Varistors that leak, political football anyone?

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The following are questions my boss has ask me to send out ... - Jul 12, 2000 by Charlie

There a few questions that I have for you. 1. What type of... - Jul 13, 2000 by

Donnie In response to your questions; 1. Flux is Multi-core... - Jul 13, 2000 by

Charlie

#3548

Varistors that leak, political football anyone? | 12 July, 2000

The following are questions my boss has ask me to send out to you folks. This is done in an effort to see "What does the rest of the world do?" [if such a world could possibly exists, in the first place]. If you would select any or all questions to respond to, we here on the other side of reality would be appreciative.

I have been tasked to do a little investigation on varistors on the bottom side of PC boards that pass through wave solder processes. We'r experiencing current leakage, and we believe it is due to the porosity of the component absorbing flux from the wave solder process ( as found by CSL). Our current corrective action is to use glue dams (a series of connected dots that would make it difficult for an electrical path to occur between the ends of the components) and a covering of latex mask to prevent the flux from reaching the component. Obviously this process is labor intensive and increases the manufacturing costs.

Are surface mount non-epoxy coated varistors placed on the bottom side of PC boards?

Are surface mount non-epoxy coated varistors placed on the bottom side of mixed technology PC boards that will pass through a wave solder process and be exposed to flux?

Do special processes exist to protect bottom side non-epoxy coated varistors from flux?

Does the amount of flux placed on a board have any impact on board operation / quality?

Do processes exist to measure the amount of flux placed on the bottom side of the board?

How much flux should be placed on the bottom side of boards? Are cleaning processes required when non-epoxy coated varistors are used on the bottom side of boards?

Are ultrasonic cleaning processes recommended to address bottom side non-epoxy coated varistors leakage issues?

Do you have any insight to non-epoxy coated varistor type manufacturability issues?

Do you recommend pursuing epoxy coated varistors for bottom side applications that would be exposed to flux?

Any feedback, or direction to a source would appreciated. thanks Ray.

Todd F. Orsini Manager, New Products Engineering

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Donnie

#3549

Re: Varistors that leak, political football anyone? | 13 July, 2000

There a few questions that I have for you. 1. What type of flux are you using? 2. What temps are your preheaters set at? 3. Can you possibly place them on after the wave? The reason I'm asking is that there may be some adjustments to the wave to burn the flux better. Yes there is a tool that can give you a visual on flux coverage. It's called a Fluxometer. It's made by ECD. what it is is a pallet that has a plate with many many small holes to simulate a pcb and there are different types of paper that set on top of the plate and depending on the amout of flux that is applied, the paper will turn color and you should be able to see the hole pattern. We have had experiences in the past that if too much flux is applied, it can cause failures at ICT or functional test. We had to wash the board with alcohol to get the excess off. As for how much flux to apply goes, that all depends on your wave process also. Are you using IR heaters or forced convection? Like I said earlier, you need to see if you're applying too much or not enough. Are you seeing any residue, and if so how much? We don't have any bottomside varistors at this time so I can't really say much on that issue. If I can be of any other assistance, please feel free to cantact me through here or via email.

Donnie

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Todd

#3550

Re: Varistors that leak, political football anyone? | 13 July, 2000

Donnie In response to your questions; 1. Flux is Multi-core X32-10M 2. Preheaters temperatures can range based on board density. the goal is to get the top side to 100C before wave. 3. Components can't be placed after wave solder, they are surface mount and this would not be cost / quality effective 4. Forced convection heaters are used 5. We are seeing some residue (weak organic acids) in the area of 229 micrograms per square in. We had been as high as 604, and it is our understanding that a level of 400 is acceptable.

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