전자부품 극소형 저항 레이저 트리밍 기술

Microelectronics Laser Trim Resistors

 

저항이 인쇄 될 때, HIC, Chip resistor 그리고 임베디드PWB 제조사는 수십 종의 잉크를 사용하여 1ohm, 10ohms, 100ohms, 1Kohms 등 최대 100Mohms를 인쇄했다. 예를 들어, 1Kohms 잉크로 인쇄하면 스퀘어 당 1Kohms의 값을 가질 것이다. 3개의 스퀘어가 직렬로 연결되면 3Kohms로 x3의 값을 가질 것이다. 따라서 저항의 형상을 변경함으로써, 임의의 값을 달성할 수 있다.

저항은 항상 목표 값보다 낮은 15~20%의 범위를 가지도록 설계되어 있다. 그 이유는 저항체를 레이저로 트리밍하여 단면적을 줄여나가면서, 목표값을 향해 위쪽으로 조정할 수 있기 때문이다.

When resistors are printed, they are printed using decade inks, 1O, 10O, 100O, 1kO etc up to 100MO. For example, one printed square of a 1kO ink will have a value of 1kO. 3 squares long will have a value of 3kO and 3 squares wide will have a value of 333O. Thus by varying the geometry of the resistor, any value can be achieved.

Resistors are always designed to fire around 15-20% low of target value. This is because you can only laser adjust upwards in value.

 

레이저 트리밍 시스템은 정밀도가 높은 브리지 측정 시스템에 연결된 프로브 카드를 사용한다. 각 저항은 링 형태의 배열을 가진 프로브 핀에 의해 측정된다. 사전 프로그램된 좌표를 사용하여 Nd:YAG 레이저가 0.001"(25.4um) 폭의 컷을 하여 저항체 물질을 기화시키고, 중첩 펄스로 일직선으로 저항체를 깍아 나간다. 필요한 값까지 증가하고 저항 값을 측정하여 도달하면 레이저가 차단된다. 레이저 트리머 시스템은 통상 10,000개/시간 의 저항을 트리밍 할 수 있다.

The laser-trimming machine incorporates a probe ring connected to a highly accurate bridge measuring system. Each resistor is probed and the value is measured by the bridge. Using pre-programmed co-ordinates, a NdYg laser is fired at the resistor in a series of overlapping pulses which vaporises the resistor material in a 0.001" wide cut. All the time, the bridge is measuring the increasing resistance value until the required value is reached and the laser is cut off. The laser trimmer machine is capable of trimming 10,000 resistors per hour.

 

컷의 여러 가지 유형은, 직선 플런지 컷, 저항의 큰 변화를 일으키는 써펜틴 컷, L 모양의 컷이 있으며, 가장 적합한 컷으로 저항 노이즈를 최소화하기 위하여 더 천천히 최종 값에 접근하고, 저항의 핫스팟과 에지 스크럽을 최소화하지만, 그 결과는 트림시간이 길어진다.

There are several different types of cut, a straight plunge cut, a serpentine cut for large changes, an L shaped cut which approaches the final value more slowly and minimises resistor hot-spots and an edge-scrub, which is best for minimising resistor noise, but takes much longer to trim.

 

Thick film resistor fabrication

두꺼운 필름의 구성 요소는 보통 알루미나와 같은 하드 기판에 도체와 저항 "페이스트"를 스크린 및 소성에 의해 만들어진다. 후막 기술에서는, 박막 (99%)에 비해 감소된 순도의 알루미나(95%)가 사용된다. 저항재질은 탄소 기반 (저항이은 검은 색으로 나타나는 이유)으로 두꺼운 필름 저항은 표면 실장 기술 (SMT)에 대한 다양한 크기의 칩 저항으로 할 수 있다. 일반적으로 850도 C 근처에서, 사용자 정의의 후막 하이브리드 네트워크 부품, 또는 저온 동시 소성 세라믹 (LTCC) 부품으로 제조된다. 만약 저렴한 표면 실장 칩 저항기를 구입한다면, 그들은 거의 대부분이 후막 공정에서 제조된 것이다.

Thick film components are made by screening and firing conductor and resistor "pastes" onto hard substrates, usually alumina. In thick-film technology, alumina of reduced purity is used (95%) compared to thin-film (99%). Firing temperature is usually around 850 degrees C. The resistor materiel is carbon-based (that's why the resistors appear black.) Thick film resistors can be made into chip resistors of various sizes for surface mount technology (SMT), or as part of custom thick-film hybrid networks, or even as part of a low-temperature co-fired ceramic (LTCC) part. When you buy inexpensive surface-mount chip resistors, they are almost always made in a thick film process.

 

감법 (화학 에칭) 대부분 박막 공정 반대로 두꺼운 필름 부착식 공법이다. 저항이 증착되기 전에 몇몇 후막 부분에서는, 도체 패턴을 인쇄하여 제 소성한다. 도체와 저항 재료는 스퀴지를 이용하여 스크린 프린팅 공정을 이용하여 증착된다. 이 박막 저항기에 비해 "낮은 기술"프로세스이며, 기판 물질은 낮은 비용 때문에, 후막 저항체는 거의 항상 박막보다 덜 비싸다.

Thick film is an additive process, as opposed to most thin-film processes which are subtractive (chemical etching). In some thick film parts, before resistors are deposited, a conductor pattern is printed and fired first. The conductor and resistor materials are deposited using a screen-printing process using a squeegee, similar to how Grateful Dead tee-shirts are made. Because it is a "low-tech" process compared to thin-film resistors, and the substrate material is lower cost, thick film resistors are almost always less expensive than thin-film.

 

후막 저항체의 시트 저항은 스퀘어 당 500K 옴까지 평방 당 20 옴에서, 박막보다 훨씬 높을 수 있다. 맞춤 작업에 대한 거의 모든 저항 값이 수용 될 수 있도록 즉, 광범위하게 변하는 저항율 여러 페이스트를 사용하는 것이 가능하고, 거의 같은 작은 영역의 각. 후막 저항체 값은 레이저 트리밍 할 수 있다. 전형적인 공정은 저항을 보호하기 위해, 600 ℃에서 소성된 유리의 오버코트를 추가한다.

Sheet resistance of thick-film resistors can be much higher than thin films, from 20 ohms per square up to 500K ohms per square. on a custom job it is possible to use multiple pastes with widely-varying sheet resistivity, so that almost any resistance value can be accommodated, each in nearly the same small area. Thick film resistor values can belaser trimmed. Typical processes add an overcoat of glass, fired at 600C, to protect the resistor.

 

 

Thick-film chip resistors

Chip resistors are used by the billions in surface-mount technology (SMT) assemblies. They come in standard sizes, in the USA the sizes are have been standardized to dimensions in mils (thousandths of an inch). Other countries use sizes in millimeters. Sizes that are most common on microwave surface-mount boards are 0603 (60 x 30 mils) and 0805 (80 x 50 mils). Smaller resistors are available in 0402 and even 0201 formats, but handling such small parts can make your surface-mount hardware more expensive. Larger resistors such as 1205 will start to cause trouble in microwave circuits at X-band and above, because they start to be large enough so that they are no longer can be modeled as lumped elements.

The ends of thick-film chip resistors are coated with conductor "wraparounds" as shown in light blue the figure below. The metal that is used is most often something that is easily solderable, and often a solder is wetted to the surface to facilitate SMT assembly.

 

 

What are the wraparounds for? Chip resistors with wraparounds can be mounted with the resistor surface (shown in orange) either up or down. Although most "low-frequency" boards mount the resistor face-up so that they can be easily inspected, in microwave applications resistor-down mounting can mean higher frequency performance.

Power rating/thermal considerations

Standard sizes for thick film chip resistors are given below, along with approximate power rating when mounted on a printed circuit board such as FR-4.

Size code	Size (mils)	Power rating (watts)
0201	20 x 10	1/20
0402	40 x 20	1/16
0603	60 x 30	1/10
0805	80 x 50	1/8
1206	120 x 60	1/4
2010	200 x 100	1/2
2512	250 x 120	1

The rated power dissipation is established by the maximum temperature that the resistor will see. This is a function of the part, as well as how it is mounted. This is a very complicated subject, and often consumes tens of thousands of dollars of analysis. Sometime soon we will provide we you the simplified version...

High-power resistors can employ high-conductivity substrates, such as BeO or AlN, which spread heat much better than alumina and reduce temperature rise. The downside to BeO is that it is a known carcinogen, so you should avoid using it in a design whenever possible. AlN is more inert, but doesn't dissipate as well as BeO.

Temperature coefficient of resistivity (TCR) for thick film resistors is usually 100 to 250 ppm. Thin-film resistors behave better over temperature.

Resistors must be linearly derated over temperature. We'll add a figure later that shows what this means...

After processing, thick and thin-film resistors will always have some statistical variation from their intended value. In the case of separate chip resistors, parts can be measured, then binned into 10%, 5% or even 1% tolerances (standard RETMA values). In larger format thick-film or thin-film networks where the resistors are just one small detail in complex artwork, often the final part will not work if the resistor value deviates too much, and out-of-spec resistors means scrapped parts. The alternative is to trim the resistors into the correct value. In laser trimming, a YAG laser is used to ablate the resistor material from the substrate, raising its resistor value.

 

There are three ways to laser-trim resistors, shown below. Plunge-cut gives the "fastest" resistor change, but from a reliability point-of-view, it may , may , L-cut, edge trim, etc.

 

In a "poor-man's version" of laser trimming that is sometimes used in prototype shops, you can abrade a resistor with emery paper to raise its value. Here you are actually decreasing the effective thickness of the resistor, rather than altering the number of squares. Thin-film resistors will have to be re-stabilized after this step, since you would be removing the oxide layer.

 

Thin-film resistors, though more expensive than thick-film, are the darling of the microwave resistor industry, because they have better electrical properties.

 

Thin-film resistor fabrication

The thin-film resistors start with a "hard" substrate such as silicon, GaAs or alumina. An extremely thin layer (hundreds of Angstroms) of resistor material is deposited over the entire substrate surface, using a sputtering process. Usually a conductor layer is deposited on top of the resistor layer. Using a photo-lithographic process the substrate is patterned and the two layers are etched away independently, so you have a part with both a conductor and resistor pattern.

Thin-film resistor materials are usually tantalum nitride (Tan) or nichrome. A limited range of sheet resistance is possible, from perhaps 5 ohms/square to 250 ohms per square. Thick film technology provides a far greater range.

Click here to learn more about tantalum nitride resistors and the skin effect.

 

Temperature coefficient of resistance (TCR)

Thin-film chip resistors usually are not offered, because thick-film is cheaper. However, in applications such as where temperature coefficient is important, thin-film chip resistors are sometimes used. Temperature coefficients lower than 25 PPM/C are possible.

According to Paul, typical TCR for nichrome resistor material is 0-50 ppm. TaN resistor material is typically -100 ppm.

 

Temperature stabilization

The sheet resistance of films such as tantalum nitride is known to increase with time as the surface layer oxidizes (the oxide is even less of a conductor than the original material). In order to minimize this effect, resistors are stabilized by baking them it temperatures as high as 400 degrees C for up to one hour. The resistance value can increase as much as 40% during the stabilization, so the original geometry must take this into account and the resistors must start out substantially below their target value. This is very effective in preventing further change in resistance over time. What is happening when resistors are stabilized? The top surface is oxidizing, which increases its resistance.