Whisker is a kind of spontaneous columnar or cylindrical filament, usually of mono-crystalline metal, emanating from the surface of a finished product. Sn whiskers have been an industrial concern and interesting problem for many years.

They are known to cause short circuits or possible reliability degradation in fine-pitch pre-tinned electrical components. Sn whiskers grow by the addition of material at their base not at their tip (i.e., they grow out of the substrate).

They can grow from as-formed electrodeposits, vapor deposited material, and intentionally deformed coatings of Sn. Similar whiskers are observed in Cd, In, and Zn. Whiskers appear to be a local response to the existence of residual stress. Compressive residual or external stress is usually considered a precondition for whisker growth.

Annealing or melting (reflow in solder terminology) may mitigate the growth for an undetermined period of time. Control methods of mitigation Sn whisker growth provided by Sunlord:

1) to select sulfonate plating liquid producing lowest compressive stress,

2) to select non-bright plating system to decrease compositions and contents of organic additives,

3) to manage and control thickness of nickel, under-layer of Sn,

4) to optimize plating process parameters to improve density and uniformity of Sn layers.

When mounting chip components on the PCB, if chips were defective such as gas or plating liquid exiting on one end of chip body, gas or plating liquid would dash out from the thinner area when the temperature is rising to some degrees.

The chips cannot keep balance because only one end is impacted. What happen next is that one end is stuck on board and the other end is upwarping. This defect is called tombstone.

Sunlord has solved this technical problem through improvement of plating and front-end process and has taken inspection method by simulating mounting machine operation and reflow to eliminate tombstone.

when loaded with large current, the permeability of magnetic material reduces, which makes the inherent inductance reduce too and thus the SRF increases, which makes the peak impedance move to higher frequency. At the frequencies lower than SRF, as the reactance comprises the majority of the impedance, the reducing inductance results in the decrease of impedance. Since the SRF of the bead is normally higher than 100MHz, and the nominal impedance decreases when loaded.

The equivalent circuit of beads is shown in the following figure.

L0 represents equivalent inductance; R0 represents equivalent resistance which includes DC resistance and AC resistance; C0 represents stray capacity. The impedance of beads includes resistance and reactance.

At the ultra-low frequencies (bellow 1MHz), AC resistance is very low and impedance is derived from the parallel of inductive reactance (wL0) and DC resistance. As the frequency is very low, the impedance of bead is very small in the ultra-low frequency.

At the low and middle frequencies (more than 1MHz to dozens of MHz), the AC loss increase gradually over the DC loss, but overall R0 is still relatively small and the inductive reactance (wL0) becomes a major part of the impedance with the increase of frequency.

In the high frequencies (dozens of MHz to hundreds of MHz, even GHz), the AC loss increases rapidly and R0 becomes a major part of the impedance. When the decreasing capacitance reactance (1/wC0) is offset by the increasing inductance reactance (wL0) at a certain frequency, this frequency is called SRF and the impedance reaches the peak value.

At the ultra-high frequencies (more than hundreds of MHz or GHz), the AC loss decreases and thus the total impedance decreases.

For EMI applications, the impedance chosen should be 2-3 times of the load impedance for that signal line. The impedance chosen should be as small as possible to suppress the EMI. This will insure that signal integrity is maintained.

Ferrites can suppress both conducted and radiated EMI. Since conducted EMI is confined to the conductor in which it is present, it is more efficacious to try to suppress the EMI close to the source on the conductor. Ferrite products are used to suppress radiated EMI. They may be placed directly on an IC or circuit trace. In either case the ferrite works by absorbing the unwanted frequencies and releasing the energy as microwatts of heat.

Figure 1 shows the typical curves of impedance vs. frequency characteristics of four types of GZ1608 beads. We can see the impedances at 100MHz of four types are all 120Ω, but the impedances at other frequencies are different. The four types of material (encoded with D, E, U and W) exhibit different electrical characteristics due to different formulation.

For example, the impedance of D material achieves the minimum value when the frequency is below 100MHz, on the other hand it achieves the maximal when the frequency is above 100MHz. The W material is the reverse. When choosing chip beads, the characteristic of circuit type must be considered.

Sunlord manufactures six series of chip beads, GZ series, SZ series, PZ series, UPZ series, HZ series and HPZ series.

GZ series generate impedance from relatively low frequencies, therefore are effective in noise suppression in a wide frequency range (30MHz-Several hundred MHz).

SZ series can minimize attenuation of the signal waveform due to its sharp impedance characteristic. Various impedances are available to match signal frequency.

PZ series can be used in high current circuits due to its low DC resistance and can match power lines to a maximum of 6A DC.

UPZ series are ultra large current beads with smaller DC resistance and larger allowable current than PZ series.

HZ series are similar to the above four series at frequencies below 100MHz, however at 1GHz the impedance is approx. 3 times larger.

HPZ series are similar to HZ series in impedance characteristics and have larger current than HZ series.

Sunlord product lines show common part numbers and its typical impedance-frequency characteristics of six series. Customers can choose suitable chip beads according to circuit types.