The balancing act: learn how to balance a line using some very simple calculations.

A fundamental aspect of automated surface-mount assembly is having a process that runs at optimal efficiency, with the least amount of waiting time between machines. Having a machine waiting for the one in front of it to supply printed circuit boards (PCBs) is very inefficient. When an automated assembly process runs at optimal efficiency with little or no wait time, we have a balanced line.

In our work, we see a lot of automated surface-mount assembly processes. What we do not see are a lot of balanced lines.

The printer at the front of the line is rarely the gating factor. We have seen lines where even semi-automatic printers keep up with lines equipped with chipshooters. Not much talent is necessary to assign the right capacity reflow oven to a particular line volume. Though specific formulas for calculating oven conveyor speed exist, basically the longer the oven, the higher the volume that can be passed through a reflow profile that takes four to five minutes.

The bottleneck in almost all lines is the pick-and-place function. Pick-and-place machines are expensive and constitute a high percentage of the investment in any line.

In a low-volume line with a single pick-and-place machine handling both passives and integrated circuits (ICs), not much can be done to tweak the line speed. Most people get in trouble when multiple placement machines, like a chipshooter and a flexible placement machine, are in the line. Common wisdom is to do the passives quickly on the chipshooter and assign the ICs to the flexible placement machine. However, this usually results in the flexible placement machine waiting an inordinate amount of time for the chipshooter to feed it a board. We dare you to walk out to your line and see if that is the case.

If so, try not to be too dismayed. A little applied math can vastly improve line efficiency. Don't worry, all we are talking about is some simple algebra, so spray some WD-40 on your brain cells and let's go.

Placement Machine Performance

First, determine the placement rate of each pick-and-place machine in the line. The rate will not appear in the manual or on the machine's spec sheet; you have to measure it. We are specifically talking about the actual placement rates for individual categories of components, such as passives, simple ICs or complex ICs. The rates should be measured while the placement machine is running continuously and should not include any stops for setup, repair, assists or component replenishment.

For example, we have a turret-style chipshooter that handles passives and simple ICs, such as SOIC8s and PLCC20s. Next in the line is a flexible placement system that can place passives--albeit quite a bit slower than the chipshooter--but can masterfully put down simple and complex ICs. The flexible system places the simple ICs at a faster rate than the chipshooter (Table 1). Although we have seemingly put the passives on the chipshooter and the ICs on the flexible placement system, appearances can be deceiving.

Bill of Materials 

Next, get out the bill of materials (BOM). You must subdivide the entire BOM into the three categories of components identified for placement systems (Table 2). As you can see, line balancing is application-specific-another likely reason why most people do not bother doing it.

Line Balancing

A line is balanced when each placement machine uses the same amount of time to place all of the components assigned to it. The placement time for each category of component is calculated by the following formula:

Placement Time = # Components/Placement Rate

Total placement time is the sum of all categories of components assigned to each machine. Using the values in our example, assume that all the passives are assigned to the chipshooter and the ICs are assigned to the flex placer. The assignment of components for the first the oretical calculation does not matter; the balancing procedure is an exact single iteration calculation regardless of the starting point.

The first step is to check whether the line is balanced initially.

Total Placement Time (Chipshooter) = 200/10,000 = 0.02 hr. (72 sec.)

Total Placement Time (Flex Placer) = 15/4,000 + 6/1,000 = 0.0038 + 0.006 = 0.0098 hr. (35.3 sec)

In this case, the flex placer is idle 50% of the time. Does that sound incredibly efficient? The solution is to move some of the passives to the flex placer, thus reducing file time for the chipshooter and increasing the time for the flex placer. When the correct number is reassigned, the total placement times are equal and the line is balanced.

Mathematically, we will equate the two equations above, adjusting both for the reassigned components. The number of components to be reassigned will be the variable x. The balancing equation is as follows:

Total Placement Time (Chipshooter) = Total Placement Time (Flex Placer) (200-X)/10,000 = 15/4,000 + 6/1,000 + X/4,000

Using simple algebra, we solve the equation lot x and find it equal to 29.3. We can then confirm the calculations and identify the actual cycle time for the product:

Total Placement Time (Chipshooter) = (200 - 29)/10,000 = 0.017 hr (61 sec.)

Total Placement Time (Flex Placer) = 15/4,000 + 6/1,000 + 29/4,000 = 0.017 hr. (61 sec.)

With a few calculations, line efficiency is radically improved and it did not cost a cent. You now know more about line balancing than most of your compatriots. By the way, line balancing is also a component of the Surface Mount Technology Association (SMTA) process engineer certification. So go ahead and try this at home to reap the rewards of having a line running optimally. Remember, we're all in this together.

TABLE 1: The measured placement rates for types of components
on two different placement machines.

Placement        Passives         Simple ICs       Complex ICs
Machine       (placements/hr)  (placements/hr)  (placemenst/hr)

Chipshooter      10,000            2,000             N/A
Flex Placer       4,000            4,000            1,000

TABLE 2: BOM totals divided into the three categories of
components.

                 Passives         Simple ICs       Complex ICs

BOM Totals          200               15                6

Phil Zarrow is president and surface-mount process consultant, and Jim Hall is a principal surface-mount process consultant--both with ITM Consulting, Durham, NH; [phone]; www.ITM Consulting.org.