The advantages and disadvantages of SMT Technology

SMT was developed to minimize manufacturing costs while making efficient use of board space. The introduction of SMT technology made it possible to manufacture smaller complex circuit boards. There are several advantages and disadvantages of surface mount technology that we will discuss throughout this article.

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The birth of surface mount technology

Surface mount technology was developed in the s and was widely used in the s. In the s, it was used in most high-end PCB assemblies.

Conventional electronic components were redesigned to include metal separators or covers that could be attached directly to the board surface. This replaced the typical wires that had to go through drilled holes.

SMT led to much smaller components and allowed for the placement of components on both sides of the board. Surface mounting allows for a greater degree of automation, minimizing labor costs and the expansion of production rates, which results in the advanced development of the plates.

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Key features of SMT and through-hole technology

SMT allows electrical components to be mounted on the board surface without any drilling. Most electronic applications prefer to use surface-mount components since they are compact and can be installed on both sides of a printed circuit board. They are suitable for applications with higher routing densities. These components are smaller or have no cables and are smaller than components with through-hole components.

The process involved in the SMT assembly is:

• Apply solder paste to the fabricated circuit board using stencils. Solder paste is composed of flux and tin particles.
• Place the mounting components on the surface.
• Use a reflow soldering method.

In through-hole technology, component cables are inserted into holes drilled in the board. These cables are then soldered to pads on the opposite side using wave soldering tools or reflow soldering tools. Because hole mounting offers strong mechanical connections, it is highly reliable. However, drilling PCBs during production tends to increase manufacturing costs. Additionally, through-hole technology limits the routing area for signal traces below the top layer of multilayer PCBs.

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Key differences between through-hole technology and surface mount technology

• SMT frees the board space limitation placed by through-hole technology.
• Through-hole components entail higher manufacturing costs than SMT components.
• Advanced design and production skills are required to utilize SMT technology compared to through-hole technology.
• SMT components may have a larger number of pins compared to hole components.
• Unlike through-hole technology, SMT allows assembly automation, which is suitable for high production volumes at lower costs compared to through-hole production.
• SMT components are more compact, which leads to a higher component density compared to mounting through holes.
• Although surface mounting leads to lower production costs, the capital investment in machines is higher than that required for through-hole technology.
• Hole mounting is more suitable for producing large, bulky components subject to periodic mechanical stress or even high-tension, high-power parts.
• SMT makes it easier to achieve higher circuit speeds due to its small size and smaller number of holes.

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Factors to consider before choosing SMT technology or through-hole technology:

• Component stability when exposed to external stresses.
• Easy thermal management/heat dissipation.
• Availability of the part and its alternative.
• Cost-effectiveness of the assembly.
• High performance and package lifespan.
• Facilitate rework in case of board failure.

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Advantages of surface mount technology:

• Surface mount technology supports microelectronics by allowing more components to be placed closer to each other on the board. This leads to lighter and more compact designs.
• The SMT production setup process is faster when compared to through-hole technology. This is due to the fact that the components are assembled using solder paste instead of instead of holes. It saves time and intensive work.
• The components can be placed on either side of the circuit board, along with a higher density of components with more possible connections per component.
• Due to the compact size of the package, the traces can be of greater density and can be accommodated on the same layer.
• The surface tension of the melted solder pulls components into alignment with the solder pads, which automatically corrects minor placement problems.
• Compared to through holes, these do not increase in size during operation. Thus, it is possible to reduce the space between packages.
• Electromagnetic compatibility is easily achieved in SMT plates due to their compact packaging and lower lead inductance.
• SMT technology allows for lower resistance and inductance in the connection. It mitigates the undesirable effects of RF signals and provides better performance at high frequencies.
• More parts can easily fit the board due to its compactness, resulting in shorter signal paths. This improves signal integrity. The heat dissipated is also lower than that of the through-hole components.
• SMT reduces plate and material handling costs.
• It allows you to have a controlled manufacturing process. This is especially suitable for the production of high-volume PCBs for better organization and appearance, since the image we pass on to those who visit us makes all the difference.

Disadvantages of surface mount technology

While SMT has several advantages, it also has some drawbacks:

• When subjecting components to mechanical stress, it is not reliable to use surface mounting as the only method of attachment to the printed circuit board. This is due to the fact that it is necessary to use component connectors to interface with external devices that are periodically removed and replaced.
• Solder connections for SMDs can be damaged by thermal cycles during operations.
• Highly qualified or specialized operators and expensive tools are required for component repair and manual prototype assembly. This is due to smaller sizes and breakthrough spaces.
• Most SMT component packages cannot be installed in sockets that allow easy installation and replacement of faulty components.
• Less solder is used for SMT solder joints, so the reliability of solder joints becomes a concern. The formation of voids can lead to failure of the welding joints.
• SMDs are typically smaller than through-hole components, leaving a surface for marking part IDs and component values. This fact makes identifying components a challenge during printed circuit board prototyping and repair.
• The solder may melt when exposed to intense heat. Therefore, SMT cannot be implemented in electrical charge circuits with high heat dissipation.
• Printed circuit boards that use this technology require more installation costs. This is due to the fact that most SMT equipment, such as the hot air rework station, pick-up and placement machine, solder paste screen printer, and reflow oven are expensive.
• The miniaturization and variety of welding joints can make the most difficult procedure and process inspection difficult.
• Due to the compact size, there is a greater likelihood of solder overflow that can result in short circuits and solder bridges.

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When to use surface mount technology?

Most of the products currently manufactured use surface mount technology.

But SMT is not suitable in all cases. Consider SMT if:

• It is necessary to accommodate a high density of components.
• The need is for a compact or small product.
• Your final product needs to be elegant and lightweight, despite the density of components.
• The requirement specifies the high speed/frequency operation of the device.
• It is necessary to produce large quantities with automated technology.
• Your product should produce very little noise (if any).

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Welding techniques used in SMT

Solder reflow and wave solder are widely used to mount components on the board. Depending on the nature of the components, the designer may choose one of these methods for surface mounting technology.

Wave soldering: Since the solder will flow through the holes to form a bond, wave soldering is primarily used for through-hole components. You can use wave soldering for most surface mount components.

Solder reflow: This process is generally preferred in SMT. Here, the solder on one pin melts and reflows faster than the other. The only disadvantage is the fact that it causes a tombstoning effect, in which the component detaches from the unmelted cushion. This effect is common for surface-mount components such as resistors, capacitors, and inductors.

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Guidelines for placing SMT components

Here are some recommendations for placing SMD to maintain good signal integrity and the energy of your board:

• Keep components as close together as possible to minimize routing distance.
• Follow the signal path according to the scheme, when placing the components.
• Never place components in the return path of sensitive signals. This leads to signal integrity issues.
• For high speed devices, place the shunt capacitors closer to their power pins. This will reduce parasitic inductance.
• Place the SMDs together in the power circuits. This will help you provide shorter cuts and reduce inductance in the connections.
• Try to keep the SMT components on one side of the board to reduce the associated costs associated with stencils and assembly.
• Maintain minimum spacing between test points and SMT components, as specified by your manufacturer. This spacing may vary depending on the height of the component.

To facilitate the assembly process, ensure that all component names, polarities, orientations, and component placements are correctly marked on the mounting drawing. The prints present in the drawings must match the actual pieces. Consult your manufacturer for their kit assembly guidelines if you are considering consigned assembly.

Efficiency improving technique for superscalar CPUs

Simultaneous multithreading (SMT) is a technique for improving the overall efficiency of superscalar CPUs with hardware multithreading. SMT permits multiple independent threads of execution to better use the resources provided by modern processor architectures.

Details

The term multithreading is ambiguous, because not only can multiple threads be executed simultaneously on one CPU core, but also multiple tasks (with different page tables, different task state segments, different protection rings, different I/O permissions, etc.). Although running on the same core, they are completely separated from each other. Multithreading is similar in concept to preemptive multitasking but is implemented at the thread level of execution in modern superscalar processors.

Simultaneous multithreading (SMT) is one of the two main implementations of multithreading, the other form being temporal multithreading (also known as super-threading). In temporal multithreading, only one thread of instructions can execute in any given pipeline stage at a time. In simultaneous multithreading, instructions from more than one thread can be executed in any given pipeline stage at a time. This is done without great changes to the basic processor architecture: the main additions needed are the ability to fetch instructions from multiple threads in a cycle, and a larger register file to hold data from multiple threads. The number of concurrent threads is decided by the chip designers. Two concurrent threads per CPU core are common, but some processors support many more.[1]

Because it inevitably increases conflict on shared resources, measuring or agreeing on its effectiveness can be difficult. However, measured energy efficiency of SMT with parallel native and managed workloads on historical 130 nm to 32 nm Intel SMT (hyper-threading) implementations found that in 45 nm and 32 nm implementations, SMT is extremely energy efficient, even with in-order Atom processors.[2] In modern systems, SMT effectively exploits concurrency with very little additional dynamic power. That is, even when performance gains are minimal the power consumption savings can be considerable.[2] Some researchers[who?] have shown that the extra threads can be used proactively to seed a shared resource like a cache, to improve the performance of another single thread, and claim this shows that SMT does not only increase efficiency. Others[who?] use SMT to provide redundant computation, for some level of error detection and recovery.

However, in most current cases, SMT is about hiding memory latency, increasing efficiency, and increasing throughput of computations per amount of hardware used.[citation needed]

Taxonomy

In processor design, there are two ways to increase on-chip parallelism with fewer resource requirements: one is superscalar technique which tries to exploit instruction-level parallelism (ILP); the other is multithreading approach exploiting thread-level parallelism (TLP).

Superscalar means executing multiple instructions at the same time while thread-level parallelism (TLP) executes instructions from multiple threads within one processor chip at the same time. There are many ways to support more than one thread within a chip, namely:

• Interleaved multithreading: Interleaved issue of multiple instructions from different threads, also referred to as temporal multithreading. It can be further divided into fine-grained multithreading or coarse-grained multithreading depending on the frequency of interleaved issues. Fine-grained multithreading&#;such as in a barrel processor&#;issues instructions for different threads after every cycle, while coarse-grained multithreading only switches to issue instructions from another thread when the current executing thread causes some long latency events (like page fault etc.). Coarse-grain multithreading is more common for less context switch between threads. For example, Intel's Montecito processor uses coarse-grained multithreading, while Sun's UltraSPARC T1 uses fine-grained multithreading. For those processors that have only one pipeline per core, interleaved multithreading is the only possible way, because it can issue at most one instruction per cycle.
• Simultaneous multithreading (SMT): Issue multiple instructions from multiple threads in one cycle. The processor must be superscalar to do so.
• Chip-level multiprocessing (CMP or multicore): integrates two or more processors into one chip, each executing threads independently.
• Any combination of multithreaded/SMT/CMP.

The key factor to distinguish them is to look at how many instructions the processor can issue in one cycle and how many threads from which the instructions come. For example, Sun Microsystems' UltraSPARC T1 is a multicore processor combined with fine-grain multithreading technique instead of simultaneous multithreading because each core can only issue one instruction at a time.

Historical implementations

While multithreading CPUs have been around since the s, simultaneous multithreading was first researched by IBM in as part of the ACS-360 project.[3] The first major commercial microprocessor developed with SMT was the Alpha (EV8). This microprocessor was developed by DEC in coordination with Dean Tullsen of the University of California, San Diego, and Susan Eggers and Henry Levy of the University of Washington. The microprocessor was never released, since the Alpha line of microprocessors was discontinued shortly before HP acquired Compaq which had in turn acquired DEC. Dean Tullsen's work was also used to develop the hyper-threaded versions of the Intel Pentium 4 microprocessors, such as the "Northwood" and "Prescott".

Modern commercial implementations

The Intel Pentium 4 was the first modern desktop processor to implement simultaneous multithreading, starting from the 3.06 GHz model released in , and since introduced into a number of their processors. Intel calls the functionality Hyper-Threading Technology, and provides a basic two-thread SMT engine. Intel claims up to a 30% speed improvement[4] compared against an otherwise identical, non-SMT Pentium 4. The performance improvement seen is very application-dependent; however, when running two programs that require full attention of the processor it can actually seem like one or both of the programs slows down slightly when Hyper-threading is turned on.[5] This is due to the replay system of the Pentium 4 tying up valuable execution resources, increasing contention for resources such as bandwidth, caches, TLBs, re-order buffer entries, and equalizing the processor resources between the two programs which adds a varying amount of execution time. The Pentium 4 Prescott core gained a replay queue, which reduces execution time needed for the replay system. This was enough to completely overcome that performance hit.[6]

The latest Imagination Technologies MIPS architecture designs include an SMT system known as "MIPS MT".[7] MIPS MT provides for both heavyweight virtual processing elements and lighter-weight hardware microthreads. RMI, a Cupertino-based startup, is the first MIPS vendor to provide a processor SOC based on eight cores, each of which runs four threads. The threads can be run in fine-grain mode where a different thread can be executed each cycle. The threads can also be assigned priorities. Imagination Technologies MIPS CPUs have two SMT threads per core.

IBM's Blue Gene/Q has 4-way SMT.

The IBM POWER5, announced in May , comes as either a dual core dual-chip module (DCM), or quad-core or oct-core multi-chip module (MCM), with each core including a two-thread SMT engine. IBM's implementation is more sophisticated than the previous ones, because it can assign a different priority to the various threads, is more fine-grained, and the SMT engine can be turned on and off dynamically, to better execute those workloads where an SMT processor would not increase performance. This is IBM's second implementation of generally available hardware multithreading. In , IBM released systems based on the POWER7 processor with eight cores with each having four Simultaneous Intelligent Threads. This switches the threading mode between one thread, two threads or four threads depending on the number of process threads being scheduled at the time. This optimizes the use of the core for minimum response time or maximum throughput. IBM POWER8 has 8 intelligent simultaneous threads per core (SMT8).

IBM Z starting with the z13 processor in has two threads per core (SMT-2).

Although many people reported that Sun Microsystems' UltraSPARC T1 (known as "Niagara" until its 14 November release) and the now defunct processor codenamed "Rock" (originally announced in , but after many delays cancelled in ) are implementations of SPARC focused almost entirely on exploiting SMT and CMP techniques, Niagara is not actually using SMT. Sun refers to these combined approaches as "CMT", and the overall concept as "Throughput Computing". The Niagara has eight cores, but each core has only one pipeline, so actually it uses fine-grained multithreading. Unlike SMT, where instructions from multiple threads share the issue window each cycle, the processor uses a round robin policy to issue instructions from the next active thread each cycle. This makes it more similar to a barrel processor. Sun Microsystems' Rock processor is different: it has more complex cores that have more than one pipeline.

The Oracle Corporation SPARC T3 has eight fine-grained threads per core; SPARC T4, SPARC T5, SPARC M5, M6 and M7 have eight fine-grained threads per core of which two can be executed simultaneously.

Fujitsu SPARC64 VI has coarse-grained Vertical Multithreading (VMT) SPARC VII and newer have 2-way SMT.

Intel Itanium Montecito uses coarse-grained multithreading and Tukwila and newer ones use 2-way SMT (with dual-domain multithreading).

Intel Xeon Phi has 4-way SMT (with time-multiplexed multithreading) with hardware-based threads which cannot be disabled, unlike regular Hyper-Threading.[8] The Intel Atom, first released in , is the first Intel product to feature 2-way SMT (marketed as Hyper-Threading) without supporting instruction reordering, speculative execution, or register renaming. Intel reintroduced Hyper-Threading with the Nehalem microarchitecture, after its absence on the Core microarchitecture.

AMD Bulldozer microarchitecture FlexFPU and Shared L2 cache are multithreaded but integer cores in module are single threaded, so it is only a partial SMT implementation.[9][10]

AMD Zen microarchitecture has 2-way SMT.

VISC architecture[11][12][13][14] uses the Virtual Software Layer (translation layer) to dispatch a single thread of instructions to the Global Front End which splits instructions into virtual hardware threadlets which are then dispatched to separate virtual cores. These virtual cores can then send them to the available resources on any of the physical cores. Multiple virtual cores can push threadlets into the reorder buffer of a single physical core, which can split partial instructions and data from multiple threadlets through the execution ports at the same time. Each virtual core keeps track of the position of the relative output. This form of multithreading can increase single threaded performance by allowing a single thread to use all resources of the CPU. The allocation of resources is dynamic on a near-single cycle latency level (1&#;4 cycles depending on the change in allocation depending on individual application needs. Therefore, if two virtual cores are competing for resources, there are appropriate algorithms in place to determine what resources are to be allocated where.

Disadvantages

Depending on the design and architecture of the processor, simultaneous multithreading can decrease performance if any of the shared resources are bottlenecks for performance.[15] Critics argue that it is a considerable burden to put on software developers that they have to test whether simultaneous multithreading is good or bad for their application in various situations and insert extra logic to turn it off if it decreases performance. Current operating systems lack convenient API calls for this purpose and for preventing processes with different priority from taking resources from each other.[16]

There is also a security concern with certain simultaneous multithreading implementations. Intel's hyperthreading in NetBurst-based processors has a vulnerability through which it is possible for one application to steal a cryptographic key from another application running in the same processor by monitoring its cache use.[17] There are also sophisticated machine learning exploits to HT implementation that were explained at Black Hat .[18]

See also

References

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