Contact an Expert

# Considerations for Surface Map Setup

The following information pertains to:

5DX Versions 6.x - 8.x

The concept of delta-Zs is perhaps the most difficult thing to understand about the surface map process. The delta-Z represents the difference between what the laser reports as the surface of the board and what the programmer believes is the best height for the testing the solder joints, that is, what the system should consider the pad level. Normally this is slightly above the actual board surface. For all points that are placed on the same substrate, whether it is ground plane or traces, the delta-Zs should be the same. Delta-Zs do not tell you anything about the warpage of the board. Board warpage information is derived from the actual laser readings for the particular board, which is combined with the delta-Zs, and stored in the MAP_RES file.

The system calculates the Z-height for the XYZ Stage for each view by using the three points that form the triangle for that view. It assumes that within each triangle the board is usually tilted, but is, at least a flat, not a curved surface. It constructs a plane based on the x, y locations and the Z-heights measured during the surface map, plus the delta-Zs (plus the board thickness if it is a bottom side view). It calculates where the center of the view is in relation to the three points on that plane and what Z-height should be used.

NOTE: Delta-Z is added to the Z-height measurement for each point before the plane is made, to determine the Z-height for each view.

If the majority of views appear to be in good focus, but a few are not, it may be that the delta-Zs are not right for the points that make up the triangle(s) involved. Another explanation is that the board may be significantly curved in the area of the triangle, or in some cases where the board is routed, there may be a discontinuity. In these cases the best approach is to move or add points, rather than recalculate delta-Zs.

If board warpage is severe, it may be seen that the solder joints in the center of the view are in good focus, but ones near the edges are not. In this case adding more surface map points will not help; the surface map correctly identified the Z-height for the center of the view. The only recourse is to decrease the area that is tested in the view, either by reducing the FOV used or by increasing the snap margin.

## Recommended Method for Setting Delta-Zs

Assuming that surface map points can all be placed on the same substrate, the recommended technique is to measure the delta-Z at 5-8 points where it can be measured accurately. Delta-Zs are most accurately measured on points near fine pitch devices, since the solder joints are thin and easy to focus on.

Average the delta-Z values (assuming that no individual measured delta-Z varies by more than a couple of mils from the average).

Use the Set All utility to set the delta-Zs for all surface map points to the average value.

The variation seen from point to point, or with consecutive measurements of the same point, is a function of how accurately the programmer can find the best plane for focus. The average value will be a better representation of the true distance than individually determined ones because the user will sometimes pick a Z-height that may be slightly above or slightly below optimal, but on average is pretty close. This is not only the fastest way to get a good surface map, it is more accurate than setting delta-Zs for each point individually.

If the application is such that it is not possible to put all points on one substrate, the same philosophy can perhaps be used. It will be necessary to keep track of which points are on what substrate, and calculate a delta-Z for each, then do the appropriate Set All.

## Problems with Setting Delta-Zs for Each Surface Map Point

When setting delta-Zs for each surface map point, there may be cases where there is nothing in the immediate area to focus on. To set the delta-Z, the programmer will typically move the XYZ Stage a short distance. By moving the XYZ Stage, the delta-Z that is measured may not be representative of the spot where the surface map point actually is. Included in the delta-Z measurement will be any board warpage between the area moved to and the actual surface map point.