Drill Program Optimisation And Hit Sequencing

A drill program is the file that tells the machine where to drill, with which tool, at what depth and in what order. The first three are fixed by the board design; the order is the part that can be optimised, and it affects the cycle time, the tool life and the hole quality.

This article covers what the programme controls, how the sequence is arranged, and how it interacts with hole quality.

What The Programme Controls

For each hole the programme defines its position, its diameter and therefore the tool, and its depth class. Holes of the same diameter are grouped into a tool table, and the machine drills them in a sequence that minimises the travel of the table and the number of tool changes. The depth is normally through the panel, but a controlled depth hole, such as a counterbore or a backdrilled via, carries its own depth instruction.

The programme also defines the drilling parameters for each tool: the spindle speed, the feed rate and the number of hits before the tool is changed. Those are process parameters rather than geometry, and they are set from the material and the hole size. The distinction matters because a change to the geometry requires a new programme, while a change to the parameters is a process change that should be recorded. The way the geometry reaches the machine from the design is described under PCB design and fabrication.

Hit Sequencing

The sequence in which the hits are made within a tool determines the distance the table travels, and therefore the cycle time. A sequence that follows a sensible path across the panel finishes the tool’s holes with little wasted movement; one that jumps from one side of the panel to the other wastes a large part of the cycle in travel. Optimising the sequence is a matter of the tool path rather than of the drilling process, and it is normally done automatically from the hole data.

The sequence also affects quality indirectly. Consecutive hits in one area heat the panel locally, and if several large holes are drilled in the same region one after another the temperature there rises and the resin is more likely to smear. Spreading the hits, or drilling the large holes in a sequence that alternates across the panel, reduces the local heating and is worth considering on a panel with a dense cluster of large holes. The consequences of the heat are described under balanced stackup and odd layer count.

Tool Change And Grouping

Every tool change costs time, so throughput depends heavily on how the holes are grouped, and the number of changes is minimised by grouping them by diameter. A board with fifteen hole sizes needs at least fifteen tool positions, and the machine changes between them as it works through its groups. The sequence of groups matters as well, because the machine can sometimes keep the next tool ready while it finishes the current one.

The grouping is limited by the tool positions available in the magazine and by the hit count. A tool that reaches its hit count must be changed even if it has more holes to drill, and the sequence is normally arranged so that the tool changes fall at natural points rather than in the middle of a dense area. On a panel with many small holes and a few large ones, the small holes dominate the cycle and the large ones are fitted in where the tool is already loaded.

Panel Stack And Depth

The stack height is part of the programme, because the drilling depth has to reach the bottom panel and the parameters change with the number of panels. A taller stack drills more slowly, wears the tool faster and needs a shorter hit count. The programme therefore carries the stack height with the parameters, and a change in stack height is a process change rather than a machine setting.

Controlled depth holes add another dimension. The depth is measured from the board surface, so a stack of panels has to be drilled one at a time for those holes, or the depth has to be set for the panel being drilled and the others masked out. A programme that mixes through holes and controlled depth holes has to handle the two differently, and the sequence has to allow for the depth change. The measurement of the resulting depth is described under multilayer prototype requirements.

Effect On Quality

The programme affects quality through the parameters it carries and through the sequence. A tool whose hit count is exceeded produces a smeared hole wall regardless of the skill of the operator, and the count is part of the programme. A sequence that concentrates the hits in one area raises the temperature, which has the same effect. A sequence that leaves a panel unsupported during a heavy cut allows it to deflect, which moves the hole.

The parameters also interact with the design. A board with a small via and a large mounting hole in the same region presents two very different drilling conditions a short distance apart, and the parameters that suit one are not ideal for the other. Where that happens, the programme may use a different feed for the two groups, which is a legitimate optimisation as long as the tool life for each is tracked separately.

Optimising For The Shop

The optimisation that matters in a shop is not the shortest path but the most stable process. A programme that produces a consistent hole and a predictable tool life is worth more than one that saves a few seconds per panel and produces an intermittent smear. That is why the parameters are established by experiment for each combination and then left alone.

Where a shop runs the same board repeatedly, the programme and its parameters should be recorded together as a single entity, with a revision number, so that a change to either can be traced. A defect that appears after a programme revision can then be related to the change immediately, which is what makes the optimisation safe to perform. The overall sequence of fabrication steps that the programme fits into is described under PCB design and fabrication.

The cycle time estimate should include the tool changes, because on a board with many diameters they can be a large fraction of the total. A programme that drills quickly but changes tools constantly may be slower overall than one with a slightly longer path and fewer changes, and the balance depends on the number of holes in each diameter group. Measuring the actual cycle time on a few panels, rather than estimating it, is what shows where the time is really going.

FAQ

Does the hit sequence affect hole quality? It can, through the local heating of the panel when many holes are drilled in one area in succession. Spreading the hits reduces the temperature and the risk of smear.

Why not use one tool size wherever possible? Because the design needs different diameters for a component lead, a via and a mounting hole. Unifying them would change the electrical and mechanical properties of the board.

Can the programme be changed without requalification? A change to the sequence alone can be assessed by inspecting the hole quality on a sample. A change to the parameters, the stack height or the tool type requires the same verification as a new process.