A design for quantitatively determining annular ring and breakout angle on PCB innerlayers.
If plated through-holes in multilayer printed circuit boards are not suitably registered, board reliability is threatened. Specifically, the annular ring on boards may be insufficient to ensure a good layer-to-layer bond and concomitant PTH integrity.
Useful data on PTH registration may be obtained using the test systems described by Paur in his two patents.1,2 These systems provide more than a simple go/no-go measurement, but do not provide continuously variable data. The misregistration data are “binned,” as the system indicates misregistration within some interval, rather than being continuous. The systems also consume some space on a board panel and entail a slight increase in drilling time and tool use.
Acceptance standards IPC-6012 and IPC-6013 define three classes of PCB reliability, in order of increasing reliability requirements, denoted as Class 1, 2 and 3. Classes 1 and 2 permit some missing annular ring on internal layers, expressed as “breakout angle.” (Figure 2 demonstrates the concept of breakout angle. As of this writing, Class 1 boards may have internal annular ring with 180˚ breakout, and Class 2 boards may have internal annular ring with 90˚.)
The IPC the acceptance standards call for vertical microsections of coupons on PCBs, which do not allow measurement of breakout in internal annular ring. As Clifford3 demonstrated, even when vertical microsections meet Class 3 acceptability requirements, the PTHs themselves in reality may not meet breakout requirements. Horizontal microsections could reveal the actual breakout, but preparing such microsections is time-consuming in comparison to vertical ones. Another serious disadvantage of horizontal microsections is loss of a retainable record, as each section is destroyed as one proceeds into layers deeper and deeper in a PCB. Fabricators and users typically want microsection records and samples retained for some time after boards are made and shipped.
This unappealing situation led us to search for an exact way to measure internal annular ring breakout.
Geometry Review and Basis
Before going into the details of the new design, it may be worthwhile to review a tool from trigonometry, the Law of Cosines. We will use the law later in this article.
Many engineers and scientists are comfortable with use of the Law of Sines, which permits calculation of the length of all sides and magnitude of all angles in a triangle if the length of one side and magnitude of two angles are known. Per Figure 1, if the length of side A and angles b and c are known, the Law of Sines allows the length of sides B and C to be calculated:
Eq. 1
Less frequently seen is the Law of Cosines, which permits calculation of the length of all sides and magnitude of all angles in a triangle, if the length of two sides and magnitude of the angle between them are known.
Referring again to Figure 1, if the length of sides A and B and the angle c between them are known, the Law of Cosines allows the length of side C to be calculated:
Eq. 2
Refer now to Figure 2, representative of a hole drilled in a circular copper pad of a board. Using Figure 2, we show below that, knowing the displacement d of the center of the drilled hole of diameter r from the center of the circular pad of radius R, one can calculate the breakout angle 2τ by using the Law of Cosines.
The displacement d of the center of the drilled hole (red dot) and the center of the copper pad (blue dot) is, in terms of x and y,
Eq. 3
In the following, the notation (a,b) indicates the angle between a and b.
Using the Law of Cosines,
Eq. 4
Hence,
Eq. 5
and, with d calculated from ,
Eq. 6
From Figure 2, notice that
Eq. 7
So
Eq. 8
How does one determine x and y when the pad is on an internal layer? The next section has the answer. It also includes some comments on measurement of R and r. The latter two parameters are known, at least approximately from the design, but users of this new system can measure true values with relative ease.
Registration System and Measurements
Consider Figure 3, which shows the circular pad and drilled hole from Figure 2 and adds a chevron and a square pad to the right and left, respectively. The chevron and square pad are the two elements of the novel registration measurement structure. The chevron and square pad are formed at the same time, from the same material, and by the same process as the circular pad. Typically, the chevron and pads would be copper foil remaining after printing and developing of copper-clad laminate.
Notice that the length of the sides of the square pad is designed to be the same as the diameter of the circular pad, R. However, any layer-to-layer variations in printing, developing and etching the internal circular pads will also affect the square pad, so the true circular pad diameter can be determined by measuring the length of a side of the square pad.
The length of the chevron is equal to the diagonal of the square, Eq. 9.
E
Eq. 9
The diameter of the drilled hole, r, can be measured directly from the microsection, as the technique and equipment are designed to take the section through the center of the drilled hole.
Figure 4 contains the same elements as Figure 3, and adds the line S representing the plane of a vertical cross-section through the center of a hole drilled in a circuit board.
The user finds the value of x to be used in Eq. 3 by measuring the distance x’ between the observed center of the drilled hole and the observed center of the square, then subtracting from x’ the distance (already known as D from the design of the pads and their locations) between the center of the circular pad and the center of the square pad. Notice that the user makes one measurement, x’, to find the value of x from Eq. 10.
Eq. 10
The value of y to be used in Eq. 3 is found by measuring the distance y’ between the center of that portion of the chevron visible in the cross-section and the observed center of the square, then subtracting 2D from the measured distance y’. Notice, once again, that just one measurement, y’, is made to find the value of y from Eq. 11.
Eq. 11
The results from Eq. 10 and 11 allow calculation of breakout angle by utilizing the Law of Cosines. Note that, by definition of internal annular ring per IPC-T-50, the radius of the drilled hole, r, must be used, not the radius of the plated hole.
Advantages and Applications
This new method has some attractive features:
- Minimal space consumption on panel may often be incorporated into an A/B coupon.
- Precision of measurement of breakout angle and misregistration is limited by uncertainty in measurements of linear dimensions in coupons.
- Sample preparation takes place concurrent with standard microsection of multilayer panels.
- Allows preservation, as part of quality records, of breakout and misregistration information of all layers.
- If space consumption on a panel is extremely critical, the chevron could be incorporated as a “negative”; i.e, by removal of copper in the square pad.
A number of applications of the new method come to mind:
- Measurement of breakout angle for conformance to IPC Class 1 or 2 requirements, from vertical microsection.
- Variables data can be used in statistical process control of PCB lamination, for displacements of innerlayers.
- Variables data can be used in designed experiments for process optimization.
- Design rules can be formulated that incorporate statistics of drilling and lamination misregistrations to determine pad size on internal layers required to ensure a desired yield in fabrication.
References
1. Tom R. Paur, US Patent 4,894,606, System for Measuring Misregistration of Printed Circuit Board Layers, January 1990.
2. Tom R. Paur, US Patent 4,918,380, System for Measuring Misregistration, April 1990.
3. Tom Clifford, “Round the Clock,” Printed Circuit Design & Manufacture, June 2004.
Ed.: This article was first published at IPC Apex Expo in April 2011 and is republished here with the authors’ permission.
Russell Dudek is advanced technology manager and Louis Hart is quality assurance manager at Compunetics (compunetics.com); This email address is being protected from spambots. You need JavaScript enabled to view it..
