|Ask the Experts|
September 10, 2006
Reflow soldering lead free components creates tombstones
We are in transition to lead free soldering. We use "lead-free" components in "leaded" assembly process. But there are many complaints of tombstone problems in production. Please suggest how to control tombstone issue of Pb Free components, which reflow soldered with leaded solder paste. This is reporting mainly 0603 components.
Siraj P P
|Expert Panel Responses|
There are several factors that impact the frequency of component tombstoning. Factors that impact tombstoning include: Printed circuit board component pad design Solder paste printing accuracy Solder paste volume consistency from one of the component's pads to the other, Component placement accuracy Reflow profile precision, etc. Using lead free components with tin lead solder paste certainly is not an ideal situation. You now have the component lead finish material, most likely 100% tin, and the tin lead solder paste reflowing at different temperatures. My suggestion is to evaluate all the factors that influence tombstoning to insure they are well understood and optimized. If you already have lead free components, unless there is a compelling reason not to do so, I would move to the conversion to lead free solder paste as soon as possible. We (Speedline Technologies) has a presentation titled "Component Tombstoning and Mid Chip Solder Balls Causes and Prevention" that I will be happy to send you if you contact me.
Tomb stoning is a problem when going to lead free. Reflow profiles and component finishes will have the biggest impact on this. A profile with a high temperature soak may solve the issue I would recommend a soak at 200C prior to ramping to spike. There are some solder pastes available with different powder distributions and alloy types that will also offer an improvement. One alloy that is known to eliminate tomb stoning is Sn100C. I would first try to profile them out using the high soak. Additionally try to identify if a specific component manufacturer has a high rate than another.
Deck Street Consultants
Tombstones are usually the result of confluence of multiple factors. Pad design, print location, print consistency, part placement and profile, to name but a few. Check the pad sizes and ensure they are not too big, check the paste deposits for location and consistency volume differences can contribute to the problem. Also check the part placement poor alignment is a contributing factor. Look to see if the board design is one end of the part attached to a heat sink that could cause differential heating in the reflow oven. Profile often is often difficult to change and is usually a marginal effect but a slow transition through the melt point is desirable. Finally there are anti-tombstone alloys with "phased reflow" which open the process window and can have a dramatic effect on the incidence of tombstones.
Senior Applications Chemist
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