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Questions and discussions about the hardware of the IPC@CHIP® products SC123, SC143, SC23 and SC24.

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Postby Branko Petljanski » 31.03.2007, 19:18

Hi all,

we have designed a board with 600+ pin Xilinx and SC143 on it. We have a problem with the unreliable mounting of SC143. We are getting the cold solder joints, and we are able to pinpoint that to SC143.

As far as we know, our assembly house is using the reflow profile published the Hardware Manual V1.03 (pg 59) , specifically the lead reflow profile. Both Xilinx and SC143 are not Pb-free.

Does anyone can point to us any additional information beside what was published in the hardware manual.

Thanks

Branko

Postby Muenchow » 01.04.2007, 10:13

Hello Branko,

can you tell, how the supposed "cold solder joints" respectively the mounting of SC143 looks like? With a correct processed BGA soldering, the device should been "sunken" due to the spherical balls having deformed to low cylinders. When the device mounting looks like this, it's very unlikely to have balls that aren't soldered correct, except for the possibilty of using unsuitable solder paste (e. g. lead free solder paste with leaded balls or vice-versa) or a PCB with oxydized surface.

In contrast, if the balls have stayed rather sperical respectively the device to board clearance after reflow soldering is too high, the necessary solder temperature apparently hasn't been reached. A special problem with SC143 is the low thermal conductance of the partially air-filled cap. Thus not only the temperature profile matters, also the utilized heating method. Particularly infrared heating mainly from topside isn't recommended to my opinion.

I assume, that you were able to identify open circuits at SC143 pins. Probably the best way to do this would be JTAG boundary scan. If the BGA soldering looks correct in visula inspection, see above, there's also a possibility of SC143 internal open circuits or PCB defects (they may have appeared during reflow process of an electrical tested board).

Regards,
Dr. Frank v. Münchow-Pohl
Beratender Ingenieur

Postby Branko Petljanski » 07.04.2007, 05:41

Thanks Mr Muenchow,

I was informed by the assembly house that they used a 'Micro-BGA' oven, which uses hot air from the top and bottom. They used normal solder paste, not lead free. They remounted the SC143 and it apparently it works fine now.

I have attached a picture of the chip, from which you observe how chip is mounted.

Can you please, if possible, comment on it.

Regards

Branko
Attachments
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Postby Muenchow » 07.04.2007, 12:47

Hello,

I wonder if the picture has been taken prior or after SC143 remounting? It seems something unusually to me in two respects: The mounting distance seems rather high, compared to BECK boards or our own circuits, although it's difficult to see the exact distance from the picture because of surface reflections. And there may be a phase boundary between ball material and solder paste although the cylindrical shape of the balls suggests they had been completely molten. If there is actually a phase boundary, as it would develop with combining leaded and lead-free alloys in BGA mounting, it could be the origin of cracks. Or is the special finish a result of remounting?

Another explanation for a higher mounting distance could be, that the balls in the package center are not completely molten, preventing the device from sinking down respectively been sucked down by capillary forces.

It also seems to me, that the pads are smaller than the BECK suggested 0,7 mm, which could also be a reason why the mounting distance is higher than usual. On first look, I would expect correct interconnections from the shown picture anyway.

BECK experts also should comment!

Regards
Dr. Frank v. Münchow-Pohl
Beratender Ingenieur

Postby Branko Petljanski » 07.04.2007, 19:02

Thanks again Mr. Muenchow,

That picture is of the newly mounted chip, which apparently works fine. Unfortunately I didn't take picture of the previously mounted chip which had the problem.

I went back to the PCB layout of board to confirm what is the size of a pad and I found out that diameter is 0.55mm. Good catch.

Once when we correct that we will know more.

Regards

Branko

Postby Muenchow » 08.04.2007, 01:06

Hello,

I reviewed different BGA pad suggestions and found that 0,55 mm isn't uncommon for 1,27 mm BGA (with 0,75 mm ball diameter), particularly for a "NMSD" (non soldermask defined) pad. NMSD means the soldermask opening is larger than the pad, thus the solder can wet also the pads edge, enclosing it from three sides. Smaller pads may also simplify pcb routing, depending on the used technology. I used 0,7 mm (NMSD) with good results, however.

I still think that a complete reflow process is most important and uncomplete reflow or "cold solder joints" most likely caused the unconnects with your boards. The new version shown in picture certainly hasn't been soldererd too hot, but the processing may be appropriate considering the devices thermal sensitivity.

Best regards
Dr. Frank v. Münchow-Pohl
Beratender Ingenieur

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