The problem is that during the same assembly, after wave soldering, some pcb’s look not clean and other pcb’s look good.
We use lead free SAC305 solder and flammable flux.
We think our process is stable, pcb’s temperature is ok after pre-heating (between 105-115ºC), wave contact about 3-3.5seconds and we always try to use the minimum flux quantity.
We suspect that the problem could be from the pcb’s solder mask, but pcb’s suppliers always said that their solder mask is “perfect”… despite in some extreme cases we have seen different color in solder mask, light green and dark green...
Someone tell us that is important the compatibility between the pcb’s solder mask and the used flux.
How could we know if the pcb’s solder mask and flux are compatible?
How could we know if pcb's solder mask is ok before we start the assembly of them?
Finally, help for those searching for compatibility A new standard and guideline will make it much simpler for users to determine whether they’ve got the right chemistry to ensure compatibility.
April 23, 2012
By Terry Costlow, IPC online editor
As design and manufacturing processes get more complex, minor issues can cause big problems. For many circuit board manufacturers, material compatibility can be an issue that causes intermittent failures.
“Making sure that your manufacturing materials and manufacturing processes are compatible with each other is a critical consideration in the development and characterization of a manufacturing process,” said Doug Pauls, one of the leaders in the 5-32b SIR and Electrochemical Migration Task Group that developed IPC-9202, Material and Process Characterization/Qualification Test Protocol for Assessing Electrochemical Performance.
Testing for material compatibility has been touched on in some standards, primarily J-STD-001. Previous versions of J-STD-001 had an appendix that provided one possible approach, but many people found it lacking when it came time to translate the lab-based tests to actual products. “There have been material qualification standards that provide performance criterion for materials like flux and conformal coatings, but there’s often a big gap between those qualification tests and how the materials work in a given design,” Pauls said.
This problem has been addressed with the completion of IPC-9202. It uses phrases like “you shall” so that it’s clear what users need to do to comply with the specification.
The issue of how to test for or demonstrate materials and process compatibility is a complex one. Consequently, the task group also is working on a how-to guideline, IPC-9203, User Guide for the IPC-B-52 Process Qualification Test Vehicle, which is now out for ballot. When published, it will serve as a user guideline for the IPC-B-52 SIR test board and how to use it.
The IPC-B-52 is the test vehicle for IPC-9203. It has been available for use for the last four to five years and is currently undergoing some minor redesign work, which should be completed this summer. The B-52 board is filled with components common to most modern electronics, which differentiates it from flat coupons used for most materials qualification tests.
Together, IPC-9202 and IPC-9203 will give users a wealth of information so they can ensure that seemingly minor changes in materials and processes don’t end up causing problems in the field. Some materials may work well under one set of circumstances but cause problems when manufacturing techniques are changed.
“You can’t separate the materials from the manufacturing processes. The industry needs information on how materials interact on a standard substrate and how they interact with each other,” said Pauls, who’s also chairman of the IPC Technical Activities Executive Committee and principal materials and process engineer at Rockwell Collins.
While this test approach can be more costly than some methods, the standard can help users rein in costs by avoiding process problems. That’s especially true for electronic manufacturing service providers and companies that have a broad range of products that use many different materials.
“For some OEMs, there are a lot of potential combinations and testing all of them would be ruinously expensive,” Pauls said. “Now OEMs can demonstrate they’ve tested a surface finish and a solder mask, for example, and found that they’re compatible. A contract manufacturer can say that they’ll manufacture using materials and processes they know to be compatible.” The standard will be the first time many users can investigate sporadic failures can be caused by incompatible materials.
“The concept of whether materials A and B are compatible is not well defined,” Pauls said. People have questions about which tests to do, whether they need to do multiple combinations of tests, whether they can set up a test matrix and how to process their test data, among other questions, he explained.
IPC-9203 will help them resolve issues like these. Pauls noted that users will need to look at the combinations as technology changes. For example, higher power requirements can mean thicker trace lines, which can prompt changes in materials and processes.
Those who take the time to run tests to ensure compatibility are likely to find that they gain a big payoff. Incompatibility issues can cause problems that only show up on occasion, making them difficult to locate and understand.
“A lot of people have been using these tests to track down pesky intermittent issues,” Pauls said.