Pb-free solder paste in enclosed print heads: as with SnPb, some do well and some do not.

Enclosed print heads were introduced a number of years ago. Within a few months, each of the major screen print equipment suppliers had rolled out their version of an enclosed print head. Today several versions of enclosed print heads are available from the major suppliers.

The primary goal of all enclosed print heads is to minimize the use of solder paste by exposing the solder paste to the atmosphere only during the actual printing process.

Initially, enclosed print heads were thought to be the solution to all printing process issues. They were touted to have a number of benefits:

* Extend the life of solder paste almost indefinitely.

* Minimize paste waste.

* Increase the speed of paste printing by several times the specified print speed of any particular material.

* Provide a higher quality and more consistent print than squeegee blades.

* Minimize operator exposure to paste.

* Enable even the oldest solder pastes to perform as well as the latest products.

To that last point, I remember several paste suppliers lamenting that customers would no longer require their latest products because the enclosed print heads would make their older products perform much better. Many believed the introduction of enclosed print heads would end the use of squeegee blades in all printing applications.

Well, as with almost all new technologies, reliability fell somewhat short of the initial claims and beliefs. Enclosed print heads do provide a number of benefits for many customers and are used with excellent results in many facilities worldwide. If operated correctly, they can reduce waste, minimize operator exposure and in some applications improve printing process performance.

However, in a number of areas enclosed print heads have not lived up to initial claims. For example:

* In general, you cannot print solder paste substantially faster than the supplier's specified speed.

* They will not work with all solder pastes.

* They will not universally improve the print quality of solder paste.

The goal of all enclosed print heads is the same, but the designs are very different (how one enclosed print head works is substantially different from the next). As such, a particular solder paste that works well in one enclosed print head product may or may not work well in another.

Thanks to extensive testing over the past several years, the major printer suppliers now understand how most commonly used SnPb pastes will behave in their products. But what about Pb-free solder pastes? Based on our experience with SnPb solder pastes, each and every Pb-free paste must be tested in the particular enclosed print head to determine how it will perform.

To that end, Speedline has undertaken an extensive study of Pb-free solder paste behavior in our enclosed print head. We have tested approximately 25 different Pb-free pastes. (In response to our requests, all the major solder suppliers submitted at least one and in most cases two or more Pb-free pastes. Testing will be ongoing as Pb-free solder pastes are introduced.) Overall, the results are positive: the majority of Pb-free solder pastes passed our tests. Consistent with our hypothesis, some Pb-free solder pastes performed very well and passed all our testing, while others did not perform well and did not pass.

Our testing involves printing 2,000 boards in a simulated high-volume production mode. The performance of the Pb-free solder pastes is evaluated by visually inspecting the quality of solder paste prints on boards no. 1, 2, 501, 502, 1001, 1002, 1501, 1502, 2001 and 2002. We analyze the metal content of the Pb-free paste by taking samples from the left, center and right sections of the pump head. The Pb-free solder paste samples are then reflowed to calculate the metal content at the start and end of the test.

The primary failure mode of either Pb-free and SnPb paste in an enclosed print head is what we call "compaction." The solder paste "dries out" as flux separates from the metal and dissipates. The paste hardens and will not print. Other failure modes exist, but compaction is most common.

Before implementing any enclosed print head, fully understand how the solder paste(s), whether Pb-free or SnPb, will behave in that particular enclosed print head. Ask the equipment supplier for details on any testing they have conducted on your particular solder paste.

Joe Belmonte is project manager, advanced process development, at Speedline Technologies (speedlinetech.com); [e-mail]. His column appears semimonthly.

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