here, I realized that I can't use this part anyways, since I would exceed the MAX Vgs of the device. So I am looking for another part, now, but it still needs to be small and will likely be another csBGA device.Thanks for the guidance. I'll post more as the project progresses.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 30, 2013, 09:04:32 am
content: Could you not just cut a small piece of kapton tape and tape it down and then rework it?if not, would it be possible to use large(er) square pads in the corners of the chip that extend some distance out from under the chip so you can "tack" it down?something like this;

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 30, 2013, 11:56:06 am
content: Quote from: jmole on April 29, 2013, 02:12:19 amFor alignment, I wonder if via-in-pad would be a good approach, or a tiny hole in the copper in the centre of the pad so it will self-centre

author: marshallh
author_rank: Unknown
timestamp: April 30, 2013, 06:13:40 pm
content: Buy your BGA parts when designing the pcb. Take them out of the moisture locking package jsut long enough to confirm that your silkscreen outline EXACTLY matches the outline of the physical package.This is the #1 most important step for manual assembly.While you can use solely flux and the provided solder balls on the package (I have soldered all my bga up until now like this), the industry in general prefers using deposited paste. However it is critical you line it up right the first time.Like the others are saying, hot air is not going to work on this small of a part, unless you can manage to hold it down with tweezers, but that is a bad idea for a part that needs to settle on the molten solder balls' surface tension. You really do need a reflow oven. Once you have one you will never look back.

author: lgbeno
author_rank: Frequent Contributor
timestamp: April 30, 2013, 06:24:20 pm
content: I think that if it was me personally, I would get a very fine syringe and needle and deposit a very small amount of paste only the pads then place the bga under a microscope looking at an angle so that I could see the solder balls align to the pads.For reflow I think it is best to do 2 toaster ovens, one for preheat and then one for reflow. I have a friend who started a cm business in his basement doing this.

author: codeboy2k
author_rank: Super Contributor
timestamp: April 30, 2013, 09:20:13 pm
content: Quote from: mikeselectricstuff on April 30, 2013, 11:56:06 amThat might be interesting, yes.. maybe the 4 corners ? interesting being the hole, not a via though

author: jmole
author_rank: Regular Contributor
timestamp: April 30, 2013, 09:22:16 pm
content: Quote from: mikeselectricstuff on April 30, 2013, 11:56:06 amI don't know about hand-reflow, but for machine reflow, via-in-pad is a terrible approach for BGAs unless you use plugged vias. Of course, this isn't from personal experience, but the hundreds of warnings I've seen about this can't be wrong. Apparently the solder balls will wick into the vias, and you can have voiding issues which leads to very bad yield.Screaming circuits has a GREAT blog, and an entire category devoted to via-in-pad layout:http://blog.screamingcircuits.com/via_in_pad/Came in very handy when I was designing QFN packages in EAGLE, since unlike altium, you actually have to design the thermal pads/vias/soldermask yourself (if you want it done right).

author: codeboy2k
author_rank: Super Contributor
timestamp: April 30, 2013, 09:28:41 pm
content: Quote from: marshallh on April 30, 2013, 06:13:40 pmI would do it with paste, like it's done on the line. I don't know for sure, but I think if I just used flux it would float around on the surface tension (?) and probably be worse off and harder to align while floating on a pool of flux. A bit of paste on the pads as others have suggested would definitely help it stick in about the right place, and it will pull in. That's what I am hoping for at least.Good point about buying the part first. I always do that, just to check the pads are right with all the parts. It's always best practice to have everything in hand before sending a board out, especially new parts I've never used before. Older parts that I've used in other designs have a higher confidence level that the pad outline will match the chip.I want an oven here at home too.. a little desktop drawer kind like Ian from Dangerous Prototypes has shown. But I already have a hot plate similar to his, and it's ok.. just have to pay attention to it..