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LGA36 6.5 x 3.5 mm

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LGA36 6.5 x 3.5 mm | 23 March, 2007

Hi, We are facing a complete new package type for us in our prototype production. RoHS LGA 36 Land Grid Array with 36 so called pads instead of bumps underneath. It is totatlly flat. This component is only 6.5 x 3.5 x 1.6 mm and we recieved them in a matrix tray. Pad dia on the comp. is 0.35mm and the pitch 0.6mm Ni/Au We have just ordered the stencil with 8% reduction of the pad size off the pcb and 0.127mm thicknes. I can already see some problems in front of me; the operators will have a difficulty to do their visual inspection to determine if the placement is ok befofore send the boards into the oven. Remember that we are only going to produce 4 prototype pcba:s at the first run (but this will increase to 284 pcba:s at the second run), before the production. I have only a couple of pcb:s and comps. and a very short time to get the process in order before that. We will also do a x-ray on these but i am not sure how well this x-ray will work except for potential solder bridges... Anybody of you guys that have some experience of this package type and if so, maybe you could share a little bit info? /Sincererly

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LGA36 6.5 x 3.5 mm | 24 March, 2007

Hi Mika,

We came across some difficulties with this type of device a couple of years back . After the proto run we needed to touch up the outside fillets to get stability and product to pass test. ( even though the manf. stated that there is no fillet required or not possible)

The way we got around this was to offset the stencil aperture by 3thou as in overprint on the outer lead pads & reduce center volume (thermal pad) by 40-50% by using a dot matrix aperture array.

We have used this as standard on all products that have these little buggers on them and no issues seen since.

The component has self centering ability due to characteristics so if your machine places it within reason the reflow should correct any minor discrepancies.

It would be worthwhile searching the archives.

Good luck,


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LGA36 6.5 x 3.5 mm | 24 March, 2007

Thanks aj, Ohh I wish that there was a way to post a picture in this forum or a possibility to attach the datasheet...

Which BTW does not have any recommendation of "how to". This package does not have the "center thermal pad" like a QFN or MLF package. In essential it looks like a QFN but it has no surrounding pads. It just looks like a uBGA with pads instead of bumps or balls.

I forgot to mention that the device has 4 outer oval dummy pads (~1.15mm away from each corner) and the size of these is 0.7 x 0.35mm. The PCB has oval solder pads for these 4 oval outer dummy pads as well. I guess that will help a littlte bit to self center. But I'm still not sure how to do the process properly. So totally: LGA 36 + 4 outer oval dummy pads which I don't understand the need for, unless it's for self centering ability. BTW, I have the package drawing, but I don't know where to put it so you can see it on the internet? /Thanks Ps. I am sorry for my poor english & grammar. Ds.

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LGA36 6.5 x 3.5 mm | 24 March, 2007



LGA36 6.5 x 3.5 mm | 24 March, 2007

I was referencing a qfn type package - apologies for any confusions.

I have no experience with the package you are refering too.

I would be interested to see how you get on.

I will have a look at the datasheet if I can find it.



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LGA36 6.5 x 3.5 mm | 24 March, 2007

Aj, Many Thanks and I will of course share our experience about this. /Mika

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LGA36 6.5 x 3.5 mm | 25 March, 2007

hopefully this help you.

also we always look on X ray for some misplacement; you can see like a dark circle and another lighter circle with an offset between both circles (meaning darker is solder and lighter is pad) so you can notice when component is displaced.

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LGA36 6.5 x 3.5 mm | 30 March, 2007

We have now ordered a another stencil with square apetures with 20% overprint just because it is so difficult to get the solder paste to stay on the pcb pads without to get it stuck within the apertures. We have set the print speed down, the separation speed down etc. etc. We are just going to try this /Sincererly

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LGA36 6.5 x 3.5 mm | 31 March, 2007

Are you observing stencil design rules with regard to aspect and area ratios?

For more: * IPC-7525 - Stencil Design Guidelines * A previous discussion * Other postings on stencil ratios in the fine SMTnet Archives can be found with SEARCH

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LGA36 6.5 x 3.5 mm | 1 April, 2007

Thanks DaveF, I think the IPC-7525 - Stencil Design Guidelines (which is overall a pretty good guideline) does not perfectly apply in this case. I would rather go for a little bit over print, as for a CCGA; unless I misinterpreted the info. I really don't know. Yet...

And also, we do have the opportunity before the second prototype run to give the design/layouters feedback on the PCB foot print, if we feel that it could be somewhat optimized. Right now the foot print on pcb is equal with the package pad layout. Comments are very much appreciated. /Sincererly

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LGA36 6.5 x 3.5 mm | 5 April, 2007

Thanks all, We have now done the first protype run with the new stencil square 0.35mm apertures and my first impression is that it works, as far as no bridging on any pad on the component > to pad on the PCB. The pick & placement as I mentioned before is OK. Now our X-Ray machine cannot see if the joint is firm whitin the PCB-pad and the LGA-pad. So I still don't know if this "gonna work" in a long term...The customers of ours doesn't want to pay for the extra "money" that I want to spend in investigation - believe that!! Our extra costs is at our site. PCB vendor says that they could not give us a proper PCB-land pattern, because it is to narrow between the "leads" and the proper LGA bigger pads. What should I tell the designers/layoters who make it so difficult for us? I whish there was a way to show You the X-ray pictures and printouts that I have. I'll guess I just have to show them the difficulties from a producton point of view...

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LGA36 6.5 x 3.5 mm | 6 April, 2007

Mika: What are the size and shape of the pads on: * Board * Interposer of component

Are either of these solder mask defined?

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LGA36 6.5 x 3.5 mm | 6 April, 2007

Thanks DaveF, Ohh I still whish there was a way to share some pictures...

* Pads on pcb are very very strange: It varies from a non uniform pattern but after we measure these I would say ~0.22 - 0.24mm dia. Haha, We have never discovered such a strange way the land pattern is done: After looking in the microscope we find out this: Tha PCB pads are actullay "a little bit" higher(!) than the solder mask! Not much but the surrounding soldermask ring look almost like a well. The leads is between the pads. PCB: 35 x 50 mm 6 layer 1.6 mm thichness on a panel with 24 boards. We will have a meeting with the customer (designer) this week, but I would like to give them a constructive feedback. I do understand that there is a difficulty to do another layout on these boards.

* Pads on the LGA itself is very consistent: This component is 6.5 x 3.5 x 1.6 mm and we recieved them in a matrix tray. Pad dia on the comp. is 0.35mm round and the pitch 0.6mm Ni/Au RoHS production of course... /Sorry for my poor english & grammar

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LGA36 6.5 x 3.5 mm | 6 April, 2007

Ah, I almost forget (forgot?) to mention that according to the Gerber files (which is the info we send to the PCB-house) the PCB land pattern for this LGA-36 0.35mm rounded pads and the boardhouse cannot achieve this. I understand them; 0.35mm pads, 0.2mm lead (threads) dia, 35 x 50 mm board and still get the signals to work... That is the reason for the "crazy" landpattern, as they tell us.

/I am not so good in English, but I hope You get the picture. Sincerlerly

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LGA36 6.5 x 3.5 mm | 6 April, 2007

Mika: Based on what we hear you saying, we're not so sure we have a problem with the board layout [1:1 between board and interposer pads, non-solder mask defines pads], but with the flawed execution of the layout. If your board fabricator decides to build a board that looks different what you expected [as described by your Gerbers], the fabricator should inform you and get your buy-off prior to delivering such a surprise to you.

Internet picture [pix] storage/posting * * * *

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LGA36 6.5 x 3.5 mm | 12 April, 2007

Many Thanks DaveF. Our Chinese PCB vendor (for this particular customer of ours) do have a huge problem to do a proper PCB production quality. Not only do they Not tell us if there is something wrong (hard to produce or whatever) with our Customer PCB gerber-files; they actually do adjustmenst on these board files on their own, whithout asking us!!!! They will never be used again! Our next (second) protype run will take place within the next week. With the very same PCB:s from the same batch as before. Nothing we can do about this.

After our meeting with the customer; they are happy to do some adjustments on the cunductor (leads/threads) width:s and shapes between the pads, that will reflect directly on the soldering yield and also get the functionality to improve (we think). Since they are having problem to get the proper isolation distance between pad > conductor on the narrowest place ~0.07mm we suggest them to go for a PCB solder pad down to 0.30mm. The Conductor width is: 0.2mm The current problem they are having now is that there will be a problem with 14V to narrow the conductor to the narrowest pad, which in turn could be cunducted. They didn't realized this before until we showed them in our measurement microscope with pictures and numbers.

The other way around is this; to suggest them to go for another layer and use vias between the pads. We did some calculation about this scenario and the conclusion; In such a case they will need micro-vias and that is Not an option because of the extra price that they are not willing to pay for the PCB:s.

The calculaion is fairly easy with a littlle bit of Trigonometri: Footprint Pad dia 0.35mm, 0.6mm pitch in a matrix, put a via hole in between the pads of let say 0.2mm hole dia. + "the annual ring" according to IPC rules; the edge from this to the pad edge will Not sum up to be able to get a proper isolatition distance!

Haha, and I was thinking that we in the production had the biggest problem...

There will now be a third and a fourth prototype run later on, with the new PCB layout according to our suggestions to go down to 0.30 mm pad dia. and also to do some earlier smaller "take down conductor (leads/threads) width:s around the curves from 0.20 to 0.12mm and then up again to 0.20mm on the straights" without to change our present stencil design: 0.13mm thichness, square aperture 0.37mm (the 0.35mm apertures I mentioned earlier in this this thread is wrong). Not to forget - a new pcb manufactor! Our uLGA process is far from established...

/Yours Sincerely

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LGA36 6.5 x 3.5 mm | 15 April, 2007

Mika, I think you will find all board houses "clean up" their customer's files to suit their shop. Depending on the severity of the change; I think they take the attitude that they will not inform you to avoid an argument going backwards and forwards between the fabricator and the design engineer. That they are doing adjustments to help you is a good sign. Maybe you should develope a protocol with your fab about changes and how you pass this resoning on to your customer to avoid the same problems in the future.

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LGA36 6.5 x 3.5 mm | 21 April, 2007

Thanks Darby, I got the point of what you are saying. Never the less, I still don't like when a pcb manufactor change the leads/threads on the pcb whitout mention this to us. In such a critical case! There should always be a close relationship between; designers, layoters, board house and pcba manufactories, unfortunately this is rare.

Once in i while, we do a PMA (Production Method Analysis) in an early stage of our customer design project. But in this case not. Our customer decided to scrap the rest the boards and we are waiting for a new delivery from another board house, with the new suggested layout. Our uLGA process is still going on... Ps. I'm sorry for my poor english & grammar Ds.

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