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BCC Technology --- Placement, Rework, Reflow



BCC Technology --- Placement, Rework, Reflow | 24 March, 2002

I am looking technology for BCC package. The preliminary datasheet from customer show that it had rectangular flat pad on the joint. The smallest pitch is 0.6mm having 96 I/Os.

I am currently putting BGA, uBGA, CSP and CBGA onto PCBs usinf no-clean process. I would like to know the preparation for this BCC package. Need advise on stencil design, SMT placement, reflow profile, rework, etc.

Any inputs will be helpful.

Thanks in advance.

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BCC Technology --- Placement, Rework, Reflow | 25 March, 2002

It maybe a stupid question but what is a BCC package??

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BCC Technology --- Placement, Rework, Reflow | 25 March, 2002

Not dumb at all.

It's a 16 pin, 0.65mm plastic SMT package with pads along the package edges proprietary to Fujitsu. Look here:

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BCC Technology --- Placement, Rework, Reflow | 25 March, 2002

We are currently applying solder bumps to hundreds of BCC's for rework. It is impossible to print to the PWB. It is recommended to apply solder bumps to the BCC's with 63/37 alloy making the rework faster and easier.

Applying the solder bump on the BCC can be tricky because you are dealing with a component that doesn't a sphere so you need to add one.

Having success on the rework depends on the consistency of the application of the solder bump. We have developed a process to apply this solder bump consistently. Once you have a BCC with the solder bump then you just apply paste flux on the solder bumps or on the board and place the BCC and reflow using a rework station.

No clean paste flux is recommended, you can create a jig to apply a consistent layer of paste flux to the solder bumps of the BCC. You can also apply the paste flux directly to the PCA. The problem with applying the paste flux to the PWA is the flux residue.

Water clean paste flux can also be used the only problem is cleaning under BCC's can be difficult.

Hope it helps a bit

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BCC Technology --- Placement, Rework, Reflow | 25 March, 2002

Some of this was copped from Fred.

There are numerous package types that now fall under the rubric of land grid array [LGA]. Land grid devices [ie, Bumped Chip Carrier� [BCC], LGA, Quad Flat-pack No-lead [QFN], MicroLeadFrame�, etc] are essentially BGA devices without balls or solder on the pads. [Dinosaurs: Think PGA w/o pins.] Generally, any device that has bare pads under them that attach to corresponding pads on the board qualifies as a land grid array.

General comments: * Apply a sufficient amount of solder to ensure proper connection to the board, since the component has no solder on the pads. A 4-6 thou thick stencil should do the job. Try to end-up with about 1-2 thou of solder between the pad and the component. * In most cases, the pads are huge [eg, 450 thou^2]. So, this gives room to tweak apertures to obtain a good solder joint without any stencil aspect issues. A LGA with flat pads on the bottom drops to a thou or two of standoff height. * Determine the proper size and shape of the brick that produces a reliable soldered connection through a little experimentation. * Profile the solder paste reflow by placing a thermal couple under the component pad, similar to a BGA. * Make sure that you know where your solder is after reflow. See BCC note below. * Treat LGA like any other plastic SMT IC device (ie, ESD, moisture, handling, etc).

If a board sees any kind of flexing, keeping the component on the board could become an issue.

With such a low standoff [eg, 1-2 thou of solder], under-fill materials take quite a bit of time to flow to the underside of a LGA and most assembly throughput cannot take that kind of time hit. So in real life, LGA are "edge" filled. But, this lack of standoff usually means that the residue from no-clean fluxes needs to be selected, so not to work against the under-fill.

Watch out for LGA with a thermal "slug" under them, like a QFN. The slug really limits the options if overprinting. If the opening for the thermal slug is too great when using blades (not a pump), the blades can "scoop" paste from the center of the deposit for the slug. Since the pads do not get scooped, the component can end-up sitting higher than the paste deposits under the heat sink. Support the board under the component if the paste opening is larger than about 100-125 thou^2. Print the heat sinks as a grid apertures that are the size of pads.

This leads to the obvious extension. Excessive pick and place pressure can schmush the paste between pads and cause bridging. This often shows-up with BCC. BCC have rectangular pads that have a radius to them. The pads are like little canoes that are only plated plastic and have no solder on them from the supplier. Too much placement force can squish the solder paste out to the sides of the print, and, if you are not careful, 0.4mm spaced pads short together very easily, becoming 0mm spaced pads. ;-)

Also, the location of microvias in the LGA pads can cause opportunities for rework. If the out-gassing of microvias is violent enough, solder can bridge between pads when the microvias on adjacent pads are positioned in the same location on pads. Staggering of the location of microvias reduces, but does not eliminate, the problem.

As mentioned by another poster on this thread, the repair process is the BIG drawback to the LGA family of devices. Removing the device and dressing the pads on the board are the easy parts. Attaching the component to the board is the tough part, since the LGA has no solder, you need to: * Print solder paste on the bottom of the component, similar to a BGA. This is not a lot of fun on the small pitch devices. OR * Have the LGA supplier provide parts with solder already applied. Not a $$$$ saver. OR * Talk someone, like Winslow, into developing a �SolderQuik� solution, like for BGA.

Some LGA have ENIG finish on them. So, soldering them to an ENIG finished board can cause problems with gold embrittlement to kill your solder connections. Search the fine SMTnet Archives for background on acceptable gold levels.

One kinda neat thing about these packages is that in a pinch you can normally eliminate solder bridging without removing the component. Just squirt some tack flux that is compatible with your normal solder paste all around the edges of the LGA. Run the board through your reflow oven again and 90% of the time the solder bridges disappear.

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BCC Technology --- Placement, Rework, Reflow | 4 April, 2002


hope this helps :

1) BCC (bump chip carrier) production for us, was with a 5mils Stencil. The outer perimeter smallish pads are not much of a problems. For the central large pad, we split the paste print (corresponds to the Stencil apperture opening) into four smaller sized quadrants. this was done to allow better Hot Air reflow movement(convection) under the BCC package. and allows the BCC package to "settle/lower" itself more uniform during the transistion stage of squishy paste into metallic solder joint. (imagine BGA self-alignment? except its at the Z-axis)

2) Linked to item 1), we also finetune the Reflow Profile to avoid "boiling" effect of the paste, that saw the BCC package flying off the board (ie. missing BCC). We used OMG Microbond WS-300 paste. *opps.... and no, am not a sales bloke* Things to look out for are insufficient/excessive reflow time and watch your peak temperature. (ask your paste supplier for help here). If you use WS-300, ask them for technical advice, they are helpful guys to have on your side...

3) Rework is helluva pain for BCC, as back in those days, we did not have easy access to X-ray. So we were extremely motivated to reduce our 1st pass yield (was around 0.2% rejects on BCC by the time we finished our SPC, reduced from 32% initial defect rate). What to look out for, are the basics such as :

a) good paste print (else wash it immediate, detect by 100% inprocess inspection before PCB goes into the mounter),

b)good component placement (first 3 panels is ok for misalignments, after that the Technical guy grows two inches shorter from QC pressure to finetune the placement. We also had 100% inprocess inspection before semi-PCBA goes into the Reflow Oven)

c) chart down the various oven setting and parameters used for each individual model that uses the BCC package. Optimize reflow output quality by individual models, and keep the QC affirmation specs within the paste supplier tech-specs. If you been running BGA, you may consider using the same Oven settings for the BCC, but with lower peak temperture. For us peak was at 215 +/-1degC. (based on the profile graph read-out).

d) we did batch sampling x-ray.

e) we did shear test (destructive test) for every 1st panel output and every 50 panels thereafter, followed by Engineer inspection of the shear test sample. (Guess these days you now have X-ray to help, eh?).

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Tom Gervascio


BCC Technology --- Placement, Rework, Reflow | 8 August, 2002

I'm interested in sharing any information on bumping of LAnd Grid Array (i.e. QFN) packages. due to PCB contraints, it's not possible to stencil paste onto sites for QFNs. That lead to a lot of experimentation with bumping of 6 mm QFN packages.In summary,

1. Tried using fixture to sodler 13 mil spheres onto QFN parts. Had problems with spheres hanging up in templates. Yields were about 15 percent.

2. Tried using Solderquik. Got about 40 percent yield- major problem is not all spheres soldered to part- major problems is fixture X-Y tolerances (difficult to ensure that tangential point of sphere contacts the QFN pad) Solderquik is great with .75 mm pitch or larger PBGAs but not a believer yet with 0.5mm QFN (especially with peripheral pads).

3. Dispense 600 cps no-clean paste onto PCB pads. THis is what we are going now- still have some problems controlling solder paste volume based upon solderpaste rheology changes during repeated dispensing operations. But it's relatively easy for an experienced person.

I would like to find a way of bumping parts in mass...


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BCC Technology --- Placement, Rework, Reflow | 8 August, 2002

On our last thing like that [5X5, 32, er 28?], we printed bumps on the QFN, but you can't do that, so there's no sense in tormenting you.

Ideas that come to mind are: * Search around SMTnet for a paper by Chrys Shea on controlling dispense shots as cartridge empties. * Were you using a tacky paste when you were trying to get the solder balls to stay on the QFN pads? Or was it that your template openings just too tight of tolerance for the solder balls? erwat?

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BCC Technology --- Placement, Rework, Reflow | 9 August, 2002

Tom, I developed a process for applying solder bumps on BCC32 and BCC48 successfully on my previous employer. The quantities were in the thousands a week, the following process is what I was using:

Insert BCC's on process carrier (carrier holds 500 BCC's). Apply a 2mil foil on to the carrier to hold down the BCC's. Screen print BCC's using 5mil stencil. Inspect print using LSM for solder paste height and consistency. Remove 2mil foil and reflow.

Note: The carrier and stencil required several hours of design and experiments. Hope It does make sense to you.

If need additional info please let me know.

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Reflow Oven