Tag Archives: BGA

Advanced PCB Technologies — High Density Interconnect

Engineers often face a peculiar dilemma. On one hand, they need to enhance the functionalities of electronic gadgets they design so that customers have more value for their money, while they are constrained to use a sleek form factor. Not only does this impose a tremendous challenge to cram many components within a highly restricted space, but the challenge extends to maintaining the quality and integrity of the design as well.

Designers meet the challenge in different ways. They use subminiature passive SMD components, often as small as 0402 (0.4×0.2 mm), special fine pitch ICs in packages such as CSP, TQFP, and BGA, and advanced printed circuit technologies that offer thin flexible, multilayer boards, especially the high density interconnect (HDI) types.

Designers use several advanced technologies in producing HDI boards. For instance, rather than using glass fibers for producing the base substrate, HDI boards use Polyimide and similar materials, as these are flexible, more durable, and can withstand very high temperatures without degenerating.

Designers use special plated through vias to interconnect different layers in a multilayer HDI board. Rather than drill holes in the PCB layers using metal drills, fabricators of HDI PCBs use lasers to drill extremely small microvia holes in the layer, which they later electroplate with copper. Since these microvias can be as small as 15-30 µm, they take up very little space on the PCB, leaving a large area for routing the traces.

Designers use traces with width as small as 20 µm to route the circuits on HDI PCBs. In combination with microvias, these thin traces allow them to achieve extremely high routing densities impossible to achieve on regular boards. This is especially helpful when designing with fine pitch ICs and high pin count BGA IC packages that have a pitch as small as 0.5 mm.

BGAs are surface mounting packages with solder ball arrays on their bottom surface. Large BGAs may have as many as 560 solder balls. With pitch size as small as 0.5 mm, it is nearly impossible for designers to run traces from each pad under the BGA. However, engineers have solved this problem in a rather unique way.

In regular PCB design, using vias within pads is taboo, as this causes dry solders. The plated through via wicks away molten solder, leaving very little solder between the pad and the IC pin. However, designers regularly use via-in-pads in HDI PCBs, as this allows them to save a lot of space that they can use for routing. Molten solder does not travel down the microvia in HDI PCBs, as fabricators fill them up and plate them over. This has another advantage, as filled vias become better conductors of heat.

Another trick a designer often uses for gaining higher routing density in HDI PCBs is placing different types of vias such as blind and buried types. Vias connecting inner layers in a multilayer PCB are buried vias, while those originating on one of the outermost layers and connecting to one of more inner layers are blind vias. Unlike a through via that passes straight through the board, designers can stagger blind and buried vias in different layers to achieve higher routing density.

Variables in Lead-Free Reflow for PCBs

Reflow ovens often show degrees of variability from profile to profile. This may depend on the distribution of components on the board, especially those that are slow heating, heat-sensitive, or of high mass. In general, reflow systems cannot generate one single reflow profile producing capable thermal results for all products.

For instance, a large BGA package on the PCB may not allow more than five degrees of variation near the peak of the reflow profile curve. Therefore, even while the BGA joints show good soldering, there is a probability of frying some other smaller components nearby the BGA package.

Variables during reflow can also be the result of several external factors. There may be limited control for some factors, but others could be uncontrollable. For instance, in some cases, the PCB may be non-uniform, its components may have varying thermal characteristics, or the tolerances of the process controller could be the major contributor. Even the exhaust could contribute as an external factor.

Oven loading is another major factor when creating custom reflow profile for a high-layer-count PCB. The reflow oven characteristics depend on the number of PCBs passing through it, as the total mass of the PCBs and their speed through the oven influences the rate of rise of temperature. Usually, the load capacity of the reflow oven is measured in boards per minute, and this value differs when only a single PCB is passing through as against a batch of several PCBs passing through at a time.

Customers often demand demonstrable settings of the custom reflow profile for their boards. It may be necessary to demonstrate that a given setting fulfills the requirements of a thermal profile for the board, without damaging any other component on it. Sometimes there are requirements to create documentation as evidence that a particular assembly is indeed within specifications. One of the advantages of creating custom profile for a board is it brings a total visibility to the lead-free reflow process when handling that board.

Automated Methods for Lead-Free Reflow

A reflow profiler, such as the one made by KIC, is the most popular method assemblers use for profiling groups of boards they assemble. The instrument works equally well when profiling individual boards automatically and continuously. Assemblers also use it for a cluster of boards consisting of two or three categories.

The KIC profiler has a Navigation prediction software accompanying it. This helps to drive the generated profile deeply within the specifications of the board. Typically, actual profiles need to be run on some boards that match the representative profile for that group. The process must be repeated periodically to ensure the settings remain valid. Used along with the Navigation prediction software, the KIC profiler saves much time and effort when creating lead-free reflow characteristics for high-layer-count PCBs.


Lead-free reflow of high-layer-count PCBs need not be a tiresome exercise provided it is possible to set up a custom reflow profile for a group of PCBs with similar thermal characteristics. Using modern thermal profilers makes the job economical and fast.