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Flex Circuits reflowing

Chris

#24383

Flex Circuits reflowing | 1 May, 2003

Are there any guidelines when building products with flex circuits? Any heat profile guidelines? Wash Guidelines? Handling Guidelines. Is it best to run on the same line?

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Jesus Fragoso

#24387

Flex Circuits reflowing | 1 May, 2003

Just take care about reflow profile with this kind of product, you can get a bad-formation in your PCB (Flexcable or your flexcircuit) however you want to call...

This bad-formations looks like a solderballs (Microscopio)

Regards Jesus

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Chris Lampron

#24396

Flex Circuits reflowing | 2 May, 2003

Hi Chris,

I had worked for a flex circuit house for 10 years. You should be able to follow your solder paste guidelines for times and temps. You may want to consider pre-baking the flex if it is made with Kapton. Kapton is Hygroscopic and will delaminate in the oven if it has absorbed too much moisture from the air. We used to bake at 100 degrees F for a min of 4 hours. This eliminated the delamination issue.

The only other concerns would be the insulating material used. Kapton should be fine, Mylar and Polyester have been used as cover coat insulation. These materials have a significantly lower processing temperature limitation and will require special consideration for temperatures. Please feel free to E-Mail me if you should have any questions.

Good Luck

Chris

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#24398

Flex Circuits reflowing | 2 May, 2003

We have been building with flex circuits for nearly three years now, its an automotive product utilizing lead free paste, I will give you a few pointers.

1. Evaluate your supplier carefully, make sure you have some kind of stretch tolerance on the print before you start production. Flex circuits are not as repeatably manufactured as FR4. 2. Pallet design, use three locating pins or tensioners instead of two, if the circuit gets a corner bent during handling you will still have two locating pins. 3. Use a vacuum pull on you support plate in the screen printer, only pull vacuum on the waste area of the array so you dont bend the flex in a critical area. 4. Number and track the pallets, as time goes by the pallets tend to grow and you will have transfer problems, have a go-no go gage and check the pallets on a regular basis. 5. Make sure the fiducials will read in all of your process equipment, I had to upgrade the camera in my screen printer in order to get repeatability. 6. You might want to consider using a cover plate after paste print, when moving down the line the flex can come off the pallet if fast conveyor speeds are used, air gets underneath the flex and it acts just like a parachute. 7. If you are using gold plated pads you might want to consider storing the opened packages in a nitrogen cabinet. 8. If processing skip mark boards be sure your placement machines can identify them repeatably. 9. Check to make sure your reflow oven can handle the additional temperature, some of the older ovens cannot. 10. Have a log and record lot #'s in case a problem arises you can easily quarantine the suspected lot and detect if the supplier tries to send in another suspect lot. 11. I would keep the product on the same line if you have that luxury. 12. Have your supplier give you the packaging specification and review them that they cannot get damaged during packing, shipment and unpacking.

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Kris

#25125

Flex Circuits reflowing | 10 July, 2003

Hi,

What would you consider significantly different about lead-free on flex circuits than tin-lead of flex circuits ? Have you published your experiances that you encountered while changing over to lead-free solder paste from a tin-lead paste ?

Thanks

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iman

#25183

Flex Circuits reflowing | 16 July, 2003

just out of curiousity, why the special nitrogen cabinet concerns for gold plated flex PCB? common "myth/fact" is gold is an "inert" coating layer that is not going to react with room environment conditions *that much?*. Am learning here, so would appreciate your engineering-view points. Thanks.

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#25186

Flex Circuits reflowing | 16 July, 2003

To answer the first post after my initial response we haven't published any of our lead free data since it is proprietary, sorry. As for the nitrogen cabinet it has been proven that over time the Gold plating can build up a layer of oxides that can cause tombstoning so we have a standard procedure of storing opened packages of flex's in the nitrogen cabinet.

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