Assembling High Current Heavy Copper PCBs

Many designers and assemblers do not know what is readily available in high current heavy copper PCB. Heavy copper boards can contain three ounces to twenty ounces of copper conductors. Quite surprisingly, a heavy copper PCB (ten ounces) can also have very fine features.

There are many variations of the heavy copper technology, including buried heavy copper, multiple levels of copper, and multilayer heavy copper. The simplest form of heavy copper is the single or double-sided PCB, containing three- to twenty-ounce thickness of copper. By starting with a heavy copper base, such as eight-ounce foil on FR4, imaging and etching the circuit, single and double-sided PCBs have the lowest cost. On all heavy copper boards, the designer has the choice of starting copper
thickness and the amount plated in the holes. The current required to be passed in the vias will determine the amount of copper plated in the hole. To plate ten ounces of copper in the holes of a twelve ounce thick PCB, the
manufacturer will start with two ounces of copper and plate up to ten ounces in the holes and on the traces.

Resolution will be quite good as the manufacturer is only etching two ounces of copper foil. To plate that thick, multiple layers of dry film will be sequentially layered on three to ten times and then imaged and developed, creating very deep dry film trenches to plate copper up. The plating of ten ounces of copper is quite slow, taking forty-five minutes per ounce, so the PCB will be expensive but can carry hundreds of amps.

Multilayer heavy copper can have four to eight layers, up to fifteen ounces of copper per layer and interconnected with heavy copper in the vias. In both multilayer and double-sided PCBs, the use of large thermal reliefs is of utmost importance for assembly. The heavy copper conducts so much heat that you would never be able to solder the components.

A very advanced heavy copper PCB is the multilayer PCB with two to five different copper levels, all interconnected and intertwined on the same layer. Note the extra vias in the large pads to pass more current and keep the pad attached, as well as providing a stronger pad for the bolt holding the cable in place.

Assembling a high current, heavy copper board can be difficult, but not impossible. Extra care and engineering when designing the board must be observed to allow the thermal transfer of the heat from a soldering iron to the pads. The use of track neck downs as well as large open thermal reliefs help the soldering iron deliver the heat needed. It is not recommended to IR or wave solder a heavy copper board. The heat picked up by the copper will be retained for so long that the heat in the FR4 joint will cause failure, and the copper traces will lift off the board.

The soldering of heavy copper PCBs can present problems if your assembly company is not experienced in trying to get enough heat into the board to solder the components. Many heavy copper PCBs need to be hand soldered on a hot plate just to get a high enough temperature to solder properly. The very thick copper in the vias can make heating the via pad to a high enough temperature difficult without losing adhesion to the FR4 and lifting the pad. Most experienced heavy copper designers will add extra vias around the pads to rivet the pad to the FR4 board. It is recommended that the PCB manufacturer use high-temperature lead-free rated laminate to help keep the pads attached with all the retained heat.

Problems encountered in assembling heavy copper PCBs are mostly based on cold solder joints, without full flow through to the other side of the via.

A new lower cost manufacturing method assists the assembly by replacing the heavy copper protruding up to twenty mils off the surface. The new buried heavy technology is flat to the surface. The buried heavy copper PCB presents a different view of the product because it looks and feels like a normal board, but has up to ten ounces of copper buried into the FR4 under the surface. This new technology helps assembly companies as the thinner one to two ounce copper pads will solder without a hot plate.

As all the tops of the traces are at the same solder height, paste stencils can be used. However, the ten ounce area will still need assistance with external heat to properly solder. Buried heavy boards can have any amount of the copper below or above the surface.

Example: Typical ten-ounce power board, eight-ounce below and two ounce copper traces above. Also, the top copper can have any amount of plating added to any tracks as well as the vias. The two-ounce SMT pads are soldered as normal after a hot plate is used to solder a few push in connectors into the ten-ounce section.

Buried heavy copper has a large number of variants, which can be used to make just about any possible combination of copper weights and technologies.

Assembly technology for heavy copper boards needs to be tested and figured out before you can solder the heat absorbing heavy copper. The
thickness of the copper varies between four ounces and twenty ounces of copper on the surface and possibly in the through holes. The heavy copper PCB can be on almost any type of laminate from flex to FR4 through polyimide. The heavy copper PCB will most likely be very heavy and thicker than a normal PCB. Your assembly company will have a learning curve and will need some solder samples to experiment with.

When trying to solder twenty ounces of copper, it can be a big surprise when placing a large soldering iron on a big pad to solder it. No matter what you do, the solder won’t melt. A large adjustable lab-style hot plate works well to preheat the heavy copper PCB. Monitor the PCB temperature with an accurate thermocouple type meter. After you have
soldered the board, let it cool slowly so as not to delaminate the board.

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