## The Self-Alignment of Reflowed Surface Mount Components

Misplacement of surface mount components by a pick and place machine or a human assembler can be inconvenient.  Fortunately, surface mount components seem to magically (and conveniently) align themselves when they go through the reflow oven. What is the cause of this favorable phenomenon? Well it’s as intuitive as being on a trampoline.

When you stand on a trampoline, the fabric bends down and exerts an upwards elastic force. You remain still because that force is balanced with the downwards gravitational force. You’ll also feel yourself leaning towards the center of the trampoline, as that is the point at which your weight would be equally distributed across the entire surface of the trampoline. Nature mostly tends towards equilibrium.  Now imagine instead of being on a trampoline, you’re on a square platform that is being balanced on four separate trampolines. Assume that none of the trampolines have a frame you could hit, and are placed symmetrically at the corners of your square platform. No matter how you jump and try to spin the platform, the corners will slide towards the center of each trampoline when you stop.

This is similar to what’s happening in the reflow oven, except instead of a platform and trampolines, it’s your surface mount component and solder deposits. When the solder turns liquid, it acts like a trampoline because of surface tension. The component is then aligned to the position which spreads its weight equally across all of the deposits. There are of course other forces at work (hydrostatic and capillary pressure, dynamic friction, gravitational), but surface tension plays the primary role in aligning the component.

When the solder becomes a liquid, it attempts to minimize its surface area (this is why it has a surface tension in the first place). The reason behind this is because all of the molecules try to have the smallest amount of free energy, which is when they are connected to/completely surrounded by other identical/similar molecules. This means that the molecules in the bulk of the material have less free energy than the molecules on the surface of the material (if the reverse were true, materials would tend towards forming surfaces rather than being in bulk). When the solder reflows, it is already inherently in favor of bonding with an identical/similar material in order to minimize the number of molecules that are on the surface and thus decreasing its free surface energy. The substrate of the QFN is made from ceramic, which holds its atoms together using ionic and covalent bonds (much stronger than metallic bonds). Trying to get the solder to bond to ceramic is like trying to mix a liquid with a solid; you might get a few drops to stick, but most of it will slide right off. On the other hand, bonding metal together is like mixing different liquids; some will separate, but most mix easily.

To what extent can the component realign itself? The answer to this question will vary depending on the geometry and size of the component. A quick experiment using mixers in 3mm (0.114”) QFN packages showed that components were able to realign themselves from up to a 9-degree offset and 0.2 mm (0.008”) horizontal translation. The next experiment showed that that a 10-degree angle could also be corrected, whereas a 12-degree angle would be more disorientated. Furthermore, any translation over 0.3 mm (0.012”) wouldn’t be realigned. What’s the significance of these values?

The value for translation is fairly easy to explain. Since the width of the pads on the QFN is 0.3 mm (0.012”), anything over 0.3 mm would mean that there is no contact between both the top and bottom pads (labelled 1 and 2 in figure 3) and the solder. Going back to the trampoline analogy, that would be like moving the platform so far to the left that the two trampolines on the right were no longer part of the system. Assuming that the platform can’t fall, it would be rebalanced on the two trampolines to left. With the QFN there are still four traces in contact with the pads, but the top and bottom pads (labeled 3 and 4 on figure 3) are not the correct ones and due to adhesive forces, pull the package off the trace on the right, and the balance is redistributed across three points of solder.

Explaining the value for the angle isn’t as simple, but still fairly intuitive. At a 10-degree angle, the surface area of the QFN’s middle pad that is overlapping with the trace is greater than the outer pad’s overlapping surface area. As a result, more of the solder adheres to the QFN’s middle pad and cohesive forces between the solder molecules pull it further into the correct position. In the case of the 12-degree angle, the overlapping surface area of the outer pad is greater than the middle pad, and so the QFN is pulled into a position that maximizes the area of contact between the outer pads and the trace.

Although we didn’t run experiments with other surface mount components, we can extrapolate from these results that self-alignment will be affected by the size of your components and traces, the number of traces required for that component, and the component’s substrate. Our QFNs have a large potential for self-correction, since they’re relatively small, have four symmetrical traces that act as alignment points, and are on a ceramic substrate that doesn’t bond well with solder. On the other hand, a larger component with only three traces on a metallic substrate would likely have a smaller potential for self-correction. What does this mean for your assembly process? If you’re not already using a reflow process, you should consider it. Not only is it forgiving in terms of component misplacement, but the tendency towards self-alignment can also reduce reflection losses from pad to trace misalignment. The relatively large margin for error shouldn’t affect the quality of your process (it doesn’t here at Marki), at least you can rest easy knowing that a small hitch usually won’t have catastrophic results when it comes to surface mount assembly.

Determining how a component will perform for your application using only a datasheet is difficult and unreliable. If you’re interested in seeing how Marki products would actually fit into your system, contact [email protected] to obtain some of our multicomponent evaluation boards. With a diverse range of configurations made up of bias tees, amplifiers, and mixers, you can find the optimal solution for your needs.

## Power Handling

Maximum power handling is a common concern of our customers. Nothing is worse than plugging in an expensive device only for it to be immediately destroyed. Understanding power handling is important to us as well so that we can address customer concerns about reliability and to develop devices that are able to tolerate higher power levels. The purpose of this document is to describe the failure mode and maximum power handling of several Marki Microwave product lines. It lists the power required to destroy the device and describes what part of the device fails at that power.

## How to Measure Mixer IP3 and Identify Potential Error Sources

Mixer linearity is continuously and permanently a critical problem faced in RF system design. The nonlinear action of all physically realizable RF mixers propagates throughout signal chains generating undesired, unfilterable output harmonics, multitone intermodulation, and nonrecoverable nonlinear signal distortion. For example, nonlinear mixing action can cause undesired output harmonics (i.e., spurs) such as the $2f_{RF} \times 2f_{LO}$ or the $2f_{RF} \times f_{LO}$ spur instead of the desired $f_{RF} \times f_{LO}$ converted signal. In multitone applications, such as data transmission and radar tracking, mixing heavily exacerbates the problem of spectral purity by not only introducing a second set of unwanted, unfilterable output harmonics but by also introducing multitone intermodulation distortion (IMD). IP3, or the 3rd order intercept point (TOI), is the figure of merit by which industry judges the linearity of all active, power-consuming RF components and their ability to maintain core linear assumptions about circuits. The mixer is no exception.

A power calibrated vector network analyzer and an external driving synthesizer is the standard tool to do IP3 measurements. While VNAs have their own errors, such as limited RF input power and poor 2nd input harmonic suppressions, it is the most convenient and time efficient way to measure IP3 over broadband frequency sweeps. Spectrum analyzers, although highly susceptible to user error, provide a secondary way of doing comparable IP3 measurements. Both measurement on a spectrum analyzer and a VNA should agree. This document serves as a tool for debugging, developing, and utilizing the spectrum analyzer, it’s supporting synthesizers, and other RF paraphernalia for mixer IP3 measurements.

## GaAs or GaN: Best Choice for Highly Linear Mixers?

Everywhere you look in the microwave industry press or at the international microwave symposium you see one topic mentioned over and over again: Gallium Nitride (GaN). There is presently a gold rush in the industry to produce new varieties of products in GaN. The wide bandgap and high electron mobility of GaN mean that it is capable of a much higher power density than Gallium Arsenide (GaAs).  The introduction and availability of a 0.15 micron commercial GaN processes means that fabless integrated circuit companies (such as Marki) can produce GaN microwave products at frequencies comparable to GaAs. This has led to a profusion of products including power amplifiers, low noise amplifiers, driver amplifiers, and many other amplifiers using GaN that set new records for power at high frequency.

One thing that is fueling the drive to GaN is the increasingly stringent linearity requirements placed on modern RF/Microwave systems. Since the noise floor of a system can only be reduced so much, the only way left to increase the dynamic range is to increase the power compression level and decrease the intermodulation products to create a wider spur free dynamic range. At the heart of most RF receivers is a mixer that limits the linearity and therefore the dynamic range of the receiver. So the next step looks obvious; GaN is increasing the dynamic range of many components in the system, so it should increase the dynamic range of the mixer as well!

There are five main reasons that this logic doesn’t work:

## An Introduction to Microwave Amplifiers Part 1: Microwave Amplifier Applications

This is a (virtually) math-free introduction to microwave amplifiers from an applications standpoint. There are many references available for the aspiring amplifier designer; this series of posts will attempt to quickly elucidate the relevant factors for the RF system design engineer working to evaluate the appropriate amplifier for her system design.

Types of Microwave Amplifiers

There are many ways to classify microwave amplifiers, but we will group them into four categories based on what role they would play in a generic superheterodyne receiver (shown above). The above system could represent a cellular, backhaul, satellite, or other radio communications link; it could also represent a radar or other imaging system. The transmitter alone could represent a jammer or exciter, and the receiver alone could represent an electronic warfare scanner or a test instrument. The requirements that drive the amplifier selection will be the same for most applications.

## How to Determine the Maximum Power Handling of an RF/Microwave Directional Coupler

The most common question we receive about our stripline directional couplers, low loss airline directional couplers, and high directivity directional bridges is ‘How much power can it handle?’. The reason is that directional couplers are frequently used for load-pull testing of amplifiers or monitoring a signal after a power amplifier. In either case, the directional coupler will be placed as close to the output of the power amplifier as possible, which means it must perform within spec at high operating powers.

Despite the importance of directional coupler performance under high power inputs, most directional coupler vendors offer a single number without any background or context. As we will show in this post, the static value commonly provided for the power handling of a directional coupler is an oversimplification of the matter.

## Driving Doublers with Amplifiers

Frequency multipliers are used to generate higher harmonics from an input sinusoid.  In particular, the job of the doubler is to output only the 2nd harmonic.  Invariably, the fundamental tone and higher harmonics will leak to the output as well; how much lower the power of these tones are compared to the 2nd harmonic is known as their “suppression”.

Doublers generally require a large input signal (greater than 12 dBm) to work properly.  To reduce stress on your signal generator, you might consider feeding a smaller signal into an amplifier, and using the amplifier to drive the doubler.  Unfortunately, you will observe that the suppressions of the fundamental and 3rd harmonic are much worse with the amplifier attached.  What happened?  Why should the amplifier make a difference?

## MM1-2567 Operation to 80 GHz

We occasionally receive requests for a mixer that will operate above our highest frequency mixer, the MM1-2567LS. The truth is that, contrary to the datasheet, the MM1-2567 actually operates above 67 GHz.  It was designed to operate up to a frequency of 80 GHz on the RF and LO sides. The issue is that our test equipment can only measure conversion losses up to 67 GHz directly. To solve this problem, I devised an experiment to prove whether the mixer frequency actually extended to it’s simulation limit or not. Here is the experimental setup:

## Repeatability of T3 Mixers and Other Handmade Microwave Components in Six Charts

T3 mixers are the highest dynamic range mixer available. They are also handbuilt parts, subject to unit to unit and lot to lot variability. In this blog post we attempt to quantify that variability. Our sample is 10 T3-08LQP mixers from 5 different date codes. All the date codes are separated by at least a month, totaling nearly two years.  Therefore, the variation you see in the plots below accurately represent the variation a designer could expect across two years in the life of their product. Of course there are always outliers, but the following represents typical performance variation.

Conversion Loss

## New MMIC Mixers from Marki Microwave Cover 3-24 GHz

Marki is bringing advanced mixer designs to a broader market with four new models of GaAs Schottky diode double balanced mixers covering S and K band applications.  These designs combine the legendary mixer design expertise of Marki Microwave with the repeatability and economies of scale intrinsic in the MMIC production method.