Testing process before PCB assembly

Question

I have a specific question regarding test and verification of circuits on PCBs such as the following one:

Since I am not from the electronics industry and never been in such a place I thought someone might have an answer to my question here.

Such a circuit on the above photo includes various components which are soldered on the PCB board above.

I am very curious how are each components or subsystems on such a a complex PCB are tested before assembling. Let's say such a circuit includes diodes and ADCs and voltage regulators ect.

Are these components tested one by one before being assembled/manufactured on the PCB? And secondly if they are tested on by one are there specific boards or probes for the tiny components which have extremely small pins?

Answer 1

The components are tested at the component manufacturer, and seldom tested in the ordinary assembly process. If the component supplier ships too many bad parts they will cease to be a component supplier for any reputable company.

Specialized (and often extremely expensive) test equipment is used to test semiconductors. There are probes that make contact with the pins. Test cost is often a significant part of the component cost. Even "almost free" parts such as resistors are tested and indeed trimmed to value, using expensive equipment.

Here is piece of automated test equipment (ATE) used for testing modern memory chips (from Advantest, a leading Japanese supplier). Naturally this sort of thing costs millions of dollars.

Parts may be tested more than once, at die level and after packaging. Sometimes tests are skipped for the toy market and such like where the packaged chip or COB PCB will simply be discarded if the IC fails to perform well enough. Sometimes test fallouts are shipped to countries with relatively low labor costs (for example, from a factory in China to customers in South Asia) where they can be sorted into "usable" and trash. This is common with LEDs, for example, where 75% usable parts may be considered okay. That would be completely unacceptable for customers of the A grade parts, of course.

Answer 2

I did my internship in an electronics company and had seen the whole process. They don't test every component. Just test a few from a batch of thousand components. For example they test 5 shift register from 1000.

And then they make 1 perfect PCB at the R&D section and place the components and test the circuit. If it is good then they start making duplicate copies of that PCB.

After manufacturing the PCB it is sent to the production line. All of the components are placed there in an effective order. Like first place the SMD, then small through-hole components, then sockets, etc.

After the complete product is manufactured (say an inverter) some workers test it. They have an automated system. So it doesn't take much time. The defective products are thrown away, which are later checked by engineers.

Answer 3

The board you see has rework (jumpers) applied to it. This means that the developers found problems in functional testing, and used the rework to correct them. This would be considered an 'alpha' quality unit in most places. Some low-end makers are willing to ship devices like this.

Chips are tested by the manufacturer on an ATE fixture. Some are tested at the wafer prior to singulation and at package level, others only at the package stage. Chip testing usually involves some level of scan, built-in self-test (BIST) and mixed-signal testing. These tests are designed to achieve maximum coverage in minimum time, usually only a few seconds for a complex IC.

At the board manufacturer, devices may undergo additional incoming screening, usually for specialty components destined high-cost or critical boards. This isn't the usual case, as this kind of testing is costly, disrupts the production flow, and can even cause problems later (moisture in BGA packages leading to 'popcorning' solder joints for example.)

Board-level testing has three main types:

• Functional Validation - exactly what you think. The unit is tested in its application for correct operation. Done on prototypes.
• Design Verification Test - an extension of Functional Verification, carried out under thermal, electrical and mechanical stress. Ensures conformance with the product spec. Done on pre-production units.
• Production Test - an optimized set of tests designed to identify likely manufacturing flaws. Done on production units.

The latter test may use a combination of techniques including:

• Automated Visual and/or x-ray inspection
• Self-test (diags)
• Boundary Scan / JTAG
• Bed-of-nails test
• Flying lead
• Limited functional test
• Burn-in / post-burn in test

Exactly what tests are done depend on the complexity of the device and the desired quality acceptance level.

Answer 4

It's not entirely clear what you are asking about.

Semiconductors (at least IC's) would typically be tested as bare dice before wire bonding to package leads since it may not be worth spending the money to bond out bad parts. Though they might bond out before testing for simple ones.

PCBs can be tested blank with spring contact probes, but often are not.

In many cases where process can be well controlled, statistically sampling rather than testing everything is done.

And the finished assembly might be functionally tested.

Or it might not be.

Buy enough cheap electronics online and you'll encounter something that shipped with obvious flaws like a part not aligned on its pads. Perhaps they didn't test. Or perhaps someone unscrupulously bought the bin of test failures and tried to sell them as good...

Answer 5

The people assembling the PCB do not test the individual components (well they might, but generally no). They generally rely on the component manufacturers to test the components before selling them. The machines used to test PCBs are magical enough so I don't know what machines the component manufacturers use. Probably super specific, and super expensive.

Testing the PCB itself after assembly is done by one of two machines:

Bed of nails: https://www.youtube.com/watch?v=ReB51Hxvmo8

Flying Leads: https://www.youtube.com/watch?v=7xunyJwjKl0

Answer 6

There are only a few rare cases where components are tested before assembly, generally it is where the cost of the board is very high, e.g. military or extremely dense computing, and a write off would be unacceptable

In these cases you have a few options, you can have carriers which you place the part in and a bed of test points push against the pads of the part, or others where you trial a limited quantity from a batch in a test pcb before doing the rest of the run,

These carriers are made for just about any kind of device you can imagine, even large FPGA's, tiny 4 ball BGA's, to individual resistors or capacitors, usually the harder part is making sure that everything aligns, and that its not too difficult for the operator to move through a set of parts quickly

In most PCB's its generally fit all the components, then test / inspect, and rework any small issues, depending on how complex things get they have optical and x-ray inspection machines where they can compare a perfect working board to the current one and flag any differences for a human to look at, this costs time, so adds some expense when you have large high density boards

Answer 7

"are there specific boards or probes for the tiny components which have extremely small pins" - I'm just answering this part.

The PCB suppliers provide stencils. It is a metal sheet having holes exactly where the component has to be soldered. The stencil is placed on the PCB and soldering lead paste is applied on the PCB through the holes.

Modern industries uses robotic arms and lasers too.

The through hole components are placed in proper position. And the circuit board it dipped into liquid soldering lead. But it is only possible for PCB having heat resistant masking. When the PCB is picked up from the liquid lead, temperature is reduced and the lead gets solid. Lead is deposited only where there is no masking.