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SMT electronics assembly manufacturing forum.


Solder Paste Flux %

#34833

Soler Paste Flux % | 9 June, 2005

I'm using Kester R520A Lead free solder paste. The flux % stated on the Certificate of Analysis(COA) when the product delivered to us is 10 %. The tolerance is 1%. But when i send the jar to an external lab to determine the flux % it is more than 11%, whereby it should not be more than 10.5%. According to Kester, the figure stated in the COA is the amount mix with the solder powder. Moreover, there will be chemical reaction between the activators in the flux & Tin (Sn) which will results in increase amount of flux. The increase is called a correction factor which we need to add up to the number in COA. The correction factor range from 0.5 to 0.8%. Does anyone have any idea about this? By the way, im trying to reduced the flux% in order to reduced the solder splattering. Will this work?

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RDR

#34846

Soler Paste Flux % | 9 June, 2005

I think I would work on the reflow profile or your paste handling procedures to prevent the splattering. I might even change paste suppliers or formulations.

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#34897

Solder Paste Flux % | 11 June, 2005

Russ, Thanks.By the way, i already look into the reflow profile & paste storage & thowing requirement. Even i cut down the stencil life to 4 hr to avoid any moisture absorbtion. If i were to change the paste formulation, what would you propose ? But must be water soluble & leadfree with type 3 powder. We reflow the paste in nitrogen atmosphere 500ppm max.Our end product is Flip Chip on Flex.

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#34899

Solder Paste Flux % | 12 June, 2005

Paramjeet Singh Gill

Flip chip with aqueous flux, that's interesting. * What is the stand-off from the board is your flip chip? * What machinery and materials do you use clean your boards? * How do determine the cleanliness of the board under the flip chip?

On to your issue with solder balls: * Moisture in the air condenses on cool solder paste. What is the temperature of the paste that you use? * OA fluxes take on moisture. You mentioned limiting stencil life to 4 hours. What is temperature and humidity in your plant? * Solder balling is often related to paste volume versus solder pad size. What are the sizes of your stencil and the pads on the board.

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#34905

Solder Paste Flux % | 13 June, 2005

Devef, The stand off height is 75micron. We only clean the product after underfill process. Dispense epoxy under the FC & cure it before aqueous wash. Samples are sent for LPC (liquid particle count) Test to ensure cleaniness.

By the way, our plant RH is 35 to 65 & temp about 23Deg C. The paste temperature on the stencil is about 25Deg C.

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RDR

#34913

Solder Paste Flux % | 13 June, 2005

I agree with Dave on the Water soluble process for this part. It is interesting that I have yet to find a Water Soluble for lead free that works. I have experienced a lot of problems with the flux on these pastes, wetting, solder balls, etc... kind of sounds like your problem. I under these circumstances cannot recommend any paste to you because of the water soluble requirement. Why the Water soluble requirement?

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#34916

Solder Paste Flux % | 13 June, 2005

I've had a lot of experience with solder spatter. It's usually related to moisture (humidity) uptake by the paste, or outgassing from plating: particularly Au/Ni. Solder spatter from the plating will change from board-vendor to board-vendor.

I should trust your vendor's numbers - there is no reliable method of measuring the %by weight of flux after the fact. Note that lead-free solder pastes usually have a higher %w/w flux content than Sn63, but this is because the lead-free alloys are lower in density. _Volumetrically_ , the flux amounts in lead-free and Sn63 pastes are identical, for the same method of application (stencil print/ screen print etc.)

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#34936

Solder Paste Flux % | 13 June, 2005

Paramjeet Singh Gill

So, you underfil before cleaning. Interesting. * What is it about your underfill that prevents the OA flux from attacking the FC solder balls? * Who is the underfill fabricator and product?

Getting back to your solder balling issue: * An earlier poster commented on the impact of the work area environment on solder ball product. You seem to have that in control. * If control of the shop enviromental is the primary driver, then solder paste formulation is a close second. [Your paste supplier should be able make suggestions. If he doesn't, his competitors certainly will.] * Poorly developed reflow recipe is another cause. How well does your recipe match with your supplier's recommendations? * Another driver is the amount of solder paste. What are the sizes of your stencil and the pads on the board?

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