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.

A plan for coordinating 2-D and 3-D x-ray and microsectioning strategies.

Last month I discussed use of “full CT” (FCT), or 3-D x-ray, for analyzing electronic components and boards. This provides a 3-D model of the sample from which “virtual x-ray microsections” can be taken at any plane within the sample for analysis. While this can give excellent information, it does take time for the quantity of 2-D x-ray images to be acquired from 360° around the sample, from which the 3-D model is produced. Furthermore, as electronic features are small, it usually requires a set of high magnification 2-D images to provide the analytical detail within the 3-D model. As I have mentioned, the geometric magnification provided by an x-ray system depends on being able to place the object/field of view (FOV) close to the x-ray tube. As the sample is rotated within the tube-detector axis for FCT, then the larger the sample, the further away the sample must be placed to prevent a collision with the tube during rotation. Although it is possible to create an FCT model by only taking images from 180° around and using careful sample placement to allow an FOV to be placed closer to the tube, detailed information is unavailable for the model because only half the potential data are taken. Looking at an individual component under full CT does permit its placement close to the x-ray tube for high-magnification images. Once that component is on a board, however, the available high magnification for full CT is likely to be lost. At this point, cutting the sample to reduce its size becomes the only realistic option to getting the required detail from the FCT model.

Your customer wants to grow. Are you ready for the transition?

One of the difficult challenges small electronics manufacturing services (EMS) companies face is the transition from a transaction-based job shop to a relationship-based, full-service EMS provider. There are a number of issues to consider prior to taking that journey.

Is it necessary? Bigger isn’t always better. The US is full of small job shops that are profitable and right-sized for their ownership and long-term strategy. At the same time, there are also companies that successfully redefine that model within their region and grow at a pace they are comfortable with. And, a restructuring in trade agreements and tax structure may drive greater opportunities for growth over the next few years as OEMs update their business models to take advantage of this change in the playing field.

Brittle materials can lead to CAF failures.