Folks,
Answers to the quiz of a few weeks back …
Phil and Rob had agreed to ask the GM if it was OK to ask the tech and engineers at some of their subcontractors to take the test anonymously. Over a period of two months Phil and Rob got 52 people to agree, almost all of them after Phil or Rob agreed to take them to lunch. They asked Patty to grade the “exams.” Today Patty would reveal the results.
“Phil, this is one of the best bets I have ever made,” teased Rob.
Everyone at the lunch table chuckled, but the look on Phil’s face said he expected to lose. Rob has said that he thought the average score would be less than 70%, Phil insisted that it would be greater than 85%. In asking the different folks to take the test, invariably Phil started asking questions not on the test. He was surprised that no one knew what tin pest was. He even asked how to time balance a chipshooter and flexible placer, only one in 20 knew.
As Patty approached the lunch table, the ensemble held their breath.
“OK, Patty, tell us the bad news,” Phil said in a resigned tone.
“Rob wins, the average score was 58%,” Patty said getting to the point. “Here are the answers and percentages on each problem,” she went on:
1. What is the composition of SAC305?
96.5% tin, 3.0% silver, 0.5% copper. 60% got this right.
2. What are tin whiskers?
Tin whiskers are metal whiskers that can “grow” from tin plating on component leads. They are mitigated by 2% bismuth in the tin, a nickel overplate of the lead copper, a matte tin finish, and a few other mitigation approaches. (40%.)
3. In a stencil aperture, what is the area ratio?
The ratio of the area of the aperture opening divided by the area of the side walls. This ratio is typically used for circular and square apertures. It is equal to D/4t, where D is the diameter of square side and t is the stencil thickness. (40%)
4. What is an approximate peak temperature for a reflow oven in lead-free assembly?
Any answer 235° to 250°C accepted. (90%)
5. A board is inspected after wave soldering and one lead is not soldered to the board. The board is run through the wave solder machine again and has the same defect on the same lead. What is the most likely cause of the defect?
a. The solder temperature is too low.
b. The pad on the board is oxidized.
c. The preheat temperature is too high.
(b. 70%)
6. What are local fiducials on a PWB for?
Local fiducials are located near the pads of a component with fine lead spacings to ensure accurate placement. (70%)
7. What does “thixotropic” mean in regard to solder pastes?
The viscosity decreases with increasing shear stress. Hence, during printing the viscosity drops as the paste is forced through the aperture, aiding good aperture fill. It increases as the printed deposit rests, minimizing slump. (20%)
8. A chipshooter places passives at a rate of 36,000 per hour. It is placing 300 passives on a PWB, how many seconds will the chipshooter take to place the passives on one board?
300/36000 = 1/120 hr = 30 seconds. (90%)
9. A reflow oven belt speed is 100 cm/min. The PWB is 40 cm long. What is the minimum cycle time that the oven can support?
The amount of time that the belt needs to cover 40 cm is 40/100 = 0.4 minutes = 24 seconds. This is the minimum cycle time the oven can support. (40%)
10. What is “tombstoning”?
Tombstoning is observed when a passive component’s terminations experience unequal wetting forces strong enough to lift one end of the passive so that it looks like a tombstone. (60%)
Overall average score: 58%.
“Wait a minute Patty; your answers are too demanding,” Phil shouted.
“Calm down Phil, I gave full credit for anything close,” Patty responded.
In unison, almost everyone at the table sighed, “Yikes.”
Patty interjected, “One person who received a 70% commented after completing problem 9 said, ‘I didn’t think I would need a Ph.D. in math to do this quiz.’ ”
All agreed that organizations committed to electronics assembly education, like the SMTA and IPC ,were more needed than ever.
Cheers,
Dr. Ron
Tribal knowledge has gotten us to where we are today.
Problem is most of the Indians in the tribe have moved on leaving tid bits of do it this way or that without explaining the how and why behind the statement.
Furthermore, many people involved in manufacturing prefer to remain in their air conditioned offices and not get their hands dirty. Many only want to hear the good news or no news at all.
There is also complacency in relying on data displayed by the machines on the line.
The whole underlying philosophy of my consulting for the past 20 years is that you can’t trust machines and must rely on independent instrumentation to quantify your processes.
That rant should solicit a few replies.
Ray C
Agree with Ray on this, I also belive that the people who run the machines should be trained to the highest level. When an operator knows whats going on, they know whats going wrong, and can mitigate problems etc.
But then I believe PCB designers should spend the first 6 months of their career assemblying and inspecting boards, and then at regular intervals from there on.
Marc.. well said.
A well trained SMT line operator .. should have been able to answer all the questions.
This statement reflects my management style.
This is NOT the norm in industry.
Most companies expect from operators not much more than a modest junior high education…
And most of the time .. they get less.
So the process engineer .. has to spec to the extreme (wasted on most of them) and continue to baby sit the production area.
As Ray and Marc stated… the pcb designer and engineering staff should be knowledgeable of the production processes and experienced with the production processes… otherwise, they will only screw things up when making changes.
Based on the test results.. the process engineers out there are barely qualified to operate a SMT line…
question 7 … only 20%? really?.. this is a really sad state of affairs.