PCBA Assembly Methods for Single-Sided and Double-Sided Boards
The assembly route for a board is decided long before the first unit is built, and it is usually fixed by the mix of components the design contains. A board with only surface-mount parts follows one path, a board with a handful of connectors follows a different one, and a board with both technologies on both sides follows a third. Choosing between the available PCBA assembly methods deliberately, while the placement is still open, avoids the situation where a design is finished and then found to require a process the assembly house cannot run efficiently.
The Six Basic Assembly Flows
Almost every production board falls into one of six flows: single-sided surface mount, single-sided through-hole, single-sided mixed technology, a board with surface mount on one side and through-hole on the other, double-sided surface mount, and double-sided mixed technology. The flows differ in how many times the board sees a reflow oven or a solder wave, and that count is the best single indicator of both cost and reliability, because every thermal excursion adds stress to the laminate and to the joints formed earlier.
Single-Sided Surface Mount
The simplest flow prints solder paste onto the pads through a stencil, places the components, and passes the board through a reflow soldering oven. The stencil aperture determines the paste volume, and the paste volume determines whether the joint forms properly, so the stencil design and the pad geometry have to be matched. This flow has the fewest process steps and the highest yield, and it is the one to aim for when a design has the freedom to use surface-mount parts throughout.
Single-Sided Through-Hole Assembly
A board that carries only through-hole components is assembled by inserting the parts and passing the board across a solder wave, which fills the barrels and forms the joints in one pass. The leads are then trimmed and the board cleaned. The flow is robust and the joints are easy to inspect, but the throughput is low and the process cannot place the fine-pitch parts that most modern designs depend on. It survives mainly in products where the mechanical strength of a through-hole connector is required, or where the assembly volume is small enough that the simplicity of the process outweighs its speed.
Single-Sided Mixed Technology
Most real designs are mixed: surface-mount devices plus a few connectors or a power device that has to be through-hole. The surface-mount parts are printed, placed and reflowed first. After inspection, the through-hole parts are inserted and the board is either passed over a wave or soldered by hand. Where the through-hole component count is small, hand soldering is usually the better choice, because tooling for a wave costs more than the labour it saves. Where the count is high, wave soldering becomes the faster option, and the layout has to keep the through-hole pads away from the surface-mount areas that must not see the wave.
Surface Mount on One Side, Through-Hole on the Other
Some boards place surface-mount parts on one side and through-hole parts on the other to use the available area more effectively. The two processes are the same as above, but the board must be supported during the wave or the reflow, since the previously assembled side cannot rest on the conveyor. A fixture that holds the board clear of the equipment is required, and its cost and its handling time should be included when the process is selected. The fixture also has to allow the board to heat evenly, or the assembly will warp during the second thermal cycle.
Double-Sided Surface Mount
Double-sided surface mount is used when the component count is too high for one side, commonly with the larger devices on the primary side and small passives on the secondary side. The process runs paste, placement and reflow twice. Two reflows are usually acceptable, but the second one subjects the joints formed in the first to another excursion, so the temperature profile has to be controlled with the assembled side in mind, and the process for the second side may require adhesive, a support pallet, or a lower peak temperature. Where a heavy component would fall from the underside during the second reflow, an adhesive is applied in addition to the paste.
Double-Sided Mixed Technology
Double-sided mixed technology is the most expensive of the six flows and the one with the most failure opportunities. One variant heats the assembly three times, which is generally considered too much, and the use of an adhesive for wave soldering on the underside often produces poor joint quality and is best avoided. The other variant suits assemblies with many surface-mount parts and few through-hole parts on both sides: the through-hole parts are soldered by hand. Where the through-hole count is genuinely high, selective soldering is a better answer than a full wave, because it applies heat only where it is needed and leaves the surface-mount side untouched. The choice between hand, wave and selective soldering is one of the layout decisions that carry into production, as described in this article on how PCB layout decisions affect production.
Stencils, Fixtures and Panels
Every flow depends on tooling that is easy to treat as an afterthought. The stencil aperture and its thickness set the paste volume; the support fixture decides whether a double-sided board survives the second pass; the panel layout decides how many boards the line processes in one cycle. Tooling should be designed from the placement data rather than from a previous product, because the aperture area ratio and the fixture keep-out are both specific to the layout in front of you. Adhesive selection matters as well, and its cure schedule has to be compatible with the solder profile used for the second side.
Choosing the Right Flow
The decision usually comes down to component mix and volume. Few through-hole parts and a single side should go through a simple SMT flow with hand soldering for the exceptions. A high through-hole count justifies wave or selective soldering. Two populated sides justify the extra tooling only when the area saving is real. Throughout, the thermal history of the board should be counted, because reliability is set by the number and severity of the excursions rather than by the elegance of the flow chart. The defects that appear when the process window is missed are described in this article on SMT component shift causes, and the alloy choices that affect the profile are covered in this article on lead-free versus leaded solder.
FAQ
How many reflow cycles can a board survive? Two is routine and three is a warning sign. Each pass adds thermal stress to the joints and the laminate, so the process should be designed to minimise the count.
When is wave soldering better than hand soldering for through-hole parts? When the through-hole component count is high enough that the labour saving exceeds the cost of tooling and the risk of heat damage to nearby surface-mount parts.
Can through-hole parts be soldered selectively instead of by wave? Yes, and selective soldering is often the better choice on a mixed board because it applies heat only to the joints that need it.
