V-Scoring Explained

This article has been condensed from www.accusystemscorp.com.

Introduction

Since you are a very busy engineer, operator or manager involved in many other important areas, we offer this concise review of the fundamentals of v-scoring, that will produce the accurate and functional product that you desire.

The benefits of v-scoring are well documented. The efficiencies gained in producing multiple assemblies from a single manufacturing cycle, ensure the continued success of this value added process. There are also several cost saving benefits to be realized by the PCB manufacturer who takes just a little bit of time to learn the basics of this versatile process.

Maybe the best way to index this topic, would be to answer your most frequently asked questions. In this way, we can imagine your interest and anticipate your needs.

V-Scoring, not unlike most fairly new processes, is not always an instant consideration in manufacturing design. So, from the viewpoint of a v-scoring service, we will highlight some of the areas that can be most challenging to discover through experience.

How Deep

This is unquestionably the most frequently asked question in the scoring business. Choosing a depth that will provide a sturdy work-piece and still separate with light to moderate pressure after assembly, is an important element in profitable manufacturing.

Specifications

Most score depth specifications make use of one of the following conventions:

1) Vee depth measured from one or both sides of the PCB surface.

Or, the more appropriate...

2) Cross-sectional view, indicating the distance between Vees. This residual material is called the Web

Optimum Web Thickness

Determining the thickness of this uniform core of material for each individual project is really very simple. We need only consider the needs of the subsequent assembly operations. Our brothers and sisters in the assembly area are looking for the following performance criteria: 1) Overall part planarity, not only for component placement, but also for solder reflow and/or wave processes. 2) Ease of de-paneling. After component attachment is complete, the individual boards should break apart with only moderate pressure.

How Wide?

Actually, almost no one asks this question... but they should. Understanding the variables that determine score width along with the associated process tolerances, will help you identify problematic specifications, and aid greatly in laying out a manufacturable product. Knowing what your score width will be is essential to design and planning. Understanding the variables that determine score line width is, once again, quite straightforward.

Variables

Score line width is determined by a combination of two elements; blade angle and blade wear. Simply stated, a blade of a larger angle will cut a wider path than a blade of a narrower angle, at the same depth. Also, the deeper the blade penetrates the surface of the board, the wider the path will be. Also, as the blade wears, the tip essentially "moves up" the width of the blade, resulting in a wider score path for the same given web. Note View A...

Score Depth	Score Width (at 'zero' tip wear) (30O blades)	Score Width (at max. tip wear) (30O blades)
.010	.005	.013
.012	.006	.015
.014	.008	.016
.016	.009	.017
.018	.010	.018
.020	.011	.019

As you can see, there is a little more to score width than meets the eye. For the sake of simplicity and real world conditions, we can be safe in saying that, a 30 degree blade, throughout its service life, will require a surface clearance of .020" +/-.004". To this number we can add equipment tolerances at +/-.002" and you fill in the next number -- which is your accumulated process tolerances, including drill and image registration, as well as material movement. Just to be safe, lets say that it is +/-.004". So, have you done the math yet? At the outside range, we can safely design and plan for a blade clearance, or "Keep Out" area, of approximately .030". Another way to look at it is that the score path is going to encroach into the top and bottom surfaces of the individual boards approximately .015" from the edge (center of the score line) into both images. So far so good? Now lets take a look at a specific project. What is the distance from the nearest surface feature to the board edge? We now know that from the center of the score line (the board edge) to the outside of the path requires approximately .015". For planning purposes then, this becomes your minimum clearance from each board edge. If the board design has this clearance, or more, from the nearest feature to the edge, then you can safely step the board correctly at "zero spacing". Remember, if you add a space between boards, you will effectively be increasing the board size!

What Tolerances can be Held?

Good Question! Most modern CNC V-Scoring Machines are designed to hold at least +/-.002" on all dimensions (X, Y and Z). Machine maintenance, condition, age and operator attention/proficiency can effect tolerances as well as drilling, imaging and laminate stability.

"Score to Fab" is a smart time saver too. Parts that do not otherwise require routing for slots, cutouts or chamfers can be v-scored instead of routed. A light pass with sand paper or equivalent on the edges, makes a smooth and serviceable finish that will rival that of a routed edge, while maintaining the programmed specifications, thereby easing the load on your busy fab area.

Some Finer Points

Cost Savings...

We haven't talked much about the material savings aspect of v-scoring because it is fairly well known that, depending on the individual board size, when you eliminate the spacing necessary for routing, (typically .062" to .250" between each board) you may find that you now have enough room to add additional rows of boards to the fabrication panel. This can effectively reduce the number of panels that need to be processed, and the amount of laminate wasted.

Assembly Rails...

While it is always a boon to increase your panel efficiencies, it is also critical that we do not lose sight of the overall yield. Surprisingly enough, many assembly situations can benefit from adding a section of throw-away material onto two, three or four sides of an assembly panel. What we are talking about here, is the growing application of break-away rails to individual circuits and/or panels. The principal usage of these rails, is to allow densely packed SMD boards to be centered in the reflow or wave equipment, and to provide a temporary fixturing area. This fixturing area also can provide a place for coupons, fiducials, etc., as well as act to prevent the possibility of heat sinking and/or heat absorption at the board edge.

Blade Angles...

You may have noticed the absence of the mention of larger blade angles (45, 60, 90 degrees etc.). Although the data we have shown for 30 degree blades can be extrapolated out for usage on other angles, we suggest you consider the following:

If there is no specific use for the bevel-type finish of a larger angle blade, we contend that the clearance/leverage advantage that they offer for separation, is a poor substitute for using the proper web thickness. If this seems a little radical...then good! There are, of course, many specific application blades that, for design reasons, must be used. Beyond that however, there seems to be a great deal of confusion over blade angles. As far as we can tell, larger blade angles appear to be a throw-back to an earlier time when space was less of an issue, and Z-axis control was not as precise as it is today. With today's designs, 30 degree blades seem to offer the optimum balance of a narrow score path, ample clearance for separation and acceptable blade life performance.

Jump Scoring...

You may have heard this popular term before. It refers to the ability to program a certain length for a score line and then "jump" over a certain programmed distance (section of the board). The use of "jump" scoring can provide a rigid assembly panel, that can be de-paneled easily without damage to even the most sensitive solder joints. With the proper application of this capability, coupled with the use of rails, virtually any assembly goal can be achieved.

Conclusion

Far and away the biggest issues associated with v-scored product are panel strength and difficult board separation after assembly. Working with your board vendor to provide an easily de-paneled product will undoubtedly give you a measurable advantage in assembly and de-paneling. We hope that the recommendations we have supplied here will help you in your quest to identify proper web thicknesses. We also hope that this approach will encourage you to question specifications, be creative, and maybe head off some potential problems at the pass.

The Dreaded Disclaimer

We have tried to get you to see a process overview. Hopefully this perspective will encourage a commonsense approach to this versatile process. This article is a recommendation only, and is based on standard .062" thick, FR-4 material with a 30 degree cutter angle, and our experience.

© 2002 Omni Graphics Ltd.