6 Questions to ask your CM/EMS about In-Circuit Testing (ICT)
Are you looking to have an electronics contract manufacture build an electronics assembly of yours that will require in-circuit testing? If so, make sure they have good controls in place for using and maintaining ICT fixtures and test software by getting answers to the 6 questions listed below.
The Hippocratic Oath for Test Engineers.
You’ve probably heard of the Hippocratic Oath which is followed by those in the healthcare industry. In essence the oath means, “first, do no harm.” Doctors should try to heal their patients, not make them worse. Similarly, Test Engineers should develop hardware and software to find defective circuit boards, not create defects.
If in-circuit Test Engineers are not careful, their test equipment could create defects. Similarly, test software changes should help catch defects, not allow true defects to escape.
1. How do you approve of an ICT fixture’s design?
The fixture is what a manufactured circuit board is placed in so that it can be probed and have measurements taken. Most companies in the electronic contract manufacturing service (EMS) industry out-source their ICT fixture development.
An EMS company should not blindly accept an ICT fixture from a fixture vendor and begin using it. Instead, The EMS company’s Test Engineers should have an approval process to go through before the fixture is deemed worthy to be used in production.
The Test Engineers should closely inspect all of the standoffs and board hold-downs. The standoffs hold the tested circuit boards up and away from the probe pins while the unit is not being tested. The hold-downs push the circuit board down onto the probe pins while testing is taking place. It is not unusual for an ICT fixture to have a hundred or more standoffs and hold-downs. A standoff or a hold-down could be placed in the fixture in such a way that when a board is being tested, it could come into contact with a component on the board – causing the component to become damaged.
Test Engineers need to make sure that all of the fixture’s standoffs and hold-downs are located sufficiently away from any component on a board that could be tested. It is not good enough to simply have a sample board and use it to confirm that the board’s components do not touch standoffs and hold-downs. This is not good enough because it does not take into account component drift. SMT components will float and move around on the PCB’s pads during reflow. Non-SMT components (i.e. handload components) will have a degree of placement variation from board to board. So, Test Engineers will need to work closely with their company’s Process Engineers to make sure that all standoffs and hold-downs are located sufficiently away from any possible component placement, taking into account component placement variation.
2. How do you make sure you don’t treat an ICT failure as a good one?
To maximize profits electronic contract manufactures need to maximize throughput. This includes having a multi-cavity ICT fixture to test as many boards as possible at the same time. Futaba has numerous ICT fixtures for the various products we assemble for our customers. Our fixtures range from 2-up to a 24-up.
If your EMS of choice does not have special automated board handling equipment to automatically separate the boards that pass ICT from the ones that failed, how do they assure that their operator manually separates them correctly?
Here at Futaba each cavity in our ICT fixture has a unique barcode. All of the units that we manufacture each have a unique barcode. Our ICT software requires the test operator to scan the barcode of the cavity that a unit will be placed in, followed by scanning the barcode of the unit that will be placed in that cavity. At the end of testing, our tracking system database is updated to reflect which units passed test, and which failed. We have other manufacturing processes after ICT. These processes also require the scanning in of a unit’s barcode. If one of these processes detects that the operator scanned in a unit that failed or skipped ICT, the operator will be shown an error message.
3. How are changes to test limits approved?
Changes need to be documented and verified. Here at Futaba (and maybe at your EMS) we have 2 shifts. If the ICT starts failing every board just outside the test limit, but the part is correct and the test limit is too tight, then the Test Engineer will update the particular test. If this happens late during 2nd Shift, chances are he is not going to be able to get everyone’s approval for a test limit change, causing the production line to be down if he cannot get everyone’s approval. Therefore, we allow the Test Engineer (since he knows best anyway) to go ahead and make the change. But we require that he document the change within the test program. In his description of the change, he should specify the date and time of the change. On the following business day, another Test Engineer must investigate and approve of the change. Why should this be done? Let’s say that the upper limit for some capacitor is 950nF (950 nano-farads) and that this upper limit is too low. He determines that it should be set to 1.2uF (a 1.0uF capacitor with a 20% tolerance) so he changes the upper test limit. However, instead of typing in 1.2uF as the upper limit, he makes a mistake in converting from nano-farads to micro-farads and types in 12.0uF. Fast-forward 9 months later and the customer discovers that we were placing 10uF capacitors instead of 1uF capacitors. Besides wanting to know how this happened, the customer will want to know how long did this loophole exist? Well, without documenting a change date, the only answer could be, “Everything we’ve ever built is suspect.” With a date, we can narrow the problem down from the date of the change. However, we’d like to avoid shipping any bad product so that is why we have another set of eyes investigate the test limit change.
4. How often is the fixture checked for maintenance issues?
At a minimum your electronics contact manufacturer should be able explain how each ICT fixture is on a maintenance plan whereby it is cleaned, worn probe pins are replaced, etc. The CM should be able to provide proof in the form of printed or electronic documents that fixtures have been maintained on schedule. If a CM can’t provide this proof, then that is a red flag.
The previous paragraph is the expected answer to this question #5. But we are looking for another answer: the fixture should be inspected at the beginning of and periodically during use on the production floor. Why is this? The short answer is to prevent fixture problems from inducing defects on manufactured boards. Sometimes a bad component could fall off of a circuit board as it is being placed in the fixture. This stray component could settle into a place such that it contacts and damages a component on all boards tested in that cavity. This stray component could “stamp” its shape on a component that it touches, indenting or cracking it.
One or more of the fixture’s hold-downs could become bent during a production shift. A bent hold-down could come into unexpected contact with a board’s component, causing damage.
By quickly finding and fixing problems with a fixture during a production run, a CM minimizes their risk of a fixture damaging a board.
5. How are probe pin replacements done?
With boards becoming increasingly dense, it is more critical than ever to make sure that when a probe is replaced, it is replaced with a correct one. If a pointy probe pin is replaced with a pin that has more of a head on it, there is the possibility that a portion of the head could rub against a component on the board, causing potential damage to it.
The CM should be able to provide a document that describes the probe type and location(s) in the ICT fixture. It is not good if they answer, “We pull the pin out, look at it and then pick the same pin out of our spare parts inventory.” The reason this is a bad answer is because the pin that was pulled out could be wrong to begin with! What if the pulled out pin should have been a pointy type but instead was one with a head on it? With this, “just pull it out and look at it” method, the CM will forever be using the wrong probes.
6. How do you maintain inventory of replacement ICT probe pins?
Ask to see your CM’s spare probe pin inventory. The probe pins should be segregated by part numbers and not “look”. Did you know that 3 probe pins could look exactly the same yet be different? Probe pins have springs within them. These springs come in different spring forces. So, 3 probe pins that look exactly the same could be different due to each having a different spring force.