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Colored Fluxes: Not Child's-Play

One of the first activities any child enjoys, once they can manipulate things, is using paints and crayons to color pictures. Stay with me: there's a reason for this introduction.

In the last six months, we have had many requests from customers concerning a flip-chip or ball-attach flux with which they are very happy, and which is qualified for their process. The customer simply asks: "By the way, can you make it colored?" The reasoning here seems to be that if a three year old can color a picture, then it should be easy for an Indium Corporation flux chemist to simply just, surely..?

The answer is that what seems simple is, in reality, very complicated, and I will touch on just some of the reasons why.

Engineers seem very surprised when you ask them, in response, why they need the color. Often, the reply comes back, "So I can see it." It is here that one of the problems starts: once you are dealing with a question of human perception (that is, the fallibility of human eyesight, plus the problems inherent in the brain processing the image), there is a large variety of variables which you then have to pin down:

 1/ What color?

 2/ How wide and thick is the deposit?

 3/ What shape is the deposit?

 4/ Which do you want to determine?: the location of the flux / how much flux is present / the shape of the flux deposit / presence or absence of flux / something else?

 5/ What standard will you use to determine 4/?

 6/ What lighting will you be using?

 7/ What optical system (microscope / cameras) will you use?

 8/ How will you benchmark the ability of different operators to see the flux?

As we have found in many instances, a flux color-level that will allow an automated replenishment system to operate may give flip-chip flux deposits that may be almost invisible to the human eye.

I haven't finished! Once this has been determined (and we will not be able to perfectly replicate the customer inspection process), then you have all the effects of adding the coloring chemical to the flux. As any of our formulation chemists will tell you: there is no such thing as a "small" change in a flux. Further questions arise:

 9/ Pre-reflow flux or post-reflow residue?

    - Post-reflow may not be feasible

 10/ Usage affects chemistry available and choice of color:

    - Color agent concentration needs to be optimized so other properties of flux are not affected
    - Experience shows thin films of colored flux are undetectable by eye or vision systems

 11/ Addition of even small quantities of the coloring agent will affect the physical properties to some extent:

    - Rheology: Tack / viscosity / pot-life (usage life)
    - Reflow / wetting / voiding
    - Coloring agent may also affect the electrical properties (SIR/ECM) of a no-clean flux!

12/ Water-insoluble color agent can stain substrates and cause cross-contamination within the reflow oven or final cleaning system.

    - NOTE: The most effective (deeply colored) color agents are not very water soluble.

13/ Color agent must be homogeneously distributed, especially for sub-100micron pitch flip-chip and copper pillar applications.

    - Manufacturing process and QA testing methods need to be developed for each flux and color chemistry

Finally: Yes, we do have several colored fluxes: some red; some blue; some black, and some fluorescent. Does this mean that they will automatically work in your inspection process? Not by a long piece of sidewalk chalk. I hope you now understand why.

Still interested? Contact me.

Cheers!  Andy