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?
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.
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.
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.
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?
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.)
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?