results with rd53a prototype modules at ethz · s5 roc1 digital switches on even when ramped but...

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Malte Backhaus, Vasilije Perovic, Branislav Ristic

26/7/2019Vasilije Perovic 1

Results with RD53A

prototype modules at ETHZ

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▪ Tests on bare HDIs already done (HV, thermal cycling, power dissipation) →

TrackerWeek November

▪ The first digital 2x2 TBPX modules successfully assembled and operated

(powering and scans) → TrackerWeek March

▪ The first chain of three digital modules successfully assembled and operated

→ TrackerWeek May


RD53A 2x2 TBPX HDI

HDI with adapter PCB HDI layout

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▪ HDIs populated by an external company

▪ Issues:

▪ Unusable pads

▪ Surface degradation

▪ FOD

▪ Flux flow

▪ Etchant damage

FOD and flux are successfully

removed during ultrasonic

cleaning

Cleaning + inspection required

to choose suitable HDIs


HDI optical inspection

and preparation

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▪ Ten digital modules built (without sensor) (S0, S1, S3, S4, S5, S6, S7, S8,

S9, S10) (three with wafer-probed chips)

▪ three with a silicon slab (glued on top of the four ROCs)

▪ four without (HDI glued directly on the ROCs)

▪ Wire bonding done at PSI for the first module, at ETH for the other nine


RD53A 2x2 TBPX module assembly

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▪ The first test is to check the performance of the

SLDOs

▪ A full VI curve is obtained by ramping the

current in predetermined steps and measuring

the input and the output voltages

Observations:

▪ If the SLDOs worked, the chip was

functional (digital scan, analog scan,

threshold tuning)

▪ Parallel powering is beneficial for the chips to

start at the same time

▪ S8, S9, S10 made using wafer-probed chips.

Now we can cross-check if anything changes

when the chips are operated in parallel on a

module.


SLDO testing

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▪ Input voltage with the spread of

~50mV at 4A → 12.5mΩ

▪ Some regulators start up at higher

input current (Parallel powering

scheme for the ROCs is beneficial)


An example of good SLDO performance (S4)

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▪ The chain consists of three

modules at the moment

▪ Option of one more with a sufficiently

long e-link

▪ Modules’ grounds are at different

potentials → thermal contact with

the cooling plate is achieved with a

thermal paste and sapphire discs


Building a serially powered chain of RD53A

digital modules

Module

1

Module

2

Module

3+

PSU

-

Module (chip side)Sapphire discThermal paste

Cooled aluminium slab

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Chain powering

Module 1

Module 2

Module 3

PSU

+ -

▪ Voltage drops are consistent with previous measurements

▪ V-I curves take the form seen on single modules

▪ Ground return resistance consistent with bare HDI

measurements → 40mΩ in total including connectors


Threshold distribution

SINGLE

MODULE

CHAIN

WITHOUT

RETUNING

CHAIN

AFTER

RETUNING

Pixel-by-pixel

difference without

retuning

Pixel-by-pixel

difference after

retuning

No change in the threshold is observed after retuning


Noise distribution

SINGLE

MODULE

CHAIN

AFTER

RETUNING

CHAIN

WITHOUT

RETUNING

No change in the noise is observed after retuning

Pixel-by-pixel

difference

without retuning

Pixel-by-pixel

difference

after retuning

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While establishing the communication to a

module, the impedance of the module seen

by the power supply changes, affecting the

potential of the modules upstream in the chain


Noise in the power

Module 1

Module 2

Module 3

PSU

+ -

COMMUNICATION


SLDO - VDDA - S8

ROC0

ROC2 ROC3

ROC1


SLDO - VDDD - S8

ROC0

ROC2 ROC3

ROC1


SLDO – VDDA - S9

ROC0

ROC2 ROC3

ROC1


SLDO – VDDD - S9

ROC0

ROC2 ROC3

ROC1


SLDO – VDDA - S10

ROC0

ROC2 ROC3

ROC1


SLDO – VDDD - S10

ROC0

ROC2 ROC3

ROC1


SLDO – VDDD – S10 – ROC2 – Ramp vs. ON-OFF

ON-OFF cycling

Ramp

SLDO consistently fails if ramped. Ramping involves increasing the

current stepwise, while with the ON-OFF cycling, the supply is switched off

and then on at every step.

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S8

▪ First observation of such problem

▪ Unusual changes in the module impedance

▪ The output is following the input or is slightly under 1V

S9

▪ Fully functional (one SLDO following the input) ROC1 – A

S10

▪ ROC2 – D – common SLDO failure where output is around 200mV if ramped

▪ Operates nominally if switched on without the slow ramp


Summary of S8, S9, S10

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▪ The behaviour of S8 does not change depending on the ramp.

▪ A hint may be in the changing impedance of the whole module

▪ S9 ROC1 – Analog SLDO works fine when not ramped


Failing SLDOs revisited

ON-OFF cyclingRamp

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▪ Similar problem observed earlier on S5. Since the chips were not wafer-

probed, we have just erroneously assumed the SLDOs did not work

▪ S5 ROC2 Analog



ON-OFF cyclingRamp

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▪ S5 ROC1 Digital

▪ Switches on even when ramped but only

at 5.3A input current (limit of the chain)

▪ Behaviour with ON-OFF cycling is normal

but the input impedance of the module

behaves differently



ON-OFF cycling

Ramp

Ramp

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▪ Obtained a programmable current source to control the current ramp speed

(Special thanks to Alvaro for providing the supply!)

▪ The idea is to check S5 and S10 (three SLDOs in total), while S8 and S9

were sent to CERN




S10 – ROC2 Digital SLDO

▪ The transition between the two behaviour modes occurs within 2ms

difference, i.e. the switch-on works reliably for ramp time of 16ms and fails

consistently for 18ms ramp time.

▪ Relatively close to the limit achieved with the power supply (approx. 5ms to

6A)


▪ A good SLDO on the other hand can easily work fine with ramp times of up to

one minute.

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▪ We did not manage to start it with the fastest ramp achievable with the power

supply. The measurement was performed with the TTI supply.

▪ Is it possible that at these ramp speeds we run into problems with

inductance? If yes, such chips should be marked during the wafer probing


S5 – ROC2 Analog

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▪ Starting with an increased current works but

1) It is pushing the current to the limit of the module/chain

2) The switch on occurs at a higher current than normally observed with the

previous SLDO tests


S5 – ROC2 Analog

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▪ Slow ramp results in a high start current. The starting itself seems insensitive

to the ramp speed provided that the current can be reached


S5 – ROC1 Digital

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▪ A fast ramp can switch the SLDO on

at a lower current as expected

▪ A delay is seen when ramping for

10ms (close to the limit)

▪ 11ms ramp results in a failure


S5 – ROC1 Digital

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▪ To do:

▪ Build more modules and continue testing

▪ Systematically measure behaviour in different failure modes

including the source of the failure (S8 and S9 are at CERN)

▪ Check for temperature dependencies etc.

▪ Check if the ramp speed limits change in a chain when different

modules are switched on and the potential changes.

▪ Wishes:

▪ SCCs with “failed” SLDOs, those with any anomalies. If you are

happy to give some for (temporary) adoption, please let us know.


Next steps

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▪ Assembly

▪ 10 digital modules built (no sensor), three of which with wafer-probed chips

▪ SLDO test showed two main modes of failure

1. Output following the input with an offset

2. Output remaining low (around 200mV)

▪ Sometimes the “failure” was due to the nature of the current ramp, except in

S8 where the failure was not resolved by increasing the ramp speed. The

impedance of the module still behaved unexpectedly.

Two ways to resolve the start-up were noted:

▪ The limit for the ramp to get the SLDOs to start is around 10ms. Possible

problems with the inductance?

▪ Increase the current (may not work when already operating close to the

limit)

We propose to implement VI scan with ON-OFF switching into the wafer

probing procedure and mark the chips that show different behaviour


Summary


S9 ROC1

Wafer probing results

results with rd53a prototype modules at ethz · s5 roc1 digital switches on even when ramped but...

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