EMS partners use thermal profiles to enhance PCB assemblies

Original equipment manufacturers (OEMs) need to keep a close watch over electronics manufacturing services (EMS) when they are being evaluated for PCB assemblies. Small differences in process, such as the use of thermal profiles, result in remarkable impact on yields, quality and cost.

As boards go through solder reflow or wave solder, the EMS must ensure that every area of the board reaches the right temperature for soldering and remains there for the right duration. They must also control the rate of heating: Heating too quickly can damage components or boil the flux, leaving splatter and solder balls; while heating too slowly can cause the flux to dry out and become inactive before the solder forms a good joint.

A PCB's thermal profile describes its temperature over time during soldering. The profile is affected by the number of layers: More layers mean more copper and a greater amount of heat absorbed. At Z-AXIS, boards range from two layers to 28 layers and there is a significant difference in thermal profiles over this range. The type of parts also affects the profile, with areas containing large or heavy parts such as BGAs heating up much more slowly than areas with smaller components.

Many high-mix EMS providers create three or four standard thermal profiles, each covering a range of values for the typical number of board layers and predominant component types they encounter. Then they apply one of these standard profiles to each new board design that enters manufacturing, assuming the standard profile will be close enough to the board's actual profile to give good results.

In our experience, a better approach is to measure, record and optimize the actual thermal profile of each board design during pre-production and use that unique profile in manufacturing.

Thermal profiling of a board is fairly easily done with a Datapaq, a device with several thermocouple sensors and data acquisition hardware in a heat-shield case. The temperature probes from the Datapaq are affixed to the bare circuit board in multiple places. Larger, heavier components that can have a significant impact on heating are modelled by adding an equivalent weight above the probe where these components will be placed.

The Datapaq is placed into its heat-shielding case and goes through the solder machine with the bare board, recording board temperatures the entire time. When removed, it gives us time vs. temperature graphs for different areas of the board, showing what temperatures were reached, for how long and at what point of the process. We analyze these graphs to ensure that: All areas of the board reached the required reflow temperature and held it for the required time, as specified by the solder data sheet; the board did not exceed its maximum rate of heating (typically 3掳C/second); and no areas of the board exceeded the temperature ratings for the components to be used.

If adjustments are needed, we can change the rate of heating by independently adjusting several different temperature zones in the reflow oven, to within 1掳C. It is occasionally necessary to adjust the speed of the conveyor, slowing it down for heavy boards. For wave solder we can control the frequency of the fluxing pumps as well. With experienced technicians, we can usually make these solder process adjustments and get a board's thermal profile exactly right on the first or second try.

These settings are stored by board part number in the memory of the solder oven or wave solder machine. Each time the board comes up for a production run, we enter the board's part number to automatically adjust the solder machine settings and optimize soldering for that board.

Optimizing the unique thermal profile for each board typically takes less than an hour of work in pre-production for each board design. Compared to using a handful of "one-size-fits-most" standard thermal profiles, it gives us lower solder defect rates to improve quality while reducing costs for the customer.

- Michael Allen
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