In the first of two parts, our columnist looks at the printability of Type 5 and 6 pastes.

In understanding the dynamics of reliably assembling metric 0201 components, an analysis of all key process steps – printing, placement and reflow – is required. While effective printing is central to the success of steps down the line, the reality is that with dimensions this small, the integrity of all elements is critical. With this as the backdrop and informed by previous analysis of metric 0201 printing, our company engaged in comprehensive evaluation of metric 0201 assembly, beginning with printing and the most viable materials for the job. This column will present findings on the stencil printing investigation, and, in a departure from the normal “printing” focus, the next column will reveal the placement and reflow study results.

To begin, we wanted to gain an appreciation regarding the effectiveness of certain solder paste materials. In full disclosure, this was not an exhaustive evaluation of multiple solder paste suppliers’ materials. Rather, our team selected two suppliers and three materials with which we have had past success. This is an important point, as not all materials are created equal and, in evaluating Type 5 and Type 6 solder pastes, it is imperative manufacturers test them under specific manufacturing conditions with specific tool sets, as all variables play a role in performance.

A poor stencil aperture seal may lead to solder bridging.

When we think of a gasket, we often relate it to something experienced in our home, perhaps a plumbing gasket or a gas pipe. A tight seal – or gasket – ensures no leaks. In SMT printing, the same theory applies: A good gasket means no solder paste leaks and a better printing result. With stencil printing, gasketing occurs when the aperture cut into the stencil is pressed firmly against the corresponding pad so it forms a seal. To achieve a good gasket, the aperture should be at least 1:1 with the pad size. If the aperture is larger than the pad, a poor gasket will result. Our company generally advises a one mil (25µm) reduction of aperture size to ensure a tight seal. So, for example, if the pad is 125µm x 125µm, an aperture of 100µm x 100µm is recommended. All things being equal, this approach will help form a good, tight gasket.

When diagnosing print problems, don’t overlook substrate support.

Substrate support is an important, yet often overlooked, element of the screen printing process. Sure, tooling is top of mind when all necessary parts of a new PCB assembly are being developed. But once it’s in the printer way down in the print nest on the table, we tend to forget about the tooling block. I suppose this out-of-sight, out-of-mind mentality is why manufacturers often point to more obvious, visible components of the printing operation as the culprit when the process shows inconsistency. Surely it must be an issue with the squeegee, the stencil or the board fabrication, right? Well, sometimes that’s the case. In many instances, however, a resolution may be as simple as a look at your tooling; something could have changed, or maybe it’s been incorrect from the start.

If the tooling isn’t manufactured properly or has been altered during production, printing inconsistency is the result – either across the board or from board to board. Paste volumes may be noticeably different in various areas of the PCB or panel; one corner may be just fine, and another has too much or too little material. This dynamic could be the first clue tooling is the cause. If the problem were an improperly manufactured stencil, for example, the issues would more likely be consistent across the entire board. Tooling errors can be extremely focused.

Why print offsets occur and how to correct them.

Printing offsets – the degree to which a material deposit is off center from the pad – can occur due to three primary elements of printing: the printed circuit board (substrate), the stencil and the printer. Each has to be manufactured and is surrounded by a process bandwidth, each with its own tolerances that can accumulate. Add to this the variables from different manufacturing methods, sites and base materials and, well, offset inevitability becomes obvious.

Let’s begin with the board and stencil. Gerber data is king; it’s where the designs begin and is the blueprint for PCB and stencil manufacture. Simply put, Gerber is an x, y coordinate and angle for a certain feature size and shape. When an offset occurs, it is the difference between what the Gerber says and what is actually produced. At the PCB level, the offsets derive from the artwork, the subtractive chemical process and the FR-4 laminate. Each of these has the potential for variability, as in the case of FR-4 that can stretch and move during temperature processing, because the coefficient of thermal expansion (CTE) is relatively poor, especially considering today’s dimensions. Given these realities, the board could be off in one corner, or it could be a gradual movement from the left corner to the right, from the center outward or just focused in one area of the board.