Best Practices for Double-Sided Mixed-Technology Board Design

PCB_CloseUp_3790As automation becomes more prevalent and sophisticated in electronics manufacturing, Pin Through Hole (PTH) designs are being phased out. PTH requires manual assembly labour that is less cost-effective than automated Surface Mount Technology (SMT) methods, and is not feasible when dealing with ultra fine pitch devices.

That said, many designs still mix PTH and SMT. PTH also remains in use for some heavy power connectors, transformers, and other devices where strong mechanical bonds are required.

This tip outlines some key best practices for double-sided, mixed-technology design.

Keep it simple SMT! On double-sided mixed boards, the solder side should be kept simple, meaning:

  • Discrete components wherever possible
  • No fine-pitch devices or ball grid arrays (BGA)
  • Component height not more than 6mmProblems in manufacturing arise when the components on the solder side of a mixed board are too heavy.

If the board is assembled using a solder reflow pass for each side, the heavy SMT parts attached to the solder side during the first pass may fall off during the second pass.

The general rule of thumb is: If the component is more massive than a PLCC44 it will not hang on by surface tension. Said more technically, the mass to solder pad surface area ratio for such devices should be lower than 4.5 gm/cm2.

The solder side of a double-sided mixed-tech board may also be soldered by wave solder if the design permits. Using only discrete devices on the solder side and avoiding BGAs and Fine Pitch keeps this option open. Success with solder joint formation using wave solder techniques is much more likely with these types of devices.

“Submarining” is the term used to describe the difficulties encountered when the leading edge of a board passes under the top of the wave on the wave solder machine. Solder will run down the top side of the card and essentially render it scrap. Very large panels with improper support are prone to sag in the wave solder preheat and any previous reflow cycles. This can increase the chances of a submarine.

Keeping a clear space around the edges of the board allows for the use of snap-on titanium stiffeners. These increase the rigidity of the panel and help prevent such occurrences.

Snap It
Another manufacturing design technique that should be observed is the use of snap-off materials on the edges of the board. As mentioned above, titanium stiffeners can then be used to help make the board more rigid. It also allows the panel to be handled smoothly in conveying and clamping systems. A 0.200–inch clearance is adequate but nothing (not even fiducial keepout areas) should be placed in this zone.

Pad It
Pad dimensions should be carefully considered on the bottom-side SMT of a double-sided mixed board. This includes extra thieving pads for wave soldered ICs and extended toe pads to facilitate solder wicking. Both can significantly improve the yield at wave soldering.

In addition, components on the board should be oriented so that the solder joints are not shadowed as the board moves through the wave solder.

Clear It
If a double-sided mixed board is so dense that the above guidelines cannot be observed, a special fixture called a selective wave fixture (a.k.a. pallet) into which the board is snapped can be used. This masks the bottom-side surface mount from exposure to the wave. To do this, there must be sufficient clearance (0.1 inch) between the PTH and SMT lands on the solder side so that the SMT can be masked by the fixture.

Keep in mind that fixtures add cost to the product; several hundreds of dollars per fixture, several fixtures are required, and they have to be cleaned, loaded, unloaded, and occasionally replaced.

Think Single-Sided
Overall, when designing a mixed board (utilizing both SMT and PTH) it pays to think single-sided! If possible, PTH parts should be placed on the same side of the board as the primary SMT devices. Double-sided PTH or PTH on the wrong side is almost certain to require manual soldering. There is just no other way to process them cost effectively with current automation.

At OCM Manufacturing, we can work with designers to ensure that their plans and prototypes are manufacturable and therefore marketable. Contact one of our Program Managers for details about how we can help.