How proper investigative work can alleviate misguided print process adjustments.

Printing is arguably one of the most sensitive processes within the entire PCB assembly operation. Not surprisingly, stencil printing’s multi-input interdependency and sensitivity have become more pronounced as miniaturization has taken hold. Even slight variations can cause process shifts, a reality our team was reminded of while conducting recent internal testing.

Our engineers set up a test with a really long board run to evaluate time to bridge, a fairly standard analysis used to understand how many PCBs can be printed for a particular product until solder paste bridging begins to appear. The evaluation, which was performed using a relatively complex ASM test board, was proceeding beautifully until we noticed a sudden shift in the output. The measurable Sigma shift went from a process running at 4 Sigma (1.33 Cpk) to 3 Sigma (1.0 Cpk). The engineer running the evaluation was looking at the process window and robustness, beginning at a 10,000 ft. view with a box plot, which gives reasonable stability insight across the entire run. When a more granular examination of the data was conducted, the data spike appeared on three boards in the batch, with one PCB being more extreme.

Build-in cleaning time during chemistry changeovers and after extended line shutdowns.

Much time and planning are invested in the choice of the ideal conformal coating material and process to adequately protect printed circuit boards. This often includes multiple qualification trials. There is also sometimes long and detailed testing in areas such as electrical performance, flame resistance, and thermal or mechanical cycling. Unfortunately, the qualification and testing process for conformal coatings is simply a snapshot of the process at the start. To maintain consistency, an often-overlooked activity remains: regular cleaning and flushing of the selective conformal coating equipment.

In general, the following comments and guidelines are designed for a discussion involving typical modern selective coating equipment (FIGURE 1). However, nearly all the principles are applicable to manual spraying operations as well.

Flux becomes increasingly tenacious the longer it sits on the board.

This month our topic is not so much a defect as something to consider when running environmental tests before any destructive analysis on solder joints. The through-hole joints shown in FIGURES 1 and 2 were soldered with a high-temperature alloy as part of our trials on robotic laser and single point soldering. The amount of flux in high-temperature cored wire tends to be higher, hence more residues after soldering. If sample boards will be exposed to high-temperature storage, in this case 200°C for 1,000 hr., or temperature cycling, clean the residues first. It is much more difficult to clean after this level of aging, and mounting samples in epoxy for microsections is much more difficult.

Many “topical” conferences are not just good at answering questions, but they also open one’s eyes to questions that have yet to be resolved.

The ITI/IPC 2019 Conference on Emerging & Critical Environmental Product Requirements is a perfect example. The two organizations caravanned across the US in June, bringing scores of environmentally conscious engineers and compliance officers up to date on the latest REACH and related regulations in the EU, UK and Asia.

What distinguishes REACH from almost any chemical safety regulation I can think of – including RoHS – is parties need to prove the safety of a substance before it’s allowed on the market, and exemptions must be justified from both a risk point-of-view and a socio-economic point-of-view. Multiple independent technical committees, appointed by the Member States, make those assessments.