SMT Design, Assembly, and Test

The IPC has defined three general end-product classes of electronic products.
• Class 1: General consumer products
• Class 2: Dedicated service electronic products—including communications, business, instrumentation,
and military products, where high performance and extended life are required, and where
uninterrupted service is desired but not critical.
• Class 3: High reliability electronic products—commercial and military products where equipment
downtime cannot be tolerated.

All three performance classes have the same needs with regard to necessary design and process functions.

1. Circuit design (not covered in this handbook)
2. Thermal design considerations
3. Substrate [typically, printed circuit board (PCB) design
4. Bare PCB fabrication and test
5. Application of solder paste
6. Placement of components in solder paste
7. Reflowing of solder paste
8. Cleaning, if necessary
9. Testing of populated PCB 

Once the circuit design is complete, substrate design and fabrication, most commonly of a printed circuit board (PCB), enter the process. Generally, PCB assemblies are classified into types and classes as described in IPC’s “Guidelines for Printed Board Component Mounting,” IPC-CM-770. It is unfortunate that the IPC chose to use the term class for both end-product classification and for this definition. The reader of this and other documents should be careful to understand which class is being referenced. The types are as follows:

1. Type 1: components (SMT and/or THT) mounted on only one side of the board
2. Type 2: components (SMT and/or THT) mounted on both sides of the board

The types are further subdivided by the types of components mounted on the board.
• A: through-hole components only
• B: surface mounted components only
• C: simple through-hole and surface mount components mixed
• X: through-hole and/or complex surface mount components, including fine pitch and BGAs
• Y: through-hole and/or complex surface mount components, including ultrafine pitch and chip
scale packages (CSPs)
• Z: through-hole and/or complex surface mount, including ultrafine pitch, chip on board (COB),
flip chip, and tape automated bonding (TAB)

The most common type combinations, and the appropriate soldering technique(s) for each, are
• 1A: THT on one side, all components inserted from top side
Wave soldering
• 1B: SMD on one side, all components placed on top side
Reflow soldering
• 1C: THT and SMD on top side only
Reflow for SMDs and wave soldering for THTs
• 2B: SMD on top and bottom
Reflow soldering
• 2C/a: THT on top side, SMD on bottom side
Wave soldering for both THTs and bottom-side SMDs
• 2C/a: THT (if present) on top side, SMD on top and bottom
Reflow and wave soldering (if THTs are present)
• 1X: THT (if present) on top side, complex SMD on top
Reflow and wave soldering (if THTs are present)
• 2X/a: THT (if present) on top side, SMD/fine pitch/BGA on top and bottom
Reflow and wave soldering (if THTs are present)
• 2Y, 2Z: THT (if present) on top side, SMD/ultrafine pitch/COB/flip chip/TAB on top and bottom
Reflow and wave soldering (if THTs are present)

The “/a” refers to the possible need to deposit adhesive prior to placing the bottom-side SMDs. If THTs are present, bottom-side SMDs will be placed in adhesive, and both the bottom-side SMDs and the protruding THT leads will be soldered by passing the assembly through a dual-wave soldering oven. If THTs are not present, the bottom-side SMDs may or may not be placed in adhesive. The surface tension of molten solder is sufficient to hold bottom-side components in place during top-side reflow. 

A Type 1B (top side SMT) bare board will first have solder paste applied to the component pads on the board. Once solder paste has been deposited, active and passive parts are placed in the paste. For prototype and low-volume lines, this can be done with manually guided x-y tables using vacuum needles to hold the components, whereas, in medium- and high-volume lines, automated placement equipment is used. This equipment will pick parts from reels, tubes, or trays and then place the components at the appropriate pad locations on the board, hence the term pick-and-place equipment.

After all parts are placed in the solder paste, the entire assembly enters a reflow oven to raise the temperature of the assembly high enough to reflow the solder paste and create acceptable solder joints at the component lead/pad transitions. Reflow ovens most commonly use convection and IR heat sources to heat the assembly above the point of solder liquidus, which for 63/37 tin-lead eutectic solder is 183° C. Due to the much higher thermal conductivity of the solder paste compared to the IC body, reflow soldering temperatures are reached at the leads/pads before the IC chip itself reaches damaging temperatures.

For a Type 2B (top and bottom SMT), the board is inverted and the process repeated. If mixed-technology Type 2C (SMD only on bottom) is being produced, the board will be inverted, an adhesive will be dispensed at the centroid of each SMD, parts will be placed, the adhesive will be cured, the assembly will be re-righted, through-hole components will be mounted, and the circuit assembly will then be wave soldered, which will create acceptable solder joints for both the through-hole components and bottom-side SMDs. It must be noted that successful wave soldering of SMDs requires a dual-wave machine with one turbulent wave and one laminar wave. For any of the type assemblies that have THT on top side and SMDs (including SMT, fine pitch, ultrafine pitch, BGA, flip chip, etc.) top and bottom, the board will first be inverted, adhesive dispensed, SMDs placed on the bottom-side of the board, the adhesive cured, the board re-righted, through-hole components placed, and the entire assembly wave soldered. It is imperative to note that only passive components and small active SMDs can be successfully bottom-side wave soldered without considerable experience on the part of the design team and the board assembly facility. It must again be noted that successful wave soldering of SMDs requires a dual-wave machine with one turbulent wave and one laminar wave. The board will then be turned upright, solder paste deposited, the top-side SMDs placed, and the assembly reflow soldered. It is common for a manufacturer of through-hole boards to convert first to a Type 2C (SMD bottom side only) substrate design before going to an all-SMD Type I design. Since this type of board requires only wave soldering, it allows amortization of through-hole insertion and wave soldering. Many factors contribute to the reality that most boards are mixed-technology boards. While most components are available in SMT packages, through-hole connectors may still be commonly used for the additional  strength that the through-hole soldering process provides, and high-power devices such as three-terminal regulators are still commonly through-hole due to off-board heat-sinking demands. Both of these issues are actively being addressed by manufacturers and solutions exist that allow all-SMT boards with connectors and power devices.

Again, it is imperative that all members of the design, build, and test teams be involved from the design stage. Today’s complex board designs mean that it is entirely possible to exceed the ability to adequately test a board if test is not designed in, or to robustly manufacture a the board if in-line inspections and handling are not adequately considered. Robustness of both test and manufacturing are only assured with full involvement of all parties to overall board design and production.

There is an older definition of types of boards that the reader will still commonly find in books and articles, including some up through 1997. For this reason, those three types will be briefly defined here, along with their soldering techniques. The reader is cautioned to be sure which definition of board types is being referred to in other publications. In these board definitions, no distinction is made among the various types of SMDs. That is, SMD could refer to standard SMT, fine-pitch, ultrafine-pitch, BGAs, etc. This older definition was conceived prior to the use of the various chip-on-board and flip chip technologies and does not consider them as special cases.

Type I, II, and III SMT circuit boards.

• Type 1: an all-SMT board, which could be single- or double-sided. Reflow soldering is used and
is one-pass for a single-sided board. For a double-sided board, several common techniques are
used:
– Invert board, deposit adhesive at centroid of parts. Deposit solder paste. Place bottom side
in eutectic paste, cure adhesive, then reflow. Invert board, place top-side parts in eutectic
paste, reflow again.
– Invert board, place bottom-side parts in eutectic paste, reflow. Invert board, place top-side
parts in eutectic paste, reflow again. Rely on the surface tension of the molten solder paste to
keep bottom-side parts in place.
– Invert board, place bottom-side parts in high-temperature paste. Reflow at appropriate high
temperature. Invert board, place top-side components in eutectic paste, reflow again. The
bottom-side paste will not melt in eutectic reflow temperatures.
• Type 2: a mixed-technology board, composed of THT components and SMD components on the
top side. If there are SMD components on the bottom side, a typical process flow will be:
– Invert board, place bottom-side SMDs in adhesive. Cure adhesive.
– Invert board, place top-side SMDs. Reflow board.
– Insert THT parts into top side.
– Wave solder both THT parts and bottom-side SMDs.
Typically, the bottom-side SMDs will only consist of chip devices and SO transistors. Some
publications will also call Type 2 a Type 2A (or Type IIA) board.
• Type 3: a mixed technology board, with SMDs only on the bottom side, and typically only chip
parts and SO transistors. The typical process flow would be:
– Invert board, place bottom-side SMDs in adhesive. Cure adhesive.
– Insert THT parts into top side.
– Wave solder both THT parts and bottom-side SMDs.
Due to this simplified process flow, and the need for only wave soldering, Type 3 boards are an
obvious first step for any board assembler moving from THT to mixed-technology boards. Some
publications will also call Type 3 a Type 2B (or Type IIB) board